CODEWITHSUNDEEP
c++lambda
Zitrax
Zitrax

Reputation: 20255

Returning lambda from function with passed in reference capture

Is it safe returning a lambda like this:

auto doIt(const Object& obj) {
    return [&obj]{ obj.doIt();};
}

It returns a lambda using local capture, but the reference is one that is passed into the function and not from a local variable.

Upvotes: 2

Views: 80

Answers (1)

WhiZTiM
WhiZTiM

Reputation: 21576

auto doIt(const Object& obj) {
    return [&obj]{ obj.doIt();};
}

That's probably not a good idea for the mere fact that doIt(const Object& obj) can bind a temporary object. Imagine doing:

auto lm = doIt(make_object());
lm();                                 //Undefined behavior

That's because the temporary returned my make_object would be destroyed at the end of the function doIt, but your returned lambda still holds a reference to the destroyed object.

Another example will be:

auto func(){
    Object obj = make_object();
    auto lm = doIt(obj);
    lm();                            //This is Ok
    return lm;
}

void something{
    auto val = func();
    val();                           //Undefined Behavior
}

EDIT: See the comments.

You can add a void doIt(const Object&&) = delete; overload to prevent temporaries; Courtesy of @StoryTeller.

Upvotes: 4

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