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Subject: [SG10] A feature macro for mandatory copy elision
From: Ville Voutilainen (ville.voutilainen_at_[hidden])
Date: 2017-10-05 05:31:18


It occurs to me that it would be useful to have a feature macro for
mandatory copy elision. This came up
in Qt: https://codereview.qt-project.org/#/c/206467/

So, we want to have a RAII handle that should not easily traverse
scopes. Since the RAII handle
wishes to store an arbitrary functor, we need to make it a template.
In order to avoid having to
force users to specify complex template arguments, we want to add a
factory function (we could
use Class Template Argument Deduction but that's another thing with
another macro). For C++17,
the factory can just return a noncopyable nonmovable type. For earlier
versions, we can befriend
the factory and require that users use lifetime-extending references
for the factory's return value.

With the envisioned macro, the class would look roughly like this,
with the template bits removed for
brevity:

struct DoesNotCopyNorMove
{
    DoesNotCopyNorMove() = default;
#ifdef __cpp_guaranteed_copy_elision
    DoesNotCopyNorMove(const DoesNotCopyNorMove&) = delete;
    DoesNotCopyNorMove(DoesNotCopyNorMove&&) = delete;
#else
private:
    DoesNotCopyNorMove(const DoesNotCopyNorMove&);
    DoesNotCopyNorMove(DoesNotCopyNorMove&&);
    friend DoesNotCopyNorMove factory();
#endif
};

Again, for pre-C++17, using it looks like this:

auto&& x = factory(); // but NOT auto x = factory();

With C++17, the same syntax can be used, but by-value use is also possible:

auto x = factory();

Thoughts?


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