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Move assignment operator

From Wikipedia, the free encyclopedia

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 Thereafter, the other object's data is no longer valid.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

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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

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  1. ^ a b "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.