C++

C++

The C++ Programming Language, written by its architect, is the seminal book on the language.
Paradigm(s)	Multi-paradigm: procedural, functional, object-oriented, generic
Appeared in	1983
Designed by	Bjarne Stroustrup
Developer	Bjarne Stroustrup Bell Labs ISO/IEC JTC1/SC22/WG21
Stable release	ISO/IEC 14882:2011 (2011)
Typing discipline	Static, unsafe, nominative
Major implementations	C++ Builder, LLVM Clang, Comeau C/C++, GCC, Intel C++ Compiler, Microsoft Visual C++, Sun Studio
Dialects	Embedded C++, Managed C++, C++/CLI, C++/CX
Influenced	Perl, LPC, Lua, Pike, Ada 95, Java, PHP, D, C99, C#, Falcon, Seed7
OS	Cross-platform (multi-platform)
Usual filename extensions	.h .hh .hpp .hxx .h++ .cc .cpp .cxx .c++
News, status & discussion about Standard C++ C++ Programming at Wikibooks

C++ (pronounced "see plus plus") is a statically typed, free-form, multi-paradigm, compiled, general-purpose programming language. It is regarded as an intermediate-level language, as it comprises both high-level and low-level language features. Developed by Bjarne Stroustrup starting in 1979 at Bell Labs, C++ was originally named C with Classes, adding object oriented features, such as classes, and other enhancements to the C programming language. The language was renamed C++ in 1983, as a pun involving the increment operator.

C++ is one of the most popular programming languages and is implemented on a wide variety of hardware and operating system platforms. As an efficient compiler to native code, its application domains include systems software, application software, device drivers, embedded software, high-performance server and client applications, and entertainment software such as video games. Several groups provide both free and proprietary C++ compiler software, including the GNU Project, Microsoft, Intel and Embarcadero Technologies. C++ has greatly influenced many other popular programming languages, most notably C# and Java. Other successful languages such as Objective-C use a very different syntax and approach to adding classes to C.

C++ is also used for hardware design, where the design is initially described in C++, then analyzed, architecturally constrained, and scheduled to create a register-transfer level hardware description language via high-level synthesis.

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 standard was amended by the 2003 technical corrigendum, ISO/IEC 14882:2003. The current standard extending C++ with new features was ratified and published by ISO in September 2011 as ISO/IEC 14882:2011 (informally known as C++11).

History

Bjarne Stroustrup, creator of C++

Bjarne Stroustrup, a Danish and British trained computer scientist, began his 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 typing, inlining, and default argument features were added to C via Stroustrup's C++ to C compiler, Cfront. The first commercial implementation of C++ was released on 14 October 1985.

In 1983, the name of the language was changed from C with Classes to C++ (++ being the increment operator in 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. Release 2.0 of C++ came in 1989 and the updated second edition of The C++ Programming Language was released in 1991. 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 feature additions included templates, exceptions, namespaces, new casts, and a Boolean type.

As the C++ language evolved, the standard library 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 a large amount of the Standard Template Library.

C++ is sometimes called a hybrid language.

It is possible to write object oriented or procedural code in the same program in C++. This has caused some concern that some C++ programmers are still writing procedural code, but are under the impression that it is object oriented, simply because they are using C++. Often it is an amalgamation of the two. This usually causes most problems when the code is revisited or the task is taken over by another coder.

C++ continues to be used and is one of the preferred programming languages to develop professional applications.

Etymology

According to Stroustrup: "the name signifies the evolutionary nature of the changes from C". 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. A joke goes that the name itself has a bug: due to the use of post-increment, which increments the value of the variable but evaluates to the unincremented value, C++ is no better than C, and the pre-increment ++C form should have been used instead. There is no language called "C plus". ABCL/c+ was the name of an earlier, unrelated programming language. A few other languages have been named similarly to C++, most notably C-- and C#.

Standardization

In 1998, the C++ standards committee (the ISO/ IEC JTC1/SC22/WG21 working group) standardized C++ and published the international standard ISO/IEC 14882:1998 (informally known as C++98). For some years after the official release of the standard, the committee processed defect reports, and in 2003 published a corrected version of the C++ standard, ISO/IEC 14882:2003. 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 specified a number of extensions to the standard library, which were expected to be included in the next version of C++.

The latest major revision of the C++ standard, C++11, (formerly known as C++0x) was approved by ISO/IEC on 12 August 2011. It has been published as 14882:2011. There are plans for a minor(C++14) and a major revision (C++17) in the future.

C++14 is the name being used for the next revision. C++14 is planned to be a small extension over C++11, featuring mainly bug fixes and small improvements, similarly to how C++03 was a small extension to C++98. While the name 'C++14' implies a release in 2014, this date is not fixed.

Philosophy

In The Design and Evolution of C++ (1994), Bjarne Stroustrup describes some rules that he used 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 compatible with C as much 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 that are not part of the standard, and, using linkage specification, libraries can even be written in languages such as BASIC, C, Fortran, or Pascal. Which of these are supported is compiler-dependent.

The C++ standard library incorporates the C standard library with some small modifications to make it optimized 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 105 standard headers, of which 27 are deprecated.

The STL was originally a third-party library from HP and later SGI, before its incorporation into the C++ standard. The main architect behind the STL is Alexander Stepanov, who experimented with generic algorithms and containers for many years. When he started with C++, he finally found a language where it was possible to create generic algorithms (e.g., STL sort) that perform even better than, for example, the C standard library qsort, thanks to C++ features like using inlining and compile-time binding instead of function pointers. The standard does not refer to it as "STL", as it is merely a part of the standard library, but the term is still widely used to distinguish it from the rest of the standard 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. The following is Bjarne Stroustrup's version of the Hello world program that uses the C++ Standard Library stream facility to write a message to standard output:

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

Within functions that define a non-void return type, failure to return a value before control reaches the end of the function results in undefined behaviour (compilers typically provide the means to issue a diagnostic in such a case). The sole exception to this rule is the main function, which implicitly returns a value of zero.

Operators and operator overloading

C++ provides more than 35 operators, covering basic arithmetic, bit manipulation, indirection, comparisons, logical operations and others. Almost all operators can be overloaded for user-defined types, with a few notable exceptions such as member access (. and .*) as well as the conditional operator. 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 by the operator, though it will be evaluated prior to execution). Overloaded "&&" and "||" operators lose their short-circuit evaluation property.

Memory management

C++ supports four types of memory management:

• Static memory allocation. A static variable is assigned a value at compile-time, and allocated storage in a fixed location along with the executable code. These are declared with the "static" keyword (in the sense of static storage, not in the sense of declaring a class variable).
• Automatic memory allocation. An automatic variable is simply declared with its class name, and storage is allocated on the stack when the value is assigned. The constructor is called when the declaration is executed, the destructor is called when the variable goes out of scope, and after the destructor the allocated memory is automatically freed.
• Dynamic memory allocation. Storage can be dynamically allocated on the heap using manual memory management - normally calls to new and delete (though old-style C calls such as malloc() and free() are still supported).
• With the use of a library, garbage collection is possible. The Boehm garbage collector is commonly used for this purpose.

The fine control over memory management is similar to C, but in contrast with languages that intend to hide such details from the programmer, such as Java, Perl, PHP, and Ruby.

Templates

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. Some substitutions are not possible; these are eliminated by an overload resolution policy described by the phrase " Substitution failure is not an error" (SFINAE). 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, because 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++ that 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

C++ introduces object-oriented programming (OOP) features to C. It offers classes, which provide the four features commonly present in OOP (and some non-OOP) languages: abstraction, encapsulation, inheritance, and polymorphism. One distinguishing feature of C++ classes compared to classes in other programming languages is support for deterministic destructors, which in turn provide support for the Resource Acquisition is Initialization (RAII) concept.

Encapsulation

Encapsulation is the hiding of information to ensure that data structures and operators are used as intended and to make the usage model more obvious to the developer. C++ provides the ability to define classes and functions as its primary encapsulation mechanisms. Within a class, members can be declared as either public, protected, or private to explicitly enforce encapsulation. 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. Therefore, 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 can hide 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 not found in most other languages, allowing a class to be derived from more than one base classes; this allows for more elaborate 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). An interface as in C# and Java can be defined in C++ as a class containing only pure virtual functions, often known as an abstract base class or "ABC". The member functions of such an abstract base class are normally explicitly defined in the derived class, not inherited implicitly. C++ virtual inheritance exhibits an ambiguity resolution feature called dominance.

Polymorphism

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 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 and would result in a compile-time error message.

When declaring a function, a programmer can specify for one or more parameters a default value. 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. Because 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. Contrary to some opinion, template code will not generate a bulk code after compilation with the proper compiler settings.

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 that 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 allow the most specific implementation of the function to be called, according to the actual run-time type of the object. In C++ implementations, 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) virtual function that 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. A class containing a pure virtual function is called an abstract data type. Objects cannot be created from abstract data types; they 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. A program that attempts to create an object of a class with a pure virtual member function or inherited pure virtual member function is ill-formed.

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.

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 were, however, attempts to standardize compilers for particular machines or operating systems (for example C++ ABI), though they seem to be largely abandoned now.

Exported templates

One particular point of contention is the export keyword, intended to allow template definitions to be separated from their declarations. The first widely available compiler to implement export was Comeau C/C++, in early 2003 (five 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. Other compilers such as GCC do not support it at all. Beginning ANSI C++ by Ivor Horton provides example code with the keyword that will not compile in most compilers, without reference to this problem. Herb Sutter, former convener of the C++ standards committee, recommended that export be removed from future versions of the C++ standard. During the March 2010 ISO C++ standards meeting, the C++ standards committee voted to remove exported templates entirely from C++11, but reserve the keyword for future use.

With C

C++ is often considered to be a superset of C, 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 or 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 1999 revision of the C standard ( C99), which now supports C++ features such as line comments (//), and mixed declarations and code. On the other hand, C99 introduced a number of new features that C++ did not support, such as variable-length arrays, native complex-number types, designated initializers, and compound literals. However, at least some of the C99-introduced features were included in the subsequent version of the C++ standard, C++11:

• C99 preprocessor (including variadic macros, wide/narrow literal concatenation, wider integer arithmetic)
• _Pragma()
• long long
• __func__
• Headers:
  • cstdbool (stdbool.h)
  • cstdint (stdint.h)
  • cinttypes (inttypes.h).

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).

Criticism

Due to its large feature set and oft-perceived "strict" syntax, the language is sometimes criticized as being overly complicated and thus difficult to fully master.

C++ is sometimes compared unfavorably with more strictly object-oriented languages on the basis that it enables programmers to "mix and match" declarative, functional, generic, modular, and procedural programming styles with object-oriented programming, rather than strictly enforcing a single style, although C++ is intentionally a multi-paradigm language. It also lacks some features such as reflection, although adding the latter has been contemplated.

A widely distributed satirical article portrayed Bjarne Stroustrup, interviewed for a 1998 issue of IEEE's Computer magazine, confessing that C++ was deliberately designed to be complex and difficult, weeding out amateur programmers and raising the salaries of the few programmers who could master the language. The FAQ section of Stroustrup's personal website contains a denial and a link to the actual interview.

Richard Stallman criticizes C++ for having ambiguous grammar and "gratuitous, trivial incompatibilities with C [...] that are of no great benefit". Linus Torvalds said in a famous email that C++ was a 'horrible language'.

Finally, several authors have remarked that C++ is not a true object-oriented language.

The same problem occurs with programming languages. As stated earlier, many C programmers moved into the realm of object orientation by migrating to C++ before being directly exposed to OO concepts. This would always come out in an interview. Many times developers who claim to be C++ programmers are simply C programmers using C++ compilers. [...] We have already mentioned that C++ is not a true object-oriented programming language but is actually an object-based programming language. Remember that C++ is considered to be object-based. Object-oriented concepts are not enforced. You can write a non-object-oriented C program using a C++ compiler.

—Matt Weisfeld, 

It is interesting to see what is being done out in the world under the name object-oriented. I have been shown some very, very strange looking pieces of code over the years, by various people, including people in universities, that they have said is OOP code written in an OOP language—and actually I made up the term object-oriented, and I can tell you, I didn't have C++ in mind.

— Alan Kay, Smalltalk co-creator,