C++

History

Bjarne Stroustrup, creator of C++
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 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 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
In 1983, the name of the language was changed from C with Classes to C++, since classes are 'one more thing' than C had (++ 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, 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.

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 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 (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.

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.

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

int main()

{ 

    std::cout << "Hello, world!\n";

} 

Language features

C++ provides more than 30 operators, covering basic arithmetic, bit manipulation, indirection, comparisons, logic 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. As a simple example, a class that represents a matrix could overload the multiplication (*) and other arithmetic operators, allowing it to be treated by application code similarly to the standard numerical types.:

matrix A, B;

matrix C = A * B

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

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 <typename T>

const 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 an argument, but does not require prior 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 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 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. 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 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 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 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.

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.

Default arguments

Default arguments are used when defining a different function is not needed when supplying a default value for an argument will suffice.

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.

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 method in a derived class overrides a method in a base class, the method to call is determined by the type of the object. A given method is overridden when there exists no difference, in the number or type of parameters, between two or more definitions of that method. 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++, 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 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 extension 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 <iostream> 

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.

Problems and controversies

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.
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, but finally the decision was made to retain it.
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); some compilers adopt a secondary standard for these items.

Main article: Criticism of C++
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 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." 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.Because of its large feature set, it can be quite 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." 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.