Is it an undefined behavior when you return a ref. to a local variable?

Question

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Returning the address of local or temporary variable
Can a local variable's memory be accessed outside its scope?
return reference to local variable

Is it an undefined behavior when you return a ref. to a local variable ?

http://ideone.com/Kz381

int & func(){

        int x = 10;
        return x;

}

int main() {

        int &y = func();
        cout << y << endl;

}

I'm not sure if "local variable" has special meaning in the specification, but I imagine it would be undefined behaviour for any local variable without static storage duration. — ta.speot.is, May 18 '12 at 08:28

score 7 · Accepted Answer · answered May 18 '12 at 08:27

7

Yes it is. The variable is no longer available when the function ends.

You're unlucky that it appears to work. The thing is, that memory isn't cleared so the 10 is still there, but it can be reclaimed at any time, so it's definitely not safe.

answered May 18 '12 at 08:27

Luchian Grigore

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The very definition of "undefined behavior". – Cody Gray - on strike May 18 '12 at 08:30
Do you mean it is ub ! and the y not always print the same result ? and the local variable will be deallocated after the func end ! – faressoft May 18 '12 at 08:39
Yes it is ub. Yes it does not always print out the same result. The local variable will go out of scope after the function ends, so it can be rewritten with any kinds of crap. – Martol1ni May 18 '12 at 08:40
@faressoft Since it is UB, anything can happen, yes. – Luchian Grigore May 18 '12 at 08:42
What about this http://ideone.com/ElcLn Is it an UN too ? – faressoft May 18 '12 at 09:11
@faressoft yes, it's also UB. – Luchian Grigore May 18 '12 at 09:13
1

A comment on the downvote would be appreciated. – Luchian Grigore May 18 '12 at 09:13

score 5 · Answer 2 · answered May 18 '12 at 08:36

5

If you want to get technical, returning it isn't undefined behavior. You only get undefined behavior when/if you try to use what was returned.

answered May 18 '12 at 08:36

Jerry Coffin

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Is it an undefined behavior when you return a ref. to a local variable?

2 Answers2