Class declaration

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:

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

Forward declaration

A declaration of the following form.

Declares a class type 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
}

Note that a new class name may also be introduced by an elaborated type specifier which appears as part of another declaration, but only if name lookup can't find a previously declared class with the same name.

class U;
namespace ns{
    class Y f(class T p); // declares function ns::f and declares ns::T and ns::Y
    class U f(); // U refers to ::U
    Y* p; T* q; // can use pointers and references to T and Y
}

Member specification

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

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;
};

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];
    }
};

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

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)
};

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

struct S {
    template<typename T>
    void f(T&& n);
 
    template<class CharT>
    struct NestedS {
        std::basic_string<CharT> s;
    };
};

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

struct S {
    template<class CharT>
    struct NestedS {
        std::basic_string<CharT> s;
    };
 
    template<class CharT>
    NestedS(std::basic_string<CharT>) -> NestedS<CharT>;
};

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 class), and inside the body of a function, in which case it defines a local class. The name of such a class only exists within the function scope, and is not accessible outside.

• A local class cannot have static data 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 other than closure types (since C++14) cannot have member templates
• Local classes cannot have friend templates
• Local classes cannot define friend functions inside the class definition
• A local class inside a function (including member function) can access the same names that the enclosing function can access.

#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 << ' ';
}

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See also