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. Like the copy assignment operator 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) provided that copy/move constructors, copy assignment operator or destructors have not been declared.[1] 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 Thereafter, the other object's data is no longer valid.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
[edit]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".
Consider the following move assignment operator for a simple string class:[2]
class String {
public:
String& operator=(String&& other) noexcept {
// If we're not trying to move the object into itself...
if (this != &other) {
delete[] this->data_; // Free this string's original data.
this->data_ = other.data_; // Copy the other string's data pointer into this string.
other.data_ = nullptr; // Finally, reset the other string's data pointer.
}
return *this;
}
private:
char* data_;
};
References
[edit]- ^ a b "Move assignment operator - cppreference.com". en.cppreference.com. Retrieved 2016-02-23.
- ^ "Move Constructors and Move Assignment Operators (C++)". msdn.microsoft.com. Retrieved 2016-02-23.