Explicitly defaulted functions (C++11)

Edit online

Explicitly defaulted function declaration is a new form of function declaration that is introduced into the C++11 standard. You can append the =default; specifier to the end of a function declaration to declare that function as an explicitly defaulted function. The compiler generates the default implementations for explicitly defaulted functions, which are more efficient than manually programmed function implementations. A function that is explicitly defaulted must be a special member function and has no default arguments. Explicitly defaulted functions can save your effort of defining those functions manually.

You can declare both inline and out-of-line explicitly defaulted functions. For example:

class A{
public:  
   A() = default;         // Inline explicitly defaulted constructor definition
   A(const A&);
   ~A() = default;        // Inline explicitly defaulted destructor definition
};

A::A(const A&) = default; // Out-of-line explicitly defaulted constructor definition

You can declare a function as an explicitly defaulted function only if the function is a special member function and has no default arguments. For example:

class B {
public:
  int func() = default;  // Error, func is not a special member function.
  B(int, int) = default; // Error, constructor B(int, int) is not 
                         // a special member function.
  B(int=0) = default;    // Error, constructor B(int=0) has a default argument. 
};

The explicitly defaulted function declarations enable more opportunities in optimization, because the compiler might treat explicitly defaulted functions as trivial.