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# Class declaration

From cppreference.com
< cpp‎ | language

Classes and structs are user-defined types, defined by class-specifier, which appears in decl-specifier-seq of the declaration syntax. The class specifier has the following syntax:

class-key attr class-head-name base-clause `{` member-specification `}` |  |

class-key | - | one of class or struct. The keywords are identical except for the default member access and the default base class access.
attr(C++11) | - | optional sequence of any number of attributes
class-head-name | - | the name of the class that's being defined. Optionally prepended by nested-name-specifier (sequence of names and scope-resolution operators, ending with scope-resolution operator), optionally followed by keyword final. The name may be omitted, in which case the class is unnamed (note that unnamed class cannot be final)
base-clause | - | optional list of one or more parent classes and the model of inheritance used for each (see derived class)
member-specification | - | list of access specifiers, member object and member function declarations and definitions (see below)

See classes for general overview of the syntax. If class-key is `union`, the declaration introduces a union type.

### [edit] Forward declaration

A declaration of the following form

class-key attr identifier `;` |  |

Declares a class or struct which will be defined later in this scope. Until the definition appears, this class name has incomplete type. This allows classes that refer to each other:

    class Vector; // forward declaration
    class Matrix {
        // ...
        friend Vector operator*(const Matrix&, const Vector&);
    };
    class Vector {
        // ...
        friend Vector operator*(const Matrix&, const Vector&);
    };

and if a particular source file only uses pointers and references to the class, this makes it possible to reduce #include dependencies:

    // in MyStruct.h
    #include <iosfwd> // contains forward declaration of std::ostream
    struct MyStruct {
        int value;
        friend std::ostream& operator<<(std::ostream& os, const S& s);
        // definition provided in MyStruct.cpp file which uses #include <ostream>
    };

If forward declaration appears in local scope, it hides previously declared class, variable, function, and all other declarations of the same name that may appear in enclosing scopes:

    struct s { int a; };
    struct s; // does nothing (s already defined in this scope)
    void g() {
        struct s; // forward declaration of a new, local struct "s"
                  // this hides global struct s until the end of this block
        s* p;     // pointer to local struct s
        struct s { char* p; }; // definitions of the local struct s
    }

A forward declaration may also appear as part of another declaration, in particular, in a parameter declaration of a function or in a decl-specifier-seq of a simple declaration.

    class Y f(class T p); // declares function f and forward-declares T and Y
    Y* p; // Y is a forward-declared class
    T* q; // T is a forward-declared class

    std::array<class T, 2>* p; // forward-declares class T
    T* q; // pointers to T can be defined

### [edit] Member specification

The member specification, or the body of a class definition, is a brace-enclosed sequence of any number of the following:

1) Member declarations, which are declarations of static and non-static data members and member functions, typedef declarations, scoped and unscoped enumerations, and other, nested, class declarations.

    class S {
        int d1; // non-static data member
        int a[10] = {1,2}; // non-static data member with initializer (C++11)
        static const int d2 = 1; // static data member with initializer
        virtual void f1(int) = 0; // pure virtual member function
        std::string d3, *d4, f2(int); // two data members and a member function
        enum {NORTH, SOUTH, EAST, WEST};
        struct NestedS {
            std::string s;
        } d5, *d6;
        typedef NestedS value_type, *pointer_type;
    };

2) Function definitions, which both declare and define member functions. A semicolon after a member function definition is optional. All member functions that are defined inside a class body are automaticaly inline.

    class M {
        std::size_t C;
        std::vector<int> data;
     public:
        M(std::size_t R, std::size_t C) : C(C), data(R*C) {} // constructor definition
        int operator()(size_t r, size_t c) const { // member function definition
            return data[r*C+c];
        }
        int& operator()(size_t r, size_t c) {  // another member function definition
            return data[r*C+c];
        }
    };

3) Access specifiers `public:`, `protected:`, and `private:`

    class S {
     public:
        S();          // public constructor
        S(const S&);  // public copy constructor
        virtual ~S(); // public virtual destructor
     private:
        int* ptr; // private data member
    };

4) Using-declarations

    class Base {
     protected:
         int d;
    };
    class Derived : public Base {
     public:
        using Base::d; // make Base's protected member d a public member of Derived
        using Base::Base; // inherit all parent's constructors (C++11)
    };

5) static_assert declarations

    template<typename T>
    struct Foo {
      static_assert(std::is_floating_point<T>::value, "Foo<T>: T must be floating point");
    };

6) member template declarations

    struct S {
        template<typename T>
        void f(T&& n);

        template<class CharT>
        struct NestedS {
            std::basic_string<CharT> s;
        };
    };

7) alias declarations

    template <typename T>
    struct identity
    {
        using type = T;
    };

### [edit] Local classes

A class declaration can appear in namespace scope (in which case it defines an ordinary class), inside another class definition (in which case it defines a nested type), and inside the body of a function, in which case it defines a local class. This class only exists within the function scope, and is not accessible outside.

- A local class cannot have static members
- Member functions of a local class have no linkage
- Member functions of a local class have to be defined entirely inside the class body
- Local classes cannot have member templates or friend templates
- A local class inside a function (including member function) can access the same names that the enclosing function can access.

local classes could not be used as template arguments | (until C++11)

Run this code

    #include <vector>
    #include <algorithm>
    #include <iostream>

    int main()
    {
        std::vector<int> v{1,2,3};
        struct Local {
           bool operator()(int n, int m) {
               return n > m;
           }
        };
        std::sort(v.begin(), v.end(), Local()); // since C++11
        for(int n: v) std::cout << n << ' ';
    }

Output:

    3 2 1

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