Move vs copy
- C++ course
- Expert
- Lesson 29/47
- 5 min
Learn the performance difference between move and copy semantics.
Move vs copy
A copy duplicates an object's resource; a move takes over the resource as is. Copying a large array is expensive; moving is cheap because only a pointer is transferred.
Since C++11 there are move constructors and std::move. std::move(a) does not actually move; it casts a to an rvalue so the move constructor is selected. After the move, the source object is valid but unspecified; usually it is nullptr.
Moving with std::move
std::move casts an lvalue to an rvalue. This makes the compiler select the move constructor. Example: std::string a = "Hello"; std::string b = std::move(a);. Here b takes over a's contents; a becomes empty.
After the move, a's state is unspecified but valid; its destructor runs safely. For large data (std::vector, std::string) move is much cheaper than copy; only three pointers/sizes are transferred, elements are not copied one by one.
Practice Move a std::string with std::move
In main define std::string a = "Hello";. Then write std::string b = std::move(a);. Finally write std::cout << b;.
Starter code
#include <iostream>
#include <string>
int main() {
// Define a, move it to b with std::move and print
return 0;
}Sample solution
#include <iostream>
#include <string>
int main() {
std::string a = "Hello";
std::string b = std::move(a);
std::cout << b;
return 0;
}Expected output
Hello
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Complete the move constructor parameter
Buffer(Buffer&& ____) noexcept : data(other.data), size(other.size) { other.data = nullptr; }Buffer(Buffer&& other) noexcept : data(other.data), size(other.size) { other.data = nullptr; }Explanation: other is the name of the moved-from resource.
2 Match each tool to its role
| Term | Meaning |
|---|---|
copy constructor |
Duplicates resource |
move constructor |
Takes over resource |
std::move |
Casts Lvalue to Rvalue |
noexcept |
Exception guarantee |
3 What does std::move do?
std::string b = std::move(a);- Casts Lvalue to Rvalue, move constructor is selected Correct answer
- Creates a new copy
- Deletes the object
- Physically moves the resource
Explanation: std::move is only a cast; the actual move is done by the move constructor.