C++

C++

Paradigm	Multi-paradigm
Appeared in	1983
Designed by	Bjarne Stroustrup
Typing discipline	Static, unsafe, nominative
Major implementations	Microsoft Visual C++, GCC, Borland C++ Builder, Intel C++ Compiler
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, Perl, D, Aikido, Dao

C++ ("C Plus Plus", pronounced /ˌsiːˌplʌsˈplʌs/) is a general-purpose programming language. It is regarded as a middle-level language, as it comprises a combination of both high-level and low-level language features.^[1] It was developed by Bjarne Stroustrup in 1979 at Bell Labs as an enhancement to the C programming language and originally named "C with Classes". It was renamed to C++ in 1983.

C++ enjoys wide use in the software industry. Some of its application domains include systems software, device drivers, embedded software, high-performance server and client applications, and entertainment software such as video games. Several groups provide both free and commercial C++ compiler software, including the GNU Project, Microsoft, Intel, Borland and others.

The language began as enhancements to C, first adding classes, then virtual functions, operator overloading, multiple inheritance, templates, and exception handling among other features. After years of development, the C++ programming language standard was ratified in 1998 as ISO/IEC 14882:1998. The current standard is the 2003 version, ISO/IEC 14882:2003. The next standard version (known informally as C++0x) is in development.

C++ is a statically typed, free-form, multi-paradigm, compiled language where compilation creates machine code for a target machine hardware. It supports procedural programming, data abstraction, object-oriented programming, and generic programming.

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 to be suitable for large software development. When Stroustrup started working in AT&T 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 was general-purpose, fast, portable and widely used. Besides C and Simula, some other languages that 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++ (++ being the increment operator in C and 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, since 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.

C++ continues to be used and is still one of the preferred programming languages to develop professional applications. The language has gone from being mostly Western, to attracting programmers from all over the world.^[3]

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.

Etymology

According to Stroustrup: "the name signifies the evolutionary nature of the changes from C".^[4] During C++'s development period, the language had been referred to as "new C", then "C with Classes". The final name is credited to Rick Mascitti (mid-1983) and was first used in December 1983. When Mascitti was questioned informally in 1992 about the naming, he indicated that it was given in a tongue-in-cheek spirit. It 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. There is no language called "C plus". ABCL/c+ was the name of an earlier, unrelated programming language.

Philosophy

In The Design and Evolution of C++ (1994), Bjarne Stroustrup describes some rules that he uses for the design 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 (the "zero-overhead principle")
• 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, however, 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 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.

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.

Language features

C++ inherits most of C's syntax and the C preprocessor. The following is a Hello world program which uses the C++ standard library stream facility to write a message to standard output:^[5][6]

#include <iostream>  // provides std::cout

int main()
{
   std::cout << "Hello, world!\n";
}

Operators and operator overloading

C++ provides more than 30 operators, covering basic arithmetic, bit manipulation, indirection, comparisons, logical operations and more. Almost all operators can be overloaded for user-defined types, with a few notable exceptions such as member access (. and .*). The rich set of overloadable operators is central to using C++ as a domain specific language. The overloadable operators are also an essential part of many advanced C++ programming techniques, such as smart pointers. 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).

Templates

See also: generic programming and template metaprogramming

C++ templates enable generic programming. C++ supports both function and class templates. Templates may be parameterized by types, compile-time constants, and other templates. C++ templates are implemented by instantiation at compile-time. To instantiate a template, compilers substitute specific arguments for a template's parameters to generate a concrete function or class instance. Templates are a powerful tool that can be used for generic programming, template metaprogramming, and code optimization, but this power implies a cost. Template use may increase code size, since each template instantiation produces a copy of the template code: one for each set of template arguments. This is in contrast to run-time generics seen in other languages (e.g. Java) where at compile-time the type is erased and a single template body is preserved.

Templates are different from macros: while both of these compile-time language features enable conditional compilation, templates are not restricted to lexical substitution. Templates are aware 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.

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, a template is a compile-time parameterized function or class written without knowledge of the specific arguments used to instantiate it. After instantiation the resulting code is equivalent to code written specifically for the passed arguments. In this manner, templates provide a way to decouple generic, broadly-applicable aspects of functions and classes (encoded in templates) from specific aspects (encoded in template parameters) without sacrificing performance due to abstraction.

Objects

Main article: C++ structures and classes

C++ introduces 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. The class can be thought of as a template from which many different individual objects may be generated as a program runs.

Encapsulation

Encapsulation is the hiding of information. 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 is accessible to any function. A private member is accessible only to functions that are members of that class and to functions and classes explicitly granted access permission by the class ("friends"). A protected member is 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 of the functions (and only the functions) that 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 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. This hides all the details of data implementation, allowing the designer to later fundamentally change the implementation without changing the interface in any way.

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, a "class" inherits privately, while a "struct" inherits publicly. 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 Java or C#, 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) polymorphisms. Compile-time polymorphism does not allow for certain run-time decisions, while run-time polymorphism typically incurs a performance penalty.

Static polymorphism

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.

When declaring a function, a programmer can specify default arguments for one or more parameters. Doing so allows the parameters with defaults to optionally be omitted when the function is called, in which case the default arguments will be used. When a function is called with fewer arguments than there are declared parameters, explicit arguments are matched to parameters in left-to-right order, with any unmatched parameters at the end of the parameter list being assigned their default arguments. In many cases, specifying default arguments in a single function declaration is preferable to providing overloaded function definitions with different numbers of parameters.

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 functions. 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) to 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 function in a derived class overrides a function in a base class, the function to call is determined by the type of the object. A given function is overridden when there exists no difference, in the number or type of parameters, between two or more definitions of that function. Hence, at compile time it may not be possible to determine the type of the object and therefore the correct function to call, given only a base class pointer; the decision is therefore put off until runtime. This is called dynamic dispatch. Virtual member functions or methods^[7] 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. If the object type is known, this may be bypassed by prepending a fully qualified class name before the function call, but in general calls to virtual functions are resolved at run time.

In addition to standard member functions, operator overloads and destructors can 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 extension copy constructors, cannot be virtual. Nonetheless 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) function 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 parenthesis and before the semicolon. Objects cannot 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 provide a non-pure definition of it (and all other pure virtual functions) before objects of the derived class can be created. 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.

Parsing and processing C++ source code

It is relatively difficult to write a good C++ parser with classic parsing algorithms such as LALR(1).^[8] 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 that supported by the GNU Compiler Collection and that of Microsoft's Visual C++) 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.

Compatibility

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.^[9]

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/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, ^[10] but finally the decision was made to retain it.^[11]

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 is 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^[12]); some compilers adopt a secondary standard for these items.

With C

For more details on this topic, see Compatibility of C and C++.

C++ is often considered to be a superset of C, but this is not strictly true.^[13] 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. 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 function declaration or definition that is to be called from/used both in C and C++ must be declared with C linkage by placing it within an extern "C" { ... } block. Such a function may not rely on features depending on name mangling (i.e., function overloading).

Adoption and reception

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. Bjarne Stroustrup 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."^[14] 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.^[15] Because of its large feature set, it is difficult to fully master C++.

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."^[16] 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.

Other criticism stems from what is missing from C++. For example, the current version of Standard C++ provides no language features to create multi-threaded software other than the volatile keyword. (The next version of C++ will introduce the thread_local keyword.) These facilities are present in some other languages including Java, Ada, and C# (see also Lock). It is possible to use operating system calls or third party libraries to do multi-threaded programming, but both approaches may create portability concerns. C++ is also sometimes compared unfavorably with single-paradigm object-oriented languages such as Smalltalk, Java or Eiffel 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.

See also: Comparison of Java and C++

See also

• Comparison of programming languages
• List of C++ compilers
• Comparison of integrated development environments for C/C++
• List of C++ template libraries
• C++0x, the planned new standard for C++

References

Further reading

External links

• A paper by Stroustrup showing the timeline of C++ evolution (1979-1991)
• Apache C++ Standard Library Documentation
• Standards Committee Page: JTC1/SC22/WG21 - C++
• C++ FAQ Lite by Marshall Cline
• Computer World interview with Bjarne Stroustrup
• CrazyEngineers.com interview with Bjarne Stroustrup
• The State of the Language: An Interview with Bjarne Stroustrup (August 15, 2008)

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