Summary - Move semantics
- C++ course
- Intermediate
- Lesson 105/116
- 5 min
Combine std::move and move ctor to write a buffer class.
Summary - Move semantics
In this group you learned move semantics. T&& is an rvalue reference that binds to temporaries, and std::move converts an lvalue to an rvalue.
The move constructor T(T&&) transfers a resource to a new object, and move assignment T& operator=(T&&) transfers to an existing object. The rule of five says to define all five special members together. Now you will consolidate what you learned with two tasks: first an rvalue reference function, then moving with std::move.
Practice Write an rvalue reference function
Define a function named show: its parameter is std::string&& s, its body is std::cout << s;. In main call show("Hello");.
Starter code
#include <iostream>
#include <string>
// Define the show function
int main() {
// Call the show function
return 0;
}Sample solution
#include <iostream>
#include <string>
void show(std::string&& s) {
std::cout << s;
}
int main() {
show("Hello");
return 0;
}Expected output
Hello
Practice Move a string with std::move
The starting code has a string a containing "World". Move the content of a into b with std::move and print b with std::cout.
Starter code
#include <iostream>
#include <string>
#include <utility>
int main() {
std::string a = "World";
// Move the content of a into b and print b
return 0;
}Sample solution
#include <iostream>
#include <string>
#include <utility>
int main() {
std::string a = "World";
std::string b = std::move(a);
std::cout << b;
return 0;
}Expected output
World
End of section
You completed move semantics. You can now use the T&& rvalue reference and move with std::move.
With the move constructor and move assignment you transfer resources efficiently. The rule of five and rule of zero tell you when to write special members. The next group covers operator overloading; you will see the move assignment operator there in more detail.
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What does std::move do?
std::string b = std::move(a);- Deletes a
- Copies a
- Returns the address of a
- Converts a to an rvalue Correct answer
Explanation: std::move converts a to an rvalue.
2 rvalue reference parameter
void show(std::string____ s) { std::cout << s; }void show(std::string&& s) { std::cout << s; }Explanation: && is the rvalue reference operator.