Is this valid C++ code? (Using local type outside scope of declaration)

Question

Is the following code valid according to (any) C++ ISO standards?

#include <functional>

auto a() {
    struct Foo {
    };
    return []() {return Foo{}; };
}

int main()
{
    auto l = a()();
    decltype(l) ll;
    //Foo f; //error: unknown type name 'Foo'
    return 0;
}

The compilers (Visual studio 2015, latest Clang and latest GCC) accept this but it seems weird that decltype should give me access to Foo.

Note that you don't need `` for lambdas. Though you do if you want to use `std::function`, which is a convenient type for storing a lambda. — François Andrieux, Mar 08 '18 at 14:34
related: https://stackoverflow.com/questions/13532784/why-can-i-use-auto-on-a-private-type — NathanOliver, Mar 08 '18 at 14:39
If you have another question, please ask it as a new question. — Baum mit Augen, Mar 08 '18 at 14:47
Was a bit miffed that @BaummitAugen [correctly] found an [accurate] duplicate after I'd answered this question. Turns out I answered the duplicate too. Winning. — Lightness Races in Orbit, Mar 08 '18 at 14:52
@LightnessRacesinOrbit It was even the *only* dupe of this I found in the first couple of pages. Tbh, I had expected there to be a couple more, but then, google was not great in understanding my query. — Baum mit Augen, Mar 08 '18 at 14:54
@BaummitAugen: Ironically, the only possible dupe I could think of, was a question that _I_ asked (see comments under my answer) — Lightness Races in Orbit, Mar 08 '18 at 14:59

score 5 · Answer 1 · answered Mar 08 '18 at 14:36

5

Yes.

It is actually the name of the type that is scoped, not the type itself.

You're not using its name, so everything is fine.

answered Mar 08 '18 at 14:36

Lightness Races in Orbit

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I once asked [a similar question](https://stackoverflow.com/q/20808905/560648). – Lightness Races in Orbit Mar 08 '18 at 14:38
Thanks, see my follow up question. – DDaniel Mar 08 '18 at 14:47

Is this valid C++ code? (Using local type outside scope of declaration)

1 Answers1