Move assignment operator

In the C++ programming language, the move assignment operator = is used for transferring a temporary object to an existing object. The move assignment operator, like most C++ operators, can be overloaded. It is a special member function.

If the move assignment operator is not explicitly defined, the compiler generates an implicit move assignment operator (C++11 and newer). The parameter of a move assignment operator is an rvalue reference (T&&) to type T, where T is 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 is called on an object created by the operation. Thereafter, the other object's data is no longer valid.

Overloading move assignment operator

To overload the move assignment operator, the signature of the function must be:^[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 must return 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.