Special member functions

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Special member functions[1] in C++ are functions which the compiler will automatically generate if they are used, but not declared explicitly by the programmer. The automatically generated special member functions are:

If a destructor is declared generation of a copy constructor is deprecated (C++11, proposal N3242 [2]).
  • Move constructor if no copy constructor, copy assignment operator, move assignment operator and destructor are explicitly declared.
  • Copy assignment operator if no move constructor and move assignment operator are explicitly declared.
If a destructor is declared, generation of a copy assignment operator is deprecated.

In these cases the compiler generated versions of these functions perform a memberwise operation. For example the compiler generated destructor will destroy each sub-object (base class or member) of the object.

The compiler generated functions will be public, non-virtual[3] and the copy constructor and assignment operators will receive const& parameters (and not be of the alternative legal forms).

Example[edit]

The following example depicts two classes: Explicit for which all special member functions are explicitly declared and Implicit for which none are declared.

#include <iostream>
#include <string>

class Explicit
{
public:
    Explicit()
    {
        std::cout << "Default constructor " << msg << '\n';
    }

    Explicit(const std::string& value)
        : msg(value)
    {
        std::cout << "Non-default constructor " << msg << '\n';
    }

    Explicit(const Explicit& other)
    {
        std::cout << "Copy constructor " << msg << '\n';
        *this = other; // invoke copy assignment operator
    }

    Explicit& operator=(const Explicit& other)
    {
        std::cout << "Copy assignment operator " << msg << '\n';
        if (this != &other) {
            msg = other.msg;
        }
        return *this;
    }

    Explicit(Explicit&& other) noexcept
    {
        std::cout << "Move constructor " << msg << '\n';
        *this = std::move(other); // invoke move assignment operator
    }

    Explicit& operator=(Explicit&& other) noexcept
    {
        std::cout << "Move assignment operator " << msg << '\n';
        if (this != &other) {
            msg = std::move(other.msg);
        }
        return *this;
    }

    ~Explicit()
    {
        std::cout << "Destructor " << msg << '\n';
    }

private:
    friend class Implicit;
    std::string msg;
};

class Implicit : public Explicit
{
public:
    void Spew()
    {
        std::cout << "Implicit(" << msg << ", " << member.msg << ")\n";
    }

private:
    Explicit member;
};

Signatures[edit]

Here are the signatures of the special member functions:

Function syntax for class MyClass
Default constructor MyClass();
Copy constructor MyClass(const MyClass& other);
Move constructor MyClass(MyClass&& other) noexcept;
Copy assignment operator MyClass& operator=(const MyClass& other);
Move assignment operator MyClass& operator=(MyClass&& other) noexcept;
Destructor ~MyClass() noexcept;

C++98[edit]

In C++98 before the introduction of move semantics the special member functions[4] were:

References[edit]

  1. ^ ISO/IEC (2011). ISO/IEC 14882:2011 (3 ed.). ISO/IEC. pp. §12.
  2. ^ http://accu.org/index.php/journals/1896
  3. ^ Except for the destructor if a base class already has a virtual destructor.
  4. ^ ISO/IEC (1998). International Standard ISO/IEC 14882: Programming languages—C++ = Languages de programmation—C++ (1 ed.). ISO/IEC. pp. §12. OCLC 71718919.