Understanding Rvalue Reference

Question

Is this undefined behavior? If not, what is the behavior?

// In some external library with, say, header "a.h" void f(int &&x) { x = 5; // Which memory does this assignment apply to? } #include "a.h" int main() { f(7); // At this point, where is the value of 5? return 0; }

Inside f, a temporary of is created with value 7, exactly as if we wrote f(const int& x), only this is not const. x refers to this temporary, and since it's not const, we can change it. However, this is only a local variable so outside f(), this value is lost. — iavr
– iavr, Commented Apr 16, 2014 at 0:40
I knew I just saw this somewhere. codepuppy.co.uk/cpptuts/Beginning/references.php — Qaz
– Qaz, Commented Apr 16, 2014 at 0:40
@Dimitrios If you pass a variable, f(int&&) simply won't match - you would need f(int), f(int&), or f(const int&). — iavr
– iavr, Commented Apr 16, 2014 at 0:52

harmic · Accepted Answer · 2014-04-16 01:00:44Z

C++11 8.5.3 describes initialization of references (including rvalue references).

It says:

Otherwise, a temporary of type “cv1 T1” is created and initialized from the initializer expression using the rules for a non-reference copy-initialization (8.5). The reference is then bound to the temporary.

So, a temporary of type int is bound to the rvalue reference, and thrown away immediately after the call returns.

Section 12.2 (Temporaries) gives examples very similar to yours.

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