rvalue references

Learn the rvalue reference (&&) concept.

rvalue references (&&)

An rvalue reference (T&&) binds to temporary values. While a normal reference (T&) binds to named variables, T&& binds only to temporary objects.

Example: void show(std::string&& s); can only be called with temporary strings: show("Hello"); works but std::string x = "Hello"; show(x); gives a compile error. This is because x is a named variable (lvalue) while "Hello" is a temporary (rvalue) string. rvalue references prevent unnecessary copies and enable move operations.

lvalue versus rvalue

In C++ every expression is either an lvalue (has a name, has an address) or an rvalue (temporary, no addressable storage).

Example: in int x = 5; the x is an lvalue and 5 is an rvalue. The expression x + 1 is also an rvalue because its result has no address. An rvalue reference (int&&) binds only to rvalues: int&& r = 5; is valid but int&& r = x; is an error. This distinction lets the compiler decide whether to copy or move.

Exercise Write an rvalue reference function

Define a function named printValue. Its parameter is std::string&& s (rvalue reference). Its body is std::cout << s;. In main call printValue("Hello");.

Show example solution

#include <iostream>
#include <string>

void printValue(std::string&& s) {
    std::cout << s;
}

int main() {
    printValue("Hello");
    return 0;
}

Test yourself

What can std::string&& bind to?
```
void show(std::string&& s);
show(______);
```
- A named variable (lvalue)
- A file
- A temporary value (rvalue)
- A pointer
Show answer

A temporary value (rvalue) — An rvalue reference binds to temporary values.

rvalue reference parameter

void show(std::string____ s) { std::cout << s; }

Show answer

void show(std::string&& s) { std::cout << s; }

Match each expression to its type

std::string&&

rvalue reference
std::string&

lvalue reference
"Hello"

rvalue
x (variable)

lvalue

Next lesson: std::move

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