Special member functions

In the C++ programming language, special member functions^[1] 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:

• Default constructor if no other constructor is explicitly declared.
• Copy constructor if no move constructor and move assignment operator are explicitly declared.

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.

• Move assignment operator if no copy constructor, copy assignment operator, move constructor and destructor are explicitly declared.
• Destructor

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).^[4]

In C++03, before the introduction of move semantics (in C++11) the special member functions consisted of:^[5]

• Default constructor (if no other constructor is explicitly declared)
• Copy constructor
• Copy assignment operator
• Destructor

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

import std;

using std::string;

class Explicit {
private:
    string message;
public:
    Explicit() {
        std::println("Default constructor {}");
    }

    explicit Explicit(string message):
        message{std::move(message)} {
        std::println("Non-default constructor {}", message);
    }

    Explicit(const Explicit& other) {
        std::println("Copy constructor {}", message);
        *this = other; // invoke copy assignment operator
    }

    Explicit& operator=(const Explicit& other) {
        std::println("Copy assignment operator {}", message);
        if (this != &other) {
            message = other.message;
        }
        return *this;
    }

    Explicit(Explicit&& other) noexcept {
        std::println("Move constructor {}", message);
        *this = std::move(other);  // invoke move assignment operator
    }

    Explicit& operator=(Explicit&& other) noexcept {
        std::println("Move assignment operator {}", message);
        if (this != &other) {
            message = std::move(other.message);
        }
        return *this;
    }

    ~Explicit() {
        std::println("Destructor {}", message);
    }
};

class Implicit : public Explicit {
private:
    Explicit member;
public:
    void spew() {
        std::println("Implicit({}, {})", message, member.message);
    }
};

Here are the signatures of the special member functions: