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c++for-loopc++11language-lawyerlifetime

What is the lifetime of the range-init in a range-based for loop?


In the latest C++ standard it implies that:

for (foo : bar)
    baz;

is equivilant to:

{
    auto && r = bar;
    for ( auto it = r.begin(), end = r.end(); it != end; ++it )
    {
        foo = *it;
        baz;
    }
}

When bar in the above is a function call that returns a collection, eg:

vector<string> boo();

ie

for (auto bo : boo())
    ...

Doesn't the line become:

auto&& r = boo();
...

And so the temporary return value of boo() is destroyed at the end of the statement auto&& r = boo(), and then r is a hanging reference at the entry of the loop. Is this reasoning correct? If not, why not?


Solution

  • Is this reasoning correct? If not, why not?

    It is correct up until this point:

    And so the temporary return value of boo() is destroyed at the end of the statement "auto&&r=boo()" [...]

    Binding a temporary to a reference extends its lifetime to be that of the reference. So the temporary lasts for the whole loop (that's also why there is an extra set of {} around the whole construct: to correctly limit the lifetime of that temporary).

    This is according to paragraph 5 of §12.2 of the C++ standard:

    The second context is when a reference is bound to a temporary. The temporary to which the reference is bound or the temporary that is the complete object of a subobject to which the reference is bound persists for the lifetime of the reference except:

    [various exceptions that don't apply here]

    This is an interesting property that allows abusing the ranged-for loop for non-rangey things: http://ideone.com/QAXNf