Move assignment operator

In the C++ programming language, the move assignment operator =, is used for transferring ownership (moving) of a temporary object to another existing object. The move assignment operator, like most of the other C++ operators, can be overloaded. It is one of the special member functions.

If the move assignment operator is not explicitly defined, then the compiler will generate an implicit move assignment operator (C++11 and newer). The parameter of a move assignment operator is an rvalue reference (T&&) to type T, T being the object that defines the move assignment operator. The move assignment operator is different than a move constructor because a move assignment operator is called on an existing object, while a move constructor would be called on an object being created. One must also signify that the other object's data is not valid anymore, and has been moved.

Overloading move assignment operator

To overload the move assignment operator, the signature of the function must be as follows:^[1]

T& operator=(T&& data)

To successfully overload the move assignment operator, the following conditions must be met:

• Check if the object calling the operator is not calling the operator on itself.
• The current object's data is de-allocated.
• The object that is being moved from must have its data marked as nullptr (or something to signify the move)
• The operator returns a reference to "*this".

An implementation of the move assignment operator:^[2]

class Resource {

public:

    Resource& operator=(Resource&& other) {
    
        if (this != &other) {           // If the object isn't being called on itself
            delete this->data;          // Delete the object's data
            this->data = other.data;    // "Move" other's data into the current object
            other.data = nullptr;       // Mark the other object as "empty"
        }
        return *this;                   // return *this
    }

    void* data;

};

A more practical example:

class String {

public:

    String& operator=(String&& otherString) {
    
        if (this != &otherString) {
            delete text;
            this->text = otherString.text;
            otherString.text = nullptr;
        }
        return *this;
    }

    char* text;

};

References

1. "Move assignment operator - cppreference.com". en.cppreference.com. Retrieved 2016-02-23.
2. "Move Constructors and Move Assignment Operators (C++)". msdn.microsoft.com. Retrieved 2016-02-23.