Move assignment
- C++ course
- Intermediate
- Lesson 103/116
- 5 min
Learn the move assignment operator.
Move assignment
Move assignment runs when an rvalue is assigned to an existing object. Its signature is T& operator=(T&& other).
Example: Bag& operator=(Bag&& other) { item = std::move(other.item); return *this; }. Here the assignment b = std::move(a); triggers move assignment. other.item is moved with std::move and *this is returned. The move constructor creates a new object while move assignment changes the content of an existing one. Both are part of move semantics.
operator= and *this
The move assignment operator returns *this; this enables chained assignment (a = b = c). Resource cleanup must also be done.
Example: Bag& operator=(Bag&& other) { item = std::move(other.item); return *this; }. The line item = std::move(other.item) releases the old content and moves the new one. return *this returns a reference to itself. If the class holds a raw pointer, the old resource must be deleted before move assignment. Classes like std::string handle this automatically, so no extra cleanup is needed.
Practice Add move assignment to the Bag class
Add move assignment to the Bag class in the starting code: Bag& operator=(Bag&& other) { item = std::move(other.item); return *this; }. In main move a into b with b = std::move(a); and print b.item with std::cout.
Starter code
#include <iostream>
#include <string>
#include <utility>
class Bag {
public:
std::string item;
Bag(std::string i) : item(i) {}
// Add move assignment
};
int main() {
Bag a("Apple");
Bag b("Banana");
// Move a into b and print b.item
return 0;
}Sample solution
#include <iostream>
#include <string>
#include <utility>
class Bag {
public:
std::string item;
Bag(std::string i) : item(i) {}
Bag& operator=(Bag&& other) {
item = std::move(other.item);
return *this;
}
};
int main() {
Bag a("Apple");
Bag b("Banana");
b = std::move(a);
std::cout << b.item;
return 0;
}Expected output
Apple
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What does b = std::move(a); trigger?
Bag a("Apple"), b("Banana");
b = std::move(a);- Copy constructor
- Move assignment Correct answer
- Copy assignment
- Move constructor
Explanation: Assigning to an existing object triggers move assignment.
2 Match each operation to its trigger
| Term | Meaning |
|---|---|
Box b(std::move(a)) |
Move constructor |
b = std::move(a) |
Move assignment |
Box b(a) |
Copy constructor |
b = a |
Copy assignment |
3 Move assignment signature
Bag& operator=(Bag____ other) { item = std::move(other.item); return *this; }Bag& operator=(Bag&& other) { item = std::move(other.item); return *this; }Explanation: Move assignment takes an rvalue reference: Bag&&.