Rvalue References (&&) and std::move Mechanics0%

Move Constructor and Move Assignment Operator

Rvalue References (&&) and std::move Mechanics

Advanced13 min read•Updated: 2026-10-22

⚡ Rvalue References (&&) and std::move Mechanics

Introduction

The introduction of rvalue references (Type&&) in C++11 solved one of C++'s oldest performance bottlenecks: unnecessary deep copying of heavy temporary objects.

🔍 The Rvalue Reference Syntax (`Type&&`)

An rvalue reference binds exclusively to rvalues (temporaries or expiring objects):

1#include <iostream>
2 
3void inspect(int& lref) {
4    std::cout << "Bound to Lvalue reference: " << lref << "\n";
5}
6 
7void inspect(int&& rref) {
8    std::cout << "Bound to Rvalue reference (Movable): " << rref << "\n";
9}
10 
11int main() {
12    int x = 42;
13    inspect(x);           // Calls inspect(int&)
14    inspect(99);          // Calls inspect(int&&) - temporary literal
15    inspect(x + 5);       // Calls inspect(int&&) - temporary result
16    inspect(std::move(x));// Calls inspect(int&&) - cast to rvalue!
17    return 0;
18}

🚚 What `std::move` Actually Does

A common misconception is that std::move moves bytes or generates machine code. In reality:

std::move does not move anything! It is simply an unconditional compile-time static_cast to an rvalue reference: static_cast<std::remove_reference_t<T>&&>(t).

1#include <iostream>
2#include <vector>
3 
4int main() {
5    std::vector<int> source = {1, 2, 3, 4, 5};
6 
7    // Deep copy: allocates new heap buffer, copies 5 integers:
8    std::vector<int> copyTarget = source;
9 
10    // Move: transfers internal pointer addresses! source becomes empty.
11    std::vector<int> moveTarget = std::move(source);
12 
13    std::cout << "moveTarget size: " << moveTarget.size() << "\n";
14    std::cout << "source size:     " << source.size() << " (moved-from valid state)\n";
15 
16    return 0;
17}

⚠️ The Moved-From State Contract

After an object is moved from, the C++ standard states it is in a "valid but unspecified state". You can safely re-assign it or let its destructor run, but you must not make assumptions about its contents.

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🔍 The Rvalue Reference Syntax (Type&&)

An rvalue reference binds exclusively to rvalues (temporaries or expiring objects):

1#include <iostream>
2 
3void inspect(int& lref) {
4    std::cout << "Bound to Lvalue reference: " << lref << "\n";
5}
6 
7void inspect(int&& rref) {
8    std::cout << "Bound to Rvalue reference (Movable): " << rref << "\n";
9}
10 
11int main() {
12    int x = 42;
13    inspect(x);           // Calls inspect(int&)
14    inspect(99);          // Calls inspect(int&&) - temporary literal
15    inspect(x + 5);       // Calls inspect(int&&) - temporary result
16    inspect(std::move(x));// Calls inspect(int&&) - cast to rvalue!
17    return 0;
18}

🚚 What std::move Actually Does

A common misconception is that std::move moves bytes or generates machine code. In reality:

std::move does not move anything! It is simply an unconditional compile-time static_cast to an rvalue reference: static_cast<std::remove_reference_t<T>&&>(t).

1#include <iostream>
2#include <vector>
3 
4int main() {
5    std::vector<int> source = {1, 2, 3, 4, 5};
6 
7    // Deep copy: allocates new heap buffer, copies 5 integers:
8    std::vector<int> copyTarget = source;
9 
10    // Move: transfers internal pointer addresses! source becomes empty.
11    std::vector<int> moveTarget = std::move(source);
12 
13    std::cout << "moveTarget size: " << moveTarget.size() << "\n";
14    std::cout << "source size:     " << source.size() << " (moved-from valid state)\n";
15 
16    return 0;
17}

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Next Lesson

Lvalues vs Rvalues, Value Categories (glvalue, prvalue, xvalue)

Move Constructor and Move Assignment Operator

⚡ Rvalue References (&&) and std::move Mechanics

Introduction

🔍 The Rvalue Reference Syntax (Type&&)

🚚 What std::move Actually Does

⚠️ The Moved-From State Contract

Move Constructor and Move Assignment Operator

Related Lessons

Practice Quiz

Loading Page Content...

⚡ Rvalue References (&&) and std::move Mechanics

Introduction

🔍 The Rvalue Reference Syntax (Type&&)

🚚 What std::move Actually Does

⚠️ The Moved-From State Contract

Move Constructor and Move Assignment Operator

Related Lessons

Practice Quiz

🔍 The Rvalue Reference Syntax (`Type&&`)

🚚 What `std::move` Actually Does

🔍 The Rvalue Reference Syntax (`Type&&`)

🚚 What `std::move` Actually Does