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C++ Paradigm: multi-paradigm
Appeared in: 1983 (1985 "The C++ Programming Language")
Designed by: Bjarne Stroustrup
Typing discipline: Static, unsafe, nominative
Major implementations: GNU Compiler Collection, Microsoft Visual C++, Borland C++ Builder
Dialects: ISO/IEC C++ 1998, ISO/IEC C++ 2003
Influenced by: C, Simula, Ada 83, ALGOL 68, CLU, ML
Influenced: Ada 95, C#, Java, PHP, D, Aikido
C++ (pronounced "see plus plus", IPA: /siː plʌs plʌs/) is a general-purpose programming language with high-level and low-level capabilities. It is a statically typed, free-form, multi-paradigm, usually compiled language supporting procedural programming, data abstraction, object-oriented programming, and generic programming.
C++ is regarded as a mid-level language. This indicates that C++ comprises a combination of both high-level and low-level language features.[1]
Dr. Bjarne Stroustrup developed C++ in 1979 at Bell Labs as an enhancement to the C programming language and named it "C with Classes". In 1983 it was renamed to C++. Enhancements started with the addition of classes, followed by, among other features, virtual functions, operator overloading, multiple inheritance, templates, and exception handling. The C++ programming language standard was ratified in 1998 as ISO/IEC 14882:1998, the current version of which is the 2003 version, ISO/IEC 14882:2003. A new version of the standard (known informally as C++0x) is being developed.
Contents [hide]
1 History
1.1 Language standard
1.2 The name "C++"
1.3 Future development
2 Philosophy
3 Standard library
4 Features introduced in C++
5 Hello world program
6 Language features
6.1 Operators
6.2 Preprocessor
6.3 Templates
6.4 Objects
6.4.1 Encapsulation
6.4.2 Inheritance
6.5 Polymorphism
6.5.1 Static polymorphism
6.5.1.1 Function overloading
6.5.1.2 Operator overloading
6.5.1.3 Default arguments
6.5.1.4 Class and function templates
6.5.2 Dynamic polymorphism
6.5.2.1 Inheritance
6.5.2.2 Virtual member functions
7 Parsing and processing C++ source code
8 Problems and controversies
8.1 Standards compliance
8.2 Criticism
9 Incompatibility with C
10 See also
11 References
12 General references
13 External links
History
Stroustrup began work on C with Classes in 1979. The idea of creating a new language originated from Stroustrup's experience in programming for his Ph.D. thesis. Stroustrup found that Simula had features that were very helpful for large software development, but the language was too slow for practical use, while BCPL was fast but too low-level and unsuitable for large software development. When Stroustrup started working in Bell Labs, he had the problem of analyzing the UNIX kernel with respect to distributed computing. Remembering his Ph.D. experience, Stroustrup set out to enhance the C language with Simula-like features. C was chosen because it is general-purpose, fast, portable and widely used. Besides C and Simula, some other languages which inspired him were ALGOL 68, Ada, CLU and ML. At first, the class, derived class, strong type checking, inlining, and default argument features were added to C via Cfront. The first commercial release occurred in October 1985.[2]
In 1983, the name of the language was changed from C with Classes to C++. New features were added including virtual functions, function name and operator overloading, references, constants, user-controlled free-store memory control, improved type checking, and BCPL style single-line comments with two forward slashes (//). In 1985, the first edition of The C++ Programming Language was released, providing an important reference to the language, as there was not yet an official standard. In 1989, Release 2.0 of C++ was released. New features included multiple inheritance, abstract classes, static member functions, const member functions, and protected members. In 1990, The Annotated C++ Reference Manual was published. This work became the basis for the future standard. Late addition of features included templates, exceptions, namespaces, new casts, and a Boolean type.
As the C++ language evolved, a standard library also evolved with it. The first addition to the C++ standard library was the stream I/O library which provided facilities to replace the traditional C functions such as printf and scanf. Later, among the most significant additions to the standard library, was the Standard Template Library.
Language standard
After years of work, a joint ANSI–ISO committee standardized C++ in 1998 (ISO/IEC 14882:1998). For some years after the official release of the standard, the committee processed defect reports, and published a corrected version of the C++ standard in 2003. In 2005, a technical report, called the "Library Technical Report 1" (often known as TR1 for short) was released. While not an official part of the standard, it gives a number of extensions to the standard library which are expected to be included in the next version of C++. Support for TR1 is growing in almost all currently maintained C++ compilers.
While the C++ language is royalty-free, the standard document itself is not freely available.
The name "C++"
This name is credited to Rick Mascitti (mid-1983) and was first used in December 1983. Earlier, during the research period, the developing language had been referred to as "new C", then "C with Classes". In computer science C++ is still referred to as a superstructure of C. The final name stems from C's "++" operator (which increments the value of a variable) and a common naming convention of using "+" to indicate an enhanced computer program. According to Stroustrup: "the name signifies the evolutionary nature of the changes from C". C+ was the name of an earlier, unrelated programming language.
Stroustrup addresses the origin of the name in Chapter 1 of his book, The C++ Programming Language, remarking that another interpretation of the C++ name could be seen from the appendix of George Orwell's Nineteen Eighty-Four. Of the three segments of the fictional language Newspeak, the "C vocabulary" is the one dedicated to technical terms and jargon. "Doubleplus" is the superlative modifier for Newspeak adjectives. Thus, "C++" might hold the meaning "most C-like" in Newspeak.
When Rick Mascitti was questioned informally in 1992 about the naming, he indicated that it was given in a tongue-in-cheek spirit. He never thought that it would become the formal name of the language.
Future development
C++ continues to evolve to meet future requirements. A new version of the C++ standard is currently being worked on, entitled C++0x, denoting that it is expected to be released before 2010. Current work indicates that C++ will continue to capitalize on its multi-paradigm nature. Notable expected improvements are native support for threading and concepts that will make working with templates easier. Adding garbage collection is currently under heavy discussion. Boost.org is a group working to make the most of C++ in its current form. They are expanding C++'s functional and metaprogramming abilities and also advise the C++ standards committee on which features work well and which need improving.
Philosophy
In The Design and Evolution of C++ (1994), Bjarne Stroustrup describes some rules that he uses for the design of C++. Knowing the rules helps to understand why C++ is the way it is. The following is a summary of the rules. Much more detail can be found in The Design and Evolution of C++.
C++ is designed to be a statically typed, general-purpose language that is as efficient and portable as C
C++ is designed to directly and comprehensively support multiple programming styles (procedural programming, data abstraction, object-oriented programming, and generic programming)
C++ is designed to give the programmer choice, even if this makes it possible for the programmer to choose incorrectly
C++ is designed to be as compatible with C as possible, therefore providing a smooth transition from C
C++ avoids features that are platform specific or not general purpose
C++ does not incur overhead for features that are not used
C++ is designed to function without a sophisticated programming environment
Inside the C++ Object Model (Lippman, 1996) describes how compilers may convert C++ program statements into an in-memory layout. Compiler authors are free to implement the standard in their own manner.
Standard library
The 1998 ANSI/ISO C++ standard consists of two parts: the core language and the C++ standard library; the latter includes most of the Standard Template Library (STL) and a slightly modified version of the C standard library. Many C++ libraries exist which are not part of the standard, and, using linkage specification, libraries can even be written in languages such as C, Fortran, Pascal, or BASIC. Which of these are supported is compiler dependent.
The C++ standard library incorporates the C standard library with some small modifications to make it work better with the C++ language. Another large part of the C++ library is based on the STL. This provides such useful tools as containers (for example vectors and lists), iterators (generalized pointers) to provide these containers with array-like access and algorithms to perform operations such as searching and sorting. Furthermore (multi)maps (associative arrays) and (multi)sets are provided, all of which export compatible interfaces. Therefore it is possible, using templates, to write generic algorithms that work with any container or on any sequence defined by iterators. As in C, the features of the library are accessed by using the #include directive to include a standard header. C++ provides 69 standard headers, of which 19 are deprecated.
Using the standard library — for example, using std::vector or std::string instead of a C-style array — can help lead to safer and more scalable software.
The STL was originally a third-party library from HP and later SGI, before its incorporation into the C++ standard. The standard does not refer to it as "STL", as it is merely a part of the standard library, but many people still use that term to distinguish it from the rest of the library (input/output streams, internationalization, diagnostics, the C library subset, etc.).
Most C++ compilers provide an implementation of the C++ standard library, including the STL. Compiler-independent implementations of the STL, such as STLPort, also exist. Other projects also produce various custom implementations of the C++ standard library and the STL with various design goals.
Features introduced in C++
Compared to the C language, C++ introduced extra features, including declarations as statements, function-like casts, new/delete, bool, reference types, inline functions, default arguments, function and operator overloading, namespaces and the scope resolution (::) operator, classes (including all class-related features such as inheritance, member functions, virtual functions, abstract classes, and constructors), templates, exception handling, runtime type identification, and the overloaded input (>>) and output (<<) operators for input and output respectively.
Contrary to popular belief, C++ did not introduce the const keyword. Const was formally added to C shortly before it was adopted by C++.
C++ also performs more type checking than C in several cases (see "Incompatibility with C" below).
Double slash comments starting with // were originally part of C's predecessor, BCPL, and were reintroduced in C++.
Several features of C++ were later adopted by C, including declarations in for loops, C++-style comments (using the // symbol), and inline, though the C99 definition of the inline keyword is not compatible with its C++ definition. However, C99 also introduced features that do not exist in C++, such as variadic macros and better handling of arrays as parameters; some C++ compilers may implement some of these features as extensions, but others are incompatible with existing C++ features.
Hello world program
The following is a Hello world program which uses the C++ standard library stream facility to write a message to standard output.[3]
#include
int main()
{
std::cout << "Hello, world!\n";
return 0;
}
Language features
[edit] Operators
Main article: Operators in C and C++
Preprocessor
C++ is principally compiled in three phases: preprocessing, translation to object code, and linking (the two last phases are what is generally thought of as the "compilation" proper). In the first phase, preprocessing, preprocessor directives apply lexical transformations to the source code, which is then fed to the compilation stage.
Preprocessor directives start with # as the first character on a line and before any spaces and work by simple substitution of tokenized character sequences for other character sequences or files, according to user-defined rules. They typically perform macro substitution, inclusion of other files (by opposition to higher-order features such as inclusion of modules/packages/units/components), conditional compilation and/or conditional inclusion. For instance:
#include
which includes (imports) all symbols from the standard library header file iostream.
Another common use is what is commonly referred to as macros:
#define MY_ASSERT(x) assert(x)
which replaces MY_ASSERT(x) as it appears in the source code with assert(x). This allows control over the use of assertions in a particular compilation unit.
Using macro definitions to emulate functions however is discouraged in practice, as it doesn't allow any type checking for the parameters and the resulting code may introduce some pitfalls.[4] Instead, the use of inline functions for this purpose is recommended.
In addition to these common directives there are several additional preprocessor directives that control the flow of compilation, conditionally include or exclude code blocks, and control various other aspects of compilation.
Traditionally the preprocessor was also used to define numerical constants, however the use of const is now preferred over #define. This provides stronger type checking and does not subvert the use of namespaces as the preprocessor does.
The goal of the standardization committee is to reduce dependency on the preprocessor. Because C++'s modular nature requires support for such directives as #include and #define, it is unlikely that it will be completely eliminated.
See also: preprocessor and C preprocessor
Templates
See also: generic programming and template metaprogramming
Templates are different from macros: while both of these compile-time language features can be used to produce conditional compilation, templates are not restricted to lexical substitution. Templates have an awareness of the semantics and type system of their companion language as well as all compile-time type definitions and can perform high-level operations including programmatic flow control based on evaluation of strictly type-checked parameters. Macros are capable of conditional control over compilation based on predetermined criteria but cannot instantiate new types, recurse or perform type evaluation and in effect are limited to pre-compilation text-substitution and text-inclusion/exclusion. In other words, macros can control compilation flow based on pre-defined symbols but cannot, unlike templates, independently instantiate new symbols. Templates are a tool for static polymorphism (see below) and generic programming. For example, a template replacing the common, but dangerous, macro #define max(x,y) ((x)>(y)?(x):(y)):
template
T max(const T& x, const T& y)
{
return x > y ? x : y;
}
This can be found in the algorithm header as std::max(). Traditionally the keyword class may also be used in place of typename.
In addition, templates are a compile time mechanism in C++ which is Turing-complete, meaning that any computation expressible by a computer program can be computed, in some form, by a template metaprogram prior to runtime.
In summary defining a template for a function or class is the equivalent of defining a function or class for each type that can be used as argument, but does not require forward knowledge of which types will be used.
Objects
C++ introduces some object-oriented (OO) features to C. It offers classes, which provide the four features commonly present in OO (and some non-OO) languages: abstraction, encapsulation, inheritance and polymorphism. Objects are instances of classes created at runtime. Think of the class as a template from which many different individual objects may be generated as a program runs.
Encapsulation
Encapsulation is the grouping together of data and functionality. C++ implements encapsulation by allowing all members of a class to be declared as either public, private, or protected. A public member of the class will be accessible to any function. A private member will only be accessible to functions that are members of that class and to functions and classes explicitly granted access permission by the class ("friends"). A protected member will be accessible to members of classes that inherit from the class in addition to the class itself and any friends.
The OO principle is that all and only the functions that can access the internal representation of a type should be encapsulated within the type definition. C++ supports this (via member functions and friend functions), but does not enforce it: the programmer can declare parts or all of the representation of a type to be public, and is also allowed to make public entities that are not part of the representation of the type. Because of this, C++ supports not just OO programming but other weaker decomposition paradigms, like modular programming.
It is generally considered good practice to make all data private or protected, and to make public only those functions that are part of a minimal interface for users of the class, that hides implementation details.
Inheritance
Inheritance allows one data type to acquire properties of other data types. Inheritance from a base class may be declared as public, protected, or private. This access specifier determines whether unrelated and derived classes can access the inherited public and protected members of the base class. Only public inheritance corresponds to what is usually meant by "inheritance". The other two forms are much less frequently used. If the access specifier is omitted, inheritance is assumed to be private for a class base and public for a struct base. Base classes may be declared as virtual; this is called virtual inheritance. Virtual inheritance ensures that only one instance of a base class exists in the inheritance graph, avoiding some of the ambiguity problems of multiple inheritance.
Multiple inheritance is a C++ feature sometimes considered controversial. Multiple inheritance allows a class to be derived from more than one base class; this can result in a complicated graph of inheritance relationships. For example, a "Flying Cat" class can inherit from both "Cat" and "Flying Mammal". Some other languages, such as C# or Java, accomplish something similar (although more limited) by allowing inheritance of multiple interfaces while restricting the number of base classes to one (interfaces, unlike classes, provide only declarations of member functions, no implementation or member data).
Polymorphism
See also: Polymorphism in object-oriented programming
Polymorphism enables one common interface for many implementations, and for objects to act differently under different circumstances.
C++ supports several kinds of static (compile-time) and dynamic (run-time) polymorphism. Compile-time polymorphism does not allow for certain run-time decisions, while run-time polymorphism typically incurs a performance penalty.
Static polymorphism
[edit] Function overloading
Function overloading allows programs to declare multiple functions having the same name (but with different arguments). The functions are distinguished by the number and/or types of their formal parameters. Thus, the same function name can refer to different functions depending on the context in which it is used. The type returned by the function is not used to distinguish overloaded functions.
Operator overloading
Similarly, operator overloading allows programs to define certain operators (such as +, !=, <, or &) to result in a function call that depends on the types of the operands they are used on. Overloading an operator does not change the precedence of calculations involving the operator, nor does it change the number of operands that the operator uses (any operand may however be ignored). The . :: .* ? operators can not be overloaded.
Default arguments
Default arguments are used when defining a different function is not needed when supplying a default value for an argument will suffice. Care should be taken when using default arguments in conjunction with overloaded functions to not cause a conflict over which function to use. For instance, the following code:
// function with default argument but also an overloaded function
int strcpy(char *str1, char *str2, short unsigned n=65535);
// second overloaded function
int strcpy(char *str1, char *str2);
will compile correctly when strcpy is used with an argument for n but not when no argument is specified. This is because the compiler has no means of knowing if the intended function is the first form with a default value of 65535 for n or the second form with no n argument .
Class and function templates
Templates in C++ provide a sophisticated mechanism for writing generic, polymorphic code. In particular, through the Curiously Recurring Template Pattern it's possible to implement a form of static polymorphism that closely mimics the syntax for overriding virtual methods (a dynamic polymorphism technique described below). Since C++ templates are type-aware and Turing-complete they can also be used to let the compiler resolve recursive conditionals and generate substantial programs through template metaprogramming.
Dynamic polymorphism
Inheritance
Variable pointers (and references) of a base class type in C++ can refer to objects of any derived classes of that type in addition to objects exactly matching the variable type. This allows arrays and other kinds of containers to hold pointers to objects of differing types. Because assignment of values to variables usually occurs at run-time, this is necessarily a run-time phenomenon.
C++ also provides a dynamic_cast operator, which allows the program to safely attempt conversion of an object into an object of a more specific object type (as opposed to conversion to a more general type, which is always allowed). This feature relies on run-time type information (RTTI). Objects known to be of a certain specific type can also be cast to that type with static_cast, a purely compile-time construct which is faster and does not require RTTI.
Virtual member functions
Ordinarily when a method in a derived class overrides a method in a base class the method to call is determined by the type of the object. Methods are overridden when, unlike with function overloading, there exists no distinction between the parameters for a given method in number or type. By virtue of inherited objects being polymorphic, it may not be possible for the compiler to determine the type of the object and therefore the correct function to call at compile time and the decision is therefore put off until runtime. This is called dynamic dispatch. Virtual member functions or methods allow the most specific implementation of the function to be called, according to the actual run-time type of the object. In C++, this is commonly done using virtual function tables. This may sometimes be bypassed by prepending a fully qualified class name before the function call, but calls to virtual functions are in general always resolved at run time.
In addition to standard member functions, operator overloads and destructors can also be virtual. A general rule of thumb is that if any functions in the class are virtual, the destructor should be as well. As the type of an object at its creation is known at compile time, constructors, and by extention copy constructors, can not be virtual. Nontheless a situation may arise where a copy of an object needs to be created when a pointer to a derived object is passed as a pointer to a base object. In such a case a common solution is to create a Clone() (or similar) method and declare that as virtual. The Clone() method creates and returns a copy of the derived class when called.
A member function can also be made "pure virtual" by appending it with = 0 after the closing bracket and before the semicolon. Objects can not be created of a class with a pure virtual function and are called abstract data types. Such abstract data types can only be derived from. Any derived class inherits the virtual function as pure and must override it (and all other pure virtual functions) with a non-pure virtual function for objects to be created from the derived class. An attempt to create an object from a class with a pure virtual function or inherited pure virtual function will be flagged as a compile-time error.
An example:
#include
class Bird // the "generic" base class
{
public:
virtual void OutputName() {std::cout << "a bird";}
virtual ~Bird() {}
};
class Swan : public Bird // Swan derives from Bird
{
public:
void OutputName() {std::cout << "a swan";} // overrides virtual function
};
int main()
{
Swan mySwan; // Creates a swan.
Bird* myBird = &mySwan; // Declares a pointer to a generic Bird,
// and sets it pointing to a newly created Swan.
myBird->OutputName(); // This will output "a swan", not "a bird".
return 0;
}
This example program makes use of virtual functions, polymorphism, and inheritance to derive new, more specific objects from a base class. In this case, the base class is a Bird, and the more specific Swan is made.
[edit] Parsing and processing C++ source code
It is relatively difficult to write a good C++ parser with classic parsing algorithms such as LALR(1) (see [1]). This is partly because the C++ grammar is not LALR. Because of this, there are very few tools for analyzing or performing non-trivial transformations (e.g., refactoring) of existing code. One way to handle this difficulty is to choose a different syntax, such as Significantly Prettier and Easier C++ Syntax, which is LALR(1) parsable. More powerful parsers, such as GLR parsers, can be substantially simpler (though slower).
Parsing (in the literal sense of producing a syntax tree) is not the most difficult problem in building a C++ processing tool. Such tools must also have the same understanding of the meaning of the identifiers in the program as a compiler might have. Practical systems for processing C++ must then not only parse the source text, but be able to resolve for each identifier precisely which definition applies (e.g. they must correctly handle C++'s complex scoping rules) and what its type is, as well as the types of larger expressions.
Finally, a practical C++ processing tool must be able to handle the variety of C++ dialects used in practice (such as GNU's and Microsoft's) and implement appropriate analyzers, source code transformers, and regenerate source text. Combining advanced parsing algorithms such as GLR with symbol table construction and program transformation machinery can enable the construction of arbitrary C++ tools.
Problems and controversies
[edit] Standards compliance
Traditionally, producing a reasonably standards-compliant C++ compiler has proven to be a difficult task for compiler vendors in general. For many years, different C++ compilers implemented the C++ language to different levels of compliance to the standard, and their implementations varied widely in some areas such as partial template specialization. Recent releases of most popular C++ compilers support almost all of the C++ 1998 standard.[5]
One particular point of contention is the export keyword, intended to allow template definitions to be separated from their declarations. The first compiler to implement export was Comeau C++, in early 2003 (5 years after the release of the standard); in 2004, the beta compiler of Borland C++ Builder X was also released with export. Both of these compilers are based on the EDG C++ front end. It should also be noted that many C++ books provide example code using the keyword export (for example, Beginning ANSI C++ by Ivor Horton) which will not compile in most compilers, but there is no reference to the problem with the keyword export mentioned. Other compilers such as GCC do not support it at all. Herb Sutter, secretary of the C++ standards committee, recommended that export be removed from future versions of the C++ standard, [6] but finally the decision was made to retain it.[7]
In order to give compiler vendors greater freedom, the C++ standards committee decided not to dictate the implementation of name mangling, exception handling, and other implementation-specific features. The downside of this decision is that object code produced by different compilers are expected to be incompatible. There are, however, third party standards for particular machines or operating systems which attempt to standardize compilers on those platforms (for example C++ ABI[8]); some compilers adopt a secondary standard for these items.
Criticism
Modern critics of the language raise several points. First, since C++ is based on and largely compatible with C, it inherits most of the criticisms leveled at that language. Taken as a whole C++ has a large feature set, including all of C, plus a large set of its own additions, in part leading to criticisms of being a "bloated" and complicated language, especially for embedded systems due to features such as exceptions and RTTI which add to code size.[citation needed] However, every compiler allows the developer to disable exceptions if desired. Bjarne Stroustrup also points out that resultant executables don't support these claims of bloat: "I have even seen the C++ version of the 'hello world' program smaller than the C version."[9] The Embedded C++ standard was specified to deal with part of this, but it received criticism for leaving out useful parts of the language that incur no runtime penalty.[10] Because of its large featureset it can be quite difficult to fully master C++, leading to programmers often bringing unnecessarily advanced or complicated solutions to simple problems.[citation needed]
While C++ is more complex than some other programming languages, Bjarne Stroustrup points out that "The programming world is far more complex today than it was 30 years ago, and modern programming languages reflect that."[11] The ISO standard of the C++ language is about 310 pages (excluding library). For comparison, the C programming language's is about 160 pages, even though it was designed more than 15 years prior and doesn't consider Object Oriented Programming. Furthermore, C#'s ECMA language definition document is about 440 pages.
C++ is also sometimes compared unfavorably with single-paradigm object-oriented languages such as Java, on the basis that it allows programmers to "mix and match" object-oriented and procedural programming, rather than strictly enforcing a single paradigm. This is part of a wider debate on the relative merits of the two programming styles.[citation needed]
Incompatibility with C
For more details on this topic, see Compatibility of C and C++.
C++ is often considered to be a superset of C,[citation needed] but this is not strictly true. Most C code can easily be made to compile correctly in C++, but there are a few differences that cause some valid C code to be invalid in C++, or to behave differently in C++.
One commonly encountered difference is that C allows implicit conversion from void* to other pointer types, but C++ does not. So, the following is valid C code:
int *i = malloc(sizeof(int) * 5); /* Implicit conversion from void* to int* */
... but to make it work in both C and C++ one would need to use an explicit cast:
int *i = (int *) malloc(sizeof(int) * 5);
...and in C++-only code, the static cast is recommended:
int *i = static_cast
Another common portability issue is that C++ defines many new keywords, such as new and class, that may be used as identifiers (e.g. variable names) in a C program.
Some incompatibilities have been removed by the latest (C99) C standard, which now supports C++ features such as // comments and mixed declarations and code. However, C99 introduced a number of new features that C++ does not support (such as variable-length arrays, native complex-number types, and compound literals), so the languages may be diverging more than they are converging. (However, at least some of the new C99 features will likely be included in the next version of the C++ standard, C++0x.)
In order to intermix C and C++ code, any C code which is to be called from/used in C++ must be declared with C linkage by placing it within an extern "C" { ... } block.
See also
C++ structures and classes
List of C++ compilers and integrated development environments
Visual C++
Boost C++ Libraries - Selection of C++ libraries designed to work with the standard C++ library
CodeSynthesis XSD - An open-source XML data binding framework for C++
Name mangling
Template metaprogramming
Comparison of Java and C++
Comparison of programming languages
Aard - A programming tool to check memory usage of C++ programs
Aikido - Scripting language with C++ like syntax
References
^ C++ The Complete Reference Third Edition, Herbert Schildt, Publisher: Osborne McGraw-Hill.
^ Bjarne Stroustrup's FAQ - When was C++ invented?. Retrieved on 2006-05-30.
^ Open issues for The C++ Programming Language (3rd Edition) - This code is copied directly from Bjarne Stroustrup's errata page (p. 633). He addresses the std::endl issue but does not speak to the definition of std::ostream, so since Stroustrup is a reliable authority on the language he created, we must assume that
^ http://www.parashift.com/c++-faq-lite/inline-functions.html#faq-9.5
^ Herb Sutter (2003-04-15). C++ Conformance Roundup. Dr. Dobb's Journal. Retrieved on 2006-05-30.
^ Why We Can’t Afford ExportPDF (266 KiB)
^ Minutes of J16 Meeting No. 36/WG21 Meeting No. 31, April 7-11, 2003 (English) (2003-04-25). Retrieved on 2006-09-04.
^ C++ ABI. Retrieved on 2006-05-30.
^ Why is the code generated for the "Hello world" program ten times larger for C++ than for C?
^ What do you think of EC++?
^ Why is C++ so BIG?
General references
Abrahams, David; Aleksey Gurtovoy. C++ Template Metaprogramming: Concepts, Tools, and Techniques from Boost and Beyond. Addison-Wesley. ISBN 0-321-22725-5.
Alexandrescu, Andrei (2001). Modern C++ Design: Generic Programming and Design Patterns Applied. Addison-Wesley. ISBN 0-201-70431-5.
Becker, Pete (2006). The C++ Standard Library Extensions : A Tutorial and Reference. Addison-Wesley. ISBN 0-321-41299-0.
Alexandrescu, Andrei; Herb Sutter (2004). C++ Design and Coding Standards: Rules and Guidelines for Writing Programs. Addison-Wesley. ISBN 0-321-11358-6.
Coplien, James O. (1992, reprinted with corrections 1994). Advanced C++: Programming Styles and Idioms. ISBN 0-201-54855-0.
Dewhurst, Stephen C. (2005). C++ Common Knowledge: Essential Intermediate Programming. Addison-Wesley. ISBN 0-321-32192-8.
Information Technology Industry Council (2003-10-15). Programming languages — C++, Second edition, Geneva: ISO/IEC. 14882:2003(E).
Josuttis, Nicolai M. The C++ Standard Library. Addison-Wesley. ISBN 0-201-37926-0.
Koenig, Andrew; Barbara E. Moo (2000). Accelerated C++ - Practical Programming by Example. Addison-Wesley. ISBN 0-201-70353-X.
Lippman, Stanley B.; Josée Lajoie, Barbara E. Moo (2005). C++ Primer. Addison-Wesley. ISBN 0-201-72148-1.
Lippman, Stanley B. (1996). Inside the C++ Object Model. Addison-Wesley. ISBN 0-201-83454-5.
Stroustrup, Bjarne (2000). The C++ Programming Language, Special Edition, Addison-Wesley. ISBN 0-201-70073-5.
Stroustrup, Bjarne (1994). The Design and Evolution of C++. Addison-Wesley. ISBN 0-201-54330-3.
Sutter, Herb (2001). More Exceptional C++: 40 New Engineering Puzzles, Programming Problems, and Solutions. Addison-Wesley. ISBN 0-201-70434-X.
Sutter, Herb (2004). Exceptional C++ Style. Addison-Wesley. ISBN 0-201-76042-8.
Vandevoorde, David; Nicolai M. Josuttis (2003). C++ Templates: The complete Guide. Addison-Wesley. ISBN 0-201-73484-2.
External links
Wikibooks has a book on the topic of
C++ ProgrammingLook up C++ in
Wiktionary, the free dictionary.A paper by Stroustrup showing the timeline of C++ evolution (1979-1991)
Open Directory on C++
Apache C++ Standard Library Documentation
Dinkumware's C++ Library Reference Manual
C/C++ Reference
Standards Committee Page: JTC1/SC22/WG21 - C++
CodePedia C++ main page
A Compiler-Comparison chart
C++ FAQ Lite by Marshall Cline
comp.lang.c++, comp.lang.c++.moderated, comp.std.c++ Newsgroups
C++ Tutorial Links
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Tuesday, September 25, 2007
c++ language
Sunday, September 23, 2007
c tutorial
C Language Tutorial
Table of Contents:
* 1. A First Program
* 2. Let's Compute
* 3. Loops
* 4. Symbolic Constants
* 5. Conditionals
* 6. Pointers
* 7. Arrays
* 8. Character Arrays
* 9. I/O Capabilities
* 10. Functions
* 11. Command-line Arguments
* 12. Graphical Interfaces: Dialog Boxes
This section contains a brief introduction to the C language. It is intended as a tutorial on the language, and aims at getting a reader new to C started as quickly as possible. It is certainly not intended as a substitute for any of the numerous textbooks on C.
The best way to learn a new ``human'' language is to speak it right from the outset, listening and repeating, leaving the intricacies of the grammar for later. The same applies to computer languages--to learn C, we must start writing C programs as quickly as possible.
An excellent textbook on C by two well-known and widely respected authors is:
The C Programming Language -- ANSI C
Brian W. C. Kernighan & Dennis M. Ritchie
Prentice Hall, 1988
Dennis Ritchie designed and implemented the first C compiler on a PDP-11 (a prehistoric machine by today's standards, yet one which had enormous influence on modern scientific computation). The C language was based on two (now defunct) languages: BCPL, written by Martin Richards, and B, written by Ken Thompson in 1970 for the first UNIX system on a PDP-7. The original ``official'' C language was the ``K & R'' C, the nickname coming from the names of the two authors of the original ``The C Programming Language''. In 1988, the American National Standards Institute (ANSI) adopted a ``new and improved'' version of C, known today as ``ANSI C''. This is the version described in the current edition of ``The C Programming Language -- ANSI C''. The ANSI version contains many revisions to the syntax and the internal workings of the language, the major ones being improved calling syntax for procedures and standarization of most (but, unfortunately, not quite all!) system libraries.
1. A First Program
Let's be polite and start by saluting the world! Type the following program into your favorite editor:
#include < stdio.h>
void main()
{
printf("\nHello World\n");
}
Save the code in the file hello.c, then compile it by typing:
gcc hello.c
This creates an executable file a.out, which is then executed simply by typing its name. The result is that the characters `` Hello World'' are printed out, preceded by an empty line.
A C program contains functions and variables. The functions specify the tasks to be performed by the program. The ``main'' function establishes the overall logic of the code. It is normally kept short and calls different functions to perform the necessary sub-tasks. All C codes must have a ``main'' function.
Our hello.c code calls printf, an output function from the I/O (input/output) library (defined in the file stdio.h). The original C language did not have any built-in I/O statements whatsoever. Nor did it have much arithmetic functionality. The original language was really not intended for ''scientific'' or ''technical'' computation.. These functions are now performed by standard libraries, which are now part of ANSI C. The K & R textbook lists the content of these and other standard libraries in an appendix.
The printf line prints the message ``Hello World'' on ``stdout'' (the output stream corresponding to the X-terminal window in which you run the code); ``\n'' prints a ``new line'' character, which brings the cursor onto the next line. By construction, printf never inserts this character on its own: the following program would produce the same result:
#include < stdio.h>
void main()
{
printf("\n");
printf("Hello World");
printf("\n");
}
Try leaving out the ``\n'' lines and see what happens.
The first statement ``#include < stdio.h>'' includes a specification of the C I/O library. All variables in C must be explicitly defined before use: the ``.h'' files are by convention ``header files'' which contain definitions of variables and functions necessary for the functioning of a program, whether it be in a user-written section of code, or as part of the standard C libaries. The directive ``#include'' tells the C compiler to insert the contents of the specified file at that point in the code. The ``< ...>'' notation instructs the compiler to look for the file in certain ``standard'' system directories.
The void preceeding ``main'' indicates that main is of ``void'' type--that is, it has no type associated with it, meaning that it cannot return a result on execution.
The ``;'' denotes the end of a statement. Blocks of statements are put in braces {...}, as in the definition of functions. All C statements are defined in free format, i.e., with no specified layout or column assignment. Whitespace (tabs or spaces) is never significant, except inside quotes as part of a character string. The following program would produce exactly the same result as our earlier example:
#include < stdio.h>
void main(){printf("\nHello World\n");}
The reasons for arranging your programs in lines and indenting to show structure should be obvious!
2. Let's Compute
The following program, sine.c, computes a table of the sine function for angles between 0 and 360 degrees.
/************************/
/* Table of */
/* Sine Function */
/************************/
/* Michel Vallieres */
/* Written: Winter 1995 */
#include < stdio.h>
#include < math.h>
void main()
{
int angle_degree;
double angle_radian, pi, value;
/* Print a header */
printf ("\nCompute a table of the sine function\n\n");
/* obtain pi once for all */
/* or just use pi = M_PI, where
M_PI is defined in math.h */
pi = 4.0*atan(1.0);
printf ( " Value of PI = %f \n\n", pi );
printf ( " angle Sine \n" );
angle_degree=0; /* initial angle value */
/* scan over angle */
while ( angle_degree <= 360 ) /* loop until angle_degree > 360 */
{
angle_radian = pi * angle_degree/180.0 ;
value = sin(angle_radian);
printf ( " %3d %f \n ", angle_degree, value );
angle_degree = angle_degree + 10; /* increment the loop index */
}
}
The code starts with a series of comments indicating its the purpose, as well as its author. It is considered good programming style to identify and document your work (although, sadly, most people only do this as an afterthought). Comments can be written anywhere in the code: any characters between /* and */ are ignored by the compiler and can be used to make the code easier to understand. The use of variable names that are meaningful within the context of the problem is also a good idea.
The #include statements now also include the header file for the standard mathematics library math.h. This statement is needed to define the calls to the trigonometric functions atan and sin. Note also that the compilation must include the mathematics library explicitly by typing
gcc sine.c -lm
Variable names are arbitrary (with some compiler-defined maximum length, typically 32 characters). C uses the following standard variable types:
int -> integer variable
short -> short integer
long -> long integer
float -> single precision real (floating point) variable
double -> double precision real (floating point) variable
char -> character variable (single byte)
The compilers checks for consistency in the types of all variables used in any code. This feature is intended to prevent mistakes, in particular in mistyping variable names. Calculations done in the math library routines are usually done in double precision arithmetic (64 bits on most workstations). The actual number of bytes used in the internal storage of these data types depends on the machine being used.
The printf function can be instructed to print integers, floats and strings properly. The general syntax is
printf( "format", variables );
where "format" specifies the converstion specification and variables is a list of quantities to print. Some useful formats are
%.nd integer (optional n = number of columns; if 0, pad with zeroes)
%m.nf float or double (optional m = number of columns,
n = number of decimal places)
%ns string (optional n = number of columns)
%c character
\n \t to introduce new line or tab
\g ring the bell (``beep'') on the terminal
3. Loops
Most real programs contain some construct that loops within the program, performing repetitive actions on a stream of data or a region of memory. There are several ways to loop in C. Two of the most common are the while loop:
while (expression)
{
...block of statements to execute...
}
and the for loop:
for (expression_1; expression_2; expression_3)
{
...block of statements to execute...
}
The while loop continues to loop until the conditional expression becomes false. The condition is tested upon entering the loop. Any logical construction (see below for a list) can be used in this context.
The for loop is a special case, and is equivalent to the following while loop:
expression_1;
while (expression_2)
{
...block of statements...
expression_3;
}
For instance, the following structure is often encountered:
i = initial_i;
while (i <= i_max)
{
...block of statements...
i = i + i_increment;
}
This structure may be rewritten in the easier syntax of the for loop as:
for (i = initial_i; i <= i_max; i = i + i_increment)
{
...block of statements...
}
Infinite loops are possible (e.g. for(;;)), but not too good for your computer budget! C permits you to write an infinite loop, and provides the break statement to ``breakout '' of the loop. For example, consider the following (admittedly not-so-clean) re-write of the previous loop:
angle_degree = 0;
for ( ; ; )
{
...block of statements...
angle_degree = angle_degree + 10;
if (angle_degree == 360) break;
}
The conditional if simply asks whether angle_degree is equal to 360 or not; if yes, the loop is stopped.
4. Symbolic Constants
You can define constants of any type by using the #define compiler directive. Its syntax is simple--for instance
#define ANGLE_MIN 0
#define ANGLE_MAX 360
would define ANGLE_MIN and ANGLE_MAX to the values 0 and 360, respectively. C distinguishes between lowercase and uppercase letters in variable names. It is customary to use capital letters in defining global constants.
5. Conditionals
Conditionals are used within the if and while constructs:
if (conditional_1)
{
...block of statements executed if conditional_1 is true...
}
else if (conditional_2)
{
...block of statements executed if conditional_2 is true...
}
else
{
...block of statements executed otherwise...
}
and any variant that derives from it, either by omitting branches or by including nested conditionals.
Conditionals are logical operations involving comparison of quantities (of the same type) using the conditional operators:
< smaller than
<= smaller than or equal to
== equal to
!= not equal to
>= greater than or equal to
> greater than
and the boolean operators
&& and
|| or
! not
Another conditional use is in the switch construct:
switch (expression)
{
case const_expression_1:
{
...block of statements...
break;
}
case const_expression_2:
{
...block of statements...
break;
}
default:
{
...block of statements..
}
}
The appropriate block of statements is executed according to the value of the expression, compared with the constant expressions in the case statement. The break statements insure that the statements in the cases following the chosen one will not be executed. If you would want to execute these statements, then you would leave out the break statements. This construct is particularly useful in handling input variables.
6. Pointers
The C language allows the programmer to ``peek and poke'' directly into memory locations. This gives great flexibility and power to the language, but it also one of the great hurdles that the beginner must overcome in using the language.
All variables in a program reside in memory; the statements
float x;
x = 6.5;
request that the compiler reserve 4 bytes of memory (on a 32-bit computer) for the floating-point variable x, then put the ``value'' 6.5 in it.
Sometimes we want to know where a variable resides in memory. The address (location in memory) of any variable is obtained by placing the operator ``&'' before its name. Therefore &x is the address of x. C allows us to go one stage further and define a variable, called a pointer, that contains the address of (i.e. ``points to'') other variables. For example:
float x;
float* px;
x = 6.5;
px = &x;
defines px to be a pointer to objects of type float, and sets it equal to the address of x:
Pointer use for a variable
The content of the memory location referenced by a pointer is obtained using the ``*'' operator (this is called dereferencing the pointer). Thus, *px refers to the value of x.
C allows us to perform arithmetic operations using pointers, but beware that the ``unit'' in pointer arithmetic is the size (in bytes) of the object to which the pointer points. For example, if px is a pointer to a variable x of type float, then the expression px + 1 refers not to the next bit or byte in memory but to the location of the next float after x (4 bytes away on most workstations); if x were of type double, then px + 1 would refer to a location 8 bytes (the size of a double)away, and so on. Only if x is of type char will px + 1 actually refer to the next byte in memory.
Thus, in
char* pc;
float* px;
float x;
x = 6.5;
px = &x;
pc = (char*) px;
(the (char*) in the last line is a ``cast'', which converts one data type to another), px and pc both point to the same location in memory--the address of x--but px + 1 and pc + 1 point to different memory locations.
Consider the following simple code.
void main()
{
float x, y; /* x and y are of float type */
float *fp, *fp2; /* fp and fp2 are pointers to float */
x = 6.5; /* x now contains the value 6.5 */
/* print contents and address of x */
printf("Value of x is %f, address of x %ld\n", x, &x);
fp = &x; /* fp now points to location of x */
/* print the contents of fp */
printf("Value in memory location fp is %f\n", *fp);
/* change content of memory location */
*fp = 9.2;
printf("New value of x is %f = %f \n", *fp, x);
/* perform arithmetic */
*fp = *fp + 1.5;
printf("Final value of x is %f = %f \n", *fp, x);
/* transfer values */
y = *fp;
fp2 = fp;
printf("Transfered value into y = %f and fp2 = %f \n", y, *fp2);
}
Run this code to see the results of these different operations. Note that, while the value of a pointer (if you print it out with printf) is typically a large integer, denoting some particular memory location in the computer, pointers are not integers--they are a completely different data type.
7. Arrays
Arrays of any type can be formed in C. The syntax is simple:
type name[dim];
In C, arrays starts at position 0. The elements of the array occupy adjacent locations in memory. C treats the name of the array as if it were a pointer to the first element--this is important in understanding how to do arithmetic with arrays. Thus, if v is an array, *v is the same thing as v[0], *(v+1) is the same thing as v[1], and so on:
Pointer use for an array
Consider the following code, which illustrates the use of pointers:
#define SIZE 3
void main()
{
float x[SIZE];
float *fp;
int i;
/* initialize the array x */
/* use a "cast" to force i */
/* into the equivalent float */
for (i = 0; i < SIZE; i++)
x[i] = 0.5*(float)i;
/* print x */
for (i = 0; i < SIZE; i++)
printf(" %d %f \n", i, x[i]);
/* make fp point to array x */
fp = x;
/* print via pointer arithmetic */
/* members of x are adjacent to */
/* each other in memory */
/* *(fp+i) refers to content of */
/* memory location (fp+i) or x[i] */
for (i = 0; i < SIZE; i++)
printf(" %d %f \n", i, *(fp+i));
}
(The expression ``i++'' is C shorthand for ``i = i + 1''.) Since x[i] means the i-th element of the array x, and fp = x points to the start of the x array, then *(fp+i) is the content of the memory address i locations beyond fp, that is, x[i].
8. Character Arrays
A string constant , such as
"I am a string"
is an array of characters. It is represented internally in C by the ASCII characters in the string, i.e., ``I'', blank, ``a'', ``m'',... for the above string, and terminated by the special null character ``\0'' so programs can find the end of the string.
String constants are often used in making the output of code intelligible using printf ;
printf("Hello, world\n");
printf("The value of a is: %f\n", a);
String constants can be associated with variables. C provides the char type variable, which can contain one character--1 byte--at a time. A character string is stored in an array of character type, one ASCII character per location. Never forget that, since strings are conventionally terminated by the null character ``\0'', we require one extra storage location in the array!
C does not provide any operator which manipulate entire strings at once. Strings are manipulated either via pointers or via special routines available from the standard string library string.h. Using character pointers is relatively easy since the name of an array is a just a pointer to its first element. Consider the following code:
void main()
{
char text_1[100], text_2[100], text_3[100];
char *ta, *tb;
int i;
/* set message to be an arrray */
/* of characters; initialize it */
/* to the constant string "..." */
/* let the compiler decide on */
/* its size by using [] */
char message[] = "Hello, I am a string; what are you?";
printf("Original message: %s\n", message);
/* copy the message to text_1 */
/* the hard way */
i=0;
while ( (text_1[i] = message[i]) != '\0' )
i++;
printf("Text_1: %s\n", text_1);
/* use explicit pointer arithmetic */
ta=message;
tb=text_2;
while ( ( *tb++ = *ta++ ) != '\0' )
;
printf("Text_2: %s\n", text_2);
}
The standard ``string'' library contains many useful functions to manipulate strings; a description of this library can be found in an appendix of the K & R textbook. Some of the most useful functions are:
char *strcpy(s,ct) -> copy ct into s, including ``\0''; return s
char *strncpy(s,ct,n) -> copy ncharcater of ct into s, return s
char *strncat(s,ct) -> concatenate ct to end of s; return s
char *strncat(s,ct,n) -> concatenate n character of ct to end
of s, terminate with ``\0''; return s
int strcmp(cs,ct) -> compare cs and ct; return 0 if cs=ct,
<0 if cs0 if cs>ct
char *strchr(cs,c) -> return pointer to first occurence of c
in cs or NULL if not encountered
size_t strlen(cs) -> return length of cs
(s and t are char*, cs and ct are const char*, c is an char converted to type int, and n is an int.)
Consider the following code which uses some of these functions:
#include < string.h>
void main()
{
char line[100], *sub_text;
/* initialize string */
strcpy(line,"hello, I am a string;");
printf("Line: %s\n", line);
/* add to end of string */
strcat(line," what are you?");
printf("Line: %s\n", line);
/* find length of string */
/* strlen brings back */
/* length as type size_t */
printf("Length of line: %d\n", (int)strlen(line));
/* find occurence of substrings */
if ( (sub_text = strchr ( line, 'W' ) )!= NULL )
printf("String starting with \"W\" ->%s\n", sub_text);
if ( ( sub_text = strchr ( line, 'w' ) )!= NULL )
printf("String starting with \"w\" ->%s\n", sub_text);
if ( ( sub_text = strchr ( sub_text, 'u' ) )!= NULL )
printf("String starting with \"w\" ->%s\n", sub_text);
}
9. I/O Capabilities
Character level I/O
C provides (through its libraries) a variety of I/O routines. At the character level, getchar() reads one character at a time from stdin, while putchar() writes one character at a time to stdout. For example, consider
#include < stdio.h>
void main()
{
int i, nc;
nc = 0;
i = getchar();
while (i != EOF) {
nc = nc + 1;
i = getchar();
}
printf("Number of characters in file = %d\n", nc);
}
This program counts the number of characters in the input stream (e.g. in a file piped into it at execution time). The code reads characters (whatever they may be) from stdin (the keyboard), uses stdout (the X-terminal you run from) for output, and writes error messages to stderr (usually also your X-terminal). These streams are always defined at run time. EOF is a special return value, defined in stdio.h, returned by getchar() when it encounters an end-of-file marker when reading. Its value is computer dependent, but the C compiler hides this fact from the user by defining the variable EOF. Thus the program reads characters from stdin and keeps adding to the counter nc, until it encounters the ``end of file''.
An experienced C programmer would probably code this example as:
#include < stdio.h>
void main()
{
int c, nc = 0;
while ( (c = getchar()) != EOF ) nc++;
printf("Number of characters in file = %d\n", nc);
}
C allows great brevity of expression, usually at the expense of readability!
The () in the statement (c = getchar()) says to execute the call to getchar() and assign the result to c before comparing it to EOF; the brackets are necessary here. Recall that nc++ (and, in fact, also ++nc) is another way of writing nc = nc + 1. (The difference between the prefix and postfix notation is that in ++nc, nc is incremented before it is used, while in nc++, nc is used before it is incremented. In this particular example, either would do.) This notation is more compact (not always an advantage, mind you), and it is often more efficiently coded by the compiler.
The UNIX command wc counts the characters, words and lines in a file. The program above can be considered as your own wc. Let's add a counter for the lines.
#include < stdio.h>
void main()
{
int c, nc = 0, nl = 0;
while ( (c = getchar()) != EOF )
{
nc++;
if (c == '\n') nl++;
}
printf("Number of characters = %d, number of lines = %d\n",
nc, nl);
}
Can you think of a way to count the number of words in the file?
Higher-Level I/O capabilities
We have already seen that printf handles formatted output to stdout. The counterpart statement for reading from stdin is scanf. The syntax
scanf("format string", variables);
resembles that of printf. The format string may contain blanks or tabs (ignored), ordinary ASCII characters, which must match those in stdin, and conversion specifications as in printf.
Equivalent statements exist to read from or write to character strings. They are:
sprintf(string, "format string", variables);
scanf(string, "format string", variables);
The ``string'' argument is the name of (i.e. a pointer to) the character array into which you want to write the information.
I/O to and from files
Similar statements also exist for handling I/O to and from files. The statements are
#include < stdio.h>
FILE *fp;
fp = fopen(name, mode);
fscanf(fp, "format string", variable list);
fprintf(fp, "format string", variable list);
fclose(fp );
The logic here is that the code must
* define a local ``pointer'' of type FILE (note that the uppercase is necessary here), which is defined in < stdio.h>
* ``open'' the file and associate it with the local pointer via fopen
* perform the I/O operations using fscanf and fprintf
* disconnect the file from the task with fclose
The ``mode'' argument in the fopen specifies the purpose/positioning in opening the file: ``r'' for reading, ``w'' for writing, and ``a'' for appending to the file. Try the following:
#include < stdio.h>
void main()
{
FILE *fp;
int i;
fp = fopen("foo.dat", "w"); /* open foo.dat for writing */
fprintf(fp, "\nSample Code\n\n"); /* write some info */
for (i = 1; i <= 10 ; i++)
fprintf(fp, "i = %d\n", i);
fclose(fp); /* close the file */
}
Compile and run this code; then use any editor to read the file foo.dat.
10. Functions
Functions are easy to use; they allow complicated programs to be parcelled up into small blocks, each of which is easier to write, read, and maintain. We have already encountered the function main and made use of I/O and mathematical routines from the standard libraries. Now let's look at some other library functions, and how to write and use our own.
Calling a Function
The call to a function in C simply entails referencing its name with the appropriate arguments. The C compiler checks for compatibility between the arguments in the calling sequence and the definition of the function.
Library functions are generally not available to us in source form. Argument type checking is accomplished through the use of header files (like stdio.h) which contain all the necessary information. For example, as we saw earlier, in order to use the standard mathematical library you must include math.h via the statement
#include < math.h>
at the top of the file containing your code. The most commonly used header files are
< stdio.h> -> defining I/O routines
< ctype.h> -> defining character manipulation routines
< string.h> -> defining string manipulation routines
< math.h> -> defining mathematical routines
< stdlib.h> -> defining number conversion, storage allocation
and similar tasks
< stdarg.h> -> defining libraries to handle routines with variable
numbers of arguments
< time.h> -> defining time-manipulation routines
In addition, the following header files exist:
< assert.h> -> defining diagnostic routines
< setjmp.h> -> defining non-local function calls
< signal.h> -> defining signal handlers
< limits.h> -> defining constants of the int type
< float.h> -> defining constants of the float type
Appendix B in the K & R book describes these libraries in great detail.
Writing Your Own Functions
A function has the following layout:
return-type function-name ( argument-list-if-necessary )
{
...local-declarations...
...statements...
return return-value;
}
If return-type is omitted, C defaults to int. The return-value must be of the declared type.
A function may simply perform a task without returning any value, in which case it has the following layout:
void function-name ( argument-list-if-necessary )
{
...local-declarations...
...statements...
}
As an example of function calls, consider the following code:
/* include headers of library */
/* defined for all routines */
/* in the file */
#include < stdio.h>
#include < string.h>
/* prototyping of functions */
/* to allow type checks by */
/* the compiler */
void main()
{
int n;
char string[50];
/* strcpy(a,b) copies string b into a */
/* defined via the stdio.h header */
strcpy(string, "Hello World");
/* call own function */
n = n_char(string);
printf("Length of string = %d\n", n);
}
/* definition of local function n_char */
int n_char(char string[])
{
/* local variable in this function */
int n;
/* strlen(a) returns the length of */
/* string a */
/* defined via the string.h header */
n = strlen(string);
if (n > 50)
printf("String is longer than 50 characters\n");
/* return the value of integer n */
return n;
}
Arguments are always passed by value in C function calls. This means that local ``copies'' of the values of the arguments are passed to the routines. Any change made to the arguments internally in the function are made only to the local copies of the arguments. In order to change (or define) an argument in the argument list, this argument must be passed as an address, thereby forcing C to change the ``real'' argument in the calling routine.
As an example, consider exchanging two numbers between variables. First let's illustrate what happen if the variables are passed by value:
#include < stdio.h>
void exchange(int a, int b);
void main()
{ /* WRONG CODE */
int a, b;
a = 5;
b = 7;
printf("From main: a = %d, b = %d\n", a, b);
exchange(a, b);
printf("Back in main: ");
printf("a = %d, b = %d\n", a, b);
}
void exchange(int a, int b)
{
int temp;
temp = a;
a = b;
b = temp;
printf(" From function exchange: ");
printf("a = %d, b = %d\n", a, b);
}
Run this code and observe that a and b are NOT exchanged! Only the copies of the arguments are exchanged. The RIGHT way to do this is of course to use pointers:
#include < stdio.h>
void exchange ( int *a, int *b );
void main()
{ /* RIGHT CODE */
int a, b;
a = 5;
b = 7;
printf("From main: a = %d, b = %d\n", a, b);
exchange(&a, &b);
printf("Back in main: ");
printf("a = %d, b = %d\n", a, b);
}
void exchange ( int *a, int *b )
{
int temp;
temp = *a;
*a = *b;
*b = temp;
printf(" From function exchange: ");
printf("a = %d, b = %d\n", *a, *b);
}
The rule of thumb here is that
* You use regular variables if the function does not change the values of those arguments
* You MUST use pointers if the function changes the values of those arguments
11. Command-line arguments
It is standard practice in UNIX for information to be passed from the command line directly into a program through the use of one or more command-line arguments, or switches. Switches are typically used to modify the behavior of a program, or to set the values of some internal parameters. You have already encountered several of these--for example, the "ls" command lists the files in your current directory, but when the switch -l is added, "ls -l" produces a so-called ``long'' listing instead. Similarly, "ls -l -a" produces a long listing, including ``hidden'' files, the command "tail -20" prints out the last 20 lines of a file (instead of the default 10), and so on.
Conceptually, switches behave very much like arguments to functions within C, and they are passed to a C program from the operating system in precisely the same way as arguments are passed between functions. Up to now, the main() statements in our programs have had nothing between the parentheses. However, UNIX actually makes available to the program (whether the programmer chooses to use the information or not) two arguments to main: an array of character strings, conventionally called argv, and an integer, usually called argc, which specifies the number of strings in that array. The full statement of the first line of the program is
main(int argc, char** argv)
(The syntax char** argv declares argv to be a pointer to a pointer to a character, that is, a pointer to a character array (a character string)--in other words, an array of character strings. You could also write this as char* argv[]. Don't worry too much about the details of the syntax, however--the use of the array will be made clearer below.)
When you run a program, the array argv contains, in order, all the information on the command line when you entered the command (strings are delineated by whitespace), including the command itself. The integer argc gives the total number of strings, and is therefore equal to equal to the number of arguments plus one. For example, if you typed
a.out -i 2 -g -x 3 4
the program would receive
argc = 7
argv[0] = "a.out"
argv[1] = "-i"
argv[2] = "2"
argv[3] = "-g"
argv[4] = "-x"
argv[5] = "3"
argv[6] = "4"
Note that the arguments, even the numeric ones, are all strings at this point. It is the programmer's job to decode them and decide what to do with them.
The following program simply prints out its own name and arguments:
#include < stdio.h>
main(int argc, char** argv)
{
int i;
printf("argc = %d\n", argc);
for (i = 0; i < argc; i++)
printf("argv[%d] = \"%s\"\n", i, argv[i]);
}
UNIX programmers have certain conventions about how to interpret the argument list. They are by no means mandatory, but it will make your program easier for others to use and understand if you stick to them. First, switches and key terms are always preceded by a ``-'' character. This makes them easy to recognize as you loop through the argument list. Then, depending on the switch, the next arguments may contain information to be interpreted as integers, floats, or just kept as character strings. With these conventions, the most common way to ``parse'' the argument list is with a for loop and a switch statement, as follows:
#include < stdio.h>
#include < stdlib.h>
main(int argc, char** argv)
{
/* Set defaults for all parameters: */
int a_value = 0;
float b_value = 0.0;
char* c_value = NULL;
int d1_value = 0, d2_value = 0;
int i;
/* Start at i = 1 to skip the command name. */
for (i = 1; i < argc; i++) {
/* Check for a switch (leading "-"). */
if (argv[i][0] == '-') {
/* Use the next character to decide what to do. */
switch (argv[i][1]) {
case 'a': a_value = atoi(argv[++i]);
break;
case 'b': b_value = atof(argv[++i]);
break;
case 'c': c_value = argv[++i];
break;
case 'd': d1_value = atoi(argv[++i]);
d2_value = atoi(argv[++i]);
break;
}
}
}
printf("a = %d\n", a_value);
printf("b = %f\n", b_value);
if (c_value != NULL) printf("c = \"%s\"\n", c_value);
printf("d1 = %d, d2 = %d\n", d1_value, d2_value);
}
Note that argv[i][j] means the j-th character of the i-th character string. The if statement checks for a leading ``-'' (character 0), then the switch statement allows various courses of action to be taken depending on the next character in the string (character 1 here). Note the use of argv[++i] to increase i before use, allowing us to access the next string in a single compact statement. The functions atoi and atof are defined in stdlib.h. They convert from character strings to ints and doubles, respectively.
A typical command line might be:
a.out -a 3 -b 5.6 -c "I am a string" -d 222 111
(The use of double quotes with -c here makes sure that the shell treats the entire string, including the spaces, as a single object.)
Arbitrarily complex command lines can be handled in this way. Finally, here's a simple program showing how to place parsing statements in a separate function whose purpose is to interpret the command line and set the values of its arguments:
/********************************/
/* */
/* Getting arguments from */
/* */
/* the Command Line */
/* */
/********************************/
/* Steve McMillan */
/* Written: Winter 1995 */
#include < stdio.h>
#include < stdlib.h>
void get_args(int argc, char** argv, int* a_value, float* b_value)
{
int i;
/* Start at i = 1 to skip the command name. */
for (i = 1; i < argc; i++) {
/* Check for a switch (leading "-"). */
if (argv[i][0] == '-') {
/* Use the next character to decide what to do. */
switch (argv[i][1]) {
case 'a': *a_value = atoi(argv[++i]);
break;
case 'b': *b_value = atof(argv[++i]);
break;
default: fprintf(stderr,
"Unknown switch %s\n", argv[i]);
}
}
}
}
main(int argc, char** argv)
{
/* Set defaults for all parameters: */
int a = 0;
float b = 0.0;
get_args(argc, argv, &a, &b);
printf("a = %d\n", a);
printf("b = %f\n", b);
}
12. Graphical Interfaces: Dialog Boxes
Suppose you don't want to deal with command line interpretation, but you still want your program to be able to change the values of certain variables in an interactive way. You could simply program in a series printf/scanf lines to quiz the user about their preferences:
.
.
.
printf("Please enter the value of n: ");
scanf("%d", &n);
printf("Please enter the value of x: ");
scanf("%f", &x);
.
.
.
and so on, but this won't work well if your program is to be used as part of a pipeline (see the UNIX primer), for example using ther graphics program plot_data, since the questions and answers will get mixed up with the data stream.
A convenient alternative is to use a simple graphical interface which generates a dialog box, offering you the option of varying key parameters in your program. Our graphics package provides a number of easy-to-use tools for constructing and using such boxes. The simplest way to set the integer variable n and the float variable x (i.e. to perform the same effect as the above lines of code) using a dialog box is as follows:
/* Simple program to illustrate use of a dialog box */
main()
{
/* Define default values: */
int n = 0;
float x = 0.0;
/* Define contents of dialog window */
create_int_dialog_entry("n", &n);
create_float_dialog_entry("x", &x);
/* Create window with name "Setup" and top-left corner at (0,0) */
set_up_dialog("Setup", 0, 0);
/* Display the window and read the results */
read_dialog_window();
/* Print out the new values */
printf("n = %d, x = %f\n", n, x);
}
Compile this program using the alias Cgfx (see the page on compilation) to link in all necessary libraries.
The two create lines define the entries in the box and the variables to be associated with them, set_up_dialog names the box and defines its location. Finally, read_dialog_window pops up a window and allows you to change the values of the variables. When the program runs, you will see a box that looks something like this:
Modify the numbers shown, click "OK" (or just hit carriage return), and the changes are made. That's all there is to it! The great advantage of this approach is that it operates independently of the flow of data through stdin/stdout. In principle, you could even control the operation of every stage in a pipeline of many chained commands, using a separate dialog box for each.
CSE JOURNALS
Legacy Directory of Computing Science Journals
This is an early example of an electronic library project. It is no longer active.
* Index of Title Words [A] [B] [C] [D] [E] [F] [G] [H] [I] [J] [K] [L] [M] [N] [O] [P] [Q] [R] [S] [T] [U] [V] [W] [X] [i]
* Alphabetical Journal List
Index of Title Words
A
ACM AI AMS APL ASIS AT Acquisition Acta Ada Adaptive Advanced Advances Agents Aided Algebra Algebraic Algorithmica Algorithmics Algorithms American Analog Analysis Animation Annals Announcements Annual Applicable Applicandae Application Applications Applied Approximation Architecture Archives Areas Artificial Aspects Association Asterisk Australian Automated Automatic Automatica Automation Autonomous
B
BIT Behavior Behavioral Behaviour Bibliometrics Biomedical Biosciences Brain Brazilian Bulletin Byte
C
C C++ CALGO CSCW CSE Canadian Categorical Categories Chicago Chinese Circuits Codes Cognition Cognitive Collaborative Collected Color Combinatorial Combinatorics Communication Communications Complex Complexity Computation Computational Computations Computer Computer-Aided Computer-Human Computer-Mediated Computers Computing Concurrency Constraints Construction Control Cooperative Cryptography Cryptologic Cryptology Currents Cybermetrics Cybernetics
D
Data Database Databases Decision Design Designs Development Digital Discovery Discrete Displays Dissemination Distributed Dynamic Dynamics
E
EDI Edge Edition Education Electronic Elektronisch Elements Embedded Empirical Engineering Entwicklung Environment Environments European Evaluation Event Evolutionary Experience Experiment Experimental Expert
F
Factors Fields Finite Formal Forschung Forth Fortran Forum Foundations Fractals From Functional Fundamenta Future Fuzziness Fuzzy
G
General Generation GeoInformatica Geographical Geometric Geometry Geosciences Graph Graphical Graphics Group
H
Harmonic Heuristics High History Human Human-Computer Humanities Hypermedia
I
IBM IEE IEEE IMA ISDN IText Image Imaging Industry Informatica Informaticae Informatics Informatik Information Informational Informatization Informetrics Infrastructure Institute Integrated Integration Integrity Intelligence Intelligent Interacting Interaction Interest Interfaces Internet Internetworking Interval Its
J
Japan Java JavaSpektrum
K
Knowledge Knowledge-Based
L
LISP Language Languages Large Law Learning Lecture Letters Life Linguistic Linguistics Lisp Literary Logic Logics
M
Machine Machines Magazine Maintenance Man Man-Machine Management Manufacturing Mathematicae Mathematical Mathematics Matrix Mechanics Media Medicine Meeting Memory Messenger Methodology Methods Micro Microcomputer Microcomputers Microelectronics Microprocessing Microprocessors Microprogramming Microsystems Mind Minds Mini Mining Missouri Mobile Modeling Modelling Models Modern Multi-Agent Multibody Multidimensional Multimedia Museum Music
N
National Natural Network Networking Networks Neural Neurocomputing Neuroscience New News Nordic Notes Numerical
O
OOPS Object Object-Oriented Objekt Online Open Operating Operations Optical Optimization Order Organization Organizational Organizations Oriental Oriented Origination Outlook
P
PC Parallel Pattern Performance Personal Personnel Philosophy Physics Pointers Policy Polytechnica Practical Practice Pragmatics Presence Probability Proceedings Process Processing Programming Prototyping Psychological Psychology Public Publishing Pure
Q
Quad Quality Quantitative Quarterly Queueing Quote
R
Random Rapid Real-Time Reasoning Recognition Record Reliability Reliable Remote Report Representation Research Resource Retrieval Review Reviews Robotic Robotica Robotics Robots
S
SIAM SIGACT SIGART SIGBIO SIGCAPH SIGCHI SIGCUE SIGDA SIGForth SIGICE SIGIR SIGLINK SIGMETRICS SIGMICRO SIGMOD SIGNUM SIGOIS SIGOPS SIGPLAN SIGSAC SIGSAM SIGSMALL SIGUCCS Safety Scandanavian Scandinavica Science Sciences Scientific Scientometrics Sector Security Selected Sensing Series Services Sets Shape Signal Simulation Smalltalk Society Software Sofware Spectrum Speech Speed Spektrum Spot Stability StandardView Standards Statistics Structures Studies Supercomputer Supercomputing Support Supported Surveys Symbolic System Systems
T
T Technical Techniques Technologies Technology Telecommunication Telematics Teleoperators Test Testing Their Theoretical Theory Tools Translation Transportation Transputer Tutoring
U
Ulam Uncertainty Understanding Universal Urban User User-Adapted
V
VLDB VLSI Verification Very Videre Virtual Vision Visual Visualization
W
Web Wide Window Wireless With Work World
X
X
i
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Thursday, September 20, 2007
World Wide Web Virtual Library
#!/usr/local/bin/eperl
Welcome to the World Wide Web Virtual Library for Civil Engineeering
This site is an extension of the WWW Virtual Library project from WWW Consortium specializing in information directly related to the field of Civil Engineering. To learn more about the WWW Virtual Library, visit http://www.vlib.org/.
We are very excited about our new look and feel for the site, and we think that the new site will prove to be more useful to everyone. However, we were unable to move the links of the old system over to our new one. If you have a link in our old pages, you can add your link to our new system. You can also view the old site for some of the links that have not been transferred over.
The information on this site is divided into two major categories: Links and Conferences.
Links:
The links listed here are listed free of charge and are freely viewable. These links are organized into Commercial, Educational, Organizational, and Journal groups. The database of links is completely searchable to provide easy viewing.
All requests to be added to the system are reviewed by our CEVL Administrators before being placed in the database for viewing by the public. When you are requesting that your site be added to the list, please keep in mind that the Administrators may take 2 to 5 days to review your request and respond.
Conference Listings
This portion of the CEVL is still in development. We apologize for any delays.
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