Move Assignment Operator in C++ 11
Last Updated :
13 Oct, 2023
In C++ programming, we have a feature called the move assignment operator, which was introduced in C++11. It helps us handle objects more efficiently, especially when it comes to managing resources like memory. In this article, we will discuss move assignment operators, when they are useful and called, and how to create user-defined move assignment operators.
Move Assignment Operator
The move assignment operator was added in C++ 11 to further strengthen the move semantics in C++. It is like a copy assignment operator but instead of copying the data, this moves the ownership of the given data to the destination object without making any additional copies. The source object is left in a valid but unspecified state.
User-Defined Move Assignment Operator
The programmer can define the move assignment operator using the syntax given below:
MyClass& operator= (MyClass&& otherObf) noexcept {
// Take resources from 'other' and make them our own
// Properly release our resources if needed
// ...
return *this;
}
As you may have noticed, the move assignment operator function uses a special && reference qualifier. It represents the r-value references (generally literals or temporary values).
Usually, it returns a reference to the object (in this case, *this) so you can chain assignments together.
The move constructor is called by the compiler when the argument is an rvalue reference which can be done by std::move() function.
Example of Move Assignment Operator
In this program, we will create a dynamic array class and create a user-defined move assignment operator.
C++
#include <iostream>
#include <memory>
#include <utility>
using namespace std;
class DynamicArray {
private :
size_t size_;
int * data_;
public :
DynamicArray( size_t size)
: size_(size)
, data_( new int [size])
{
for ( size_t i = 0; i < size; ++i) {
data_[i] = 0;
}
}
DynamicArray& operator=(DynamicArray&& other) noexcept
{
if ( this != &other) {
delete [] data_;
size_ = other.size_;
data_ = other.data_;
other.size_ = 0;
other.data_ = nullptr;
}
return * this ;
}
~DynamicArray() { delete [] data_; }
void print() const
{
for ( size_t i = 0; i < size_; ++i) {
cout << data_[i] << " " ;
}
cout << endl;
}
};
int main()
{
DynamicArray arr1(5);
DynamicArray arr2(10);
cout << "====== BEFORE MOVING ======" << endl;
cout << "arr1: " ;
arr1.print();
cout << "arr2: " ;
arr2.print();
arr2 = std::move(arr1);
cout << endl << "====== AFTER MOVING ======" << endl;
cout << "arr1: " ;
arr1.print();
cout << "arr2: " ;
arr2.print();
return 0;
}
|
Output
====== BEFORE MOVING ======
arr1: 0 0 0 0 0
arr2: 0 0 0 0 0 0 0 0 0 0
====== AFTER MOVING ======
arr1:
arr2: 0 0 0 0 0
Explanation
The move assignment operator (operator=) is used to transfer resources from one DynamicArray object to another. It releases the current resources of the destination object, takes the resources from the source object, and leaves the source object in a valid but unspecified state.
In the main function:
- We create two DynamicArray objects, arr1 with 5 elements and arr2 with 10 elements. We print their initial states.
- We use the move assignment operator to transfer the resources from arr1 to arr2 by calling arr2 = std::move(arr1).
- After the move, we print the states of both arr1 and arr2. arr1 is now in a valid but unspecified state, and arr2 contains the resources of arr1.
Note: We can also define a move constructor with a move assignment operator and reduce the code redundancy by calling the move assignment operator from the move constructor. To know about move constructor, refer to the article – Move Constructors in C++
Implicit Definition of Move Assignment Operator
The compiler also defines a default move assignment operator implicitly when the following conditions are satisfied:
- No user-defined copy constructor is present.
- No user-defined destructor is present.
- No user-defined move constructor is present.
Need of Move Assignment Operator
In C++, objects can manage resources like memory. When we copy an object, it can be quite slow, especially if the object holds a lot of resources. Traditional copy operations create new copies, which can lead to unnecessary memory usage and slow down your program.
This is where move assignment operators come to the rescue. They let one object take over the resources of another, without making extra copies. This can significantly boost performance in scenarios like resizing arrays or returning objects from functions.
In contrast to the copy assignment operator, the end result of the move assignment operator will be a single copy of the source object.
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