Re: [boost] [result_of] Allow result_of to work with C++11 lambdas

Subject: Re: [boost] [result_of] Allow result_of to work with C++11 lambdas
From: Nathan Crookston (nathan.crookston_at_[hidden])
Date: 2013-04-10 13:12:04

Hi Vicente,

Vicente J. Botet Escriba wrote:

> The problem is that result_of for compatibility problems is not able to
> choose the version of result_of that works the better. So the user needs to
> define one of these macros to select the best adapted to his needs. But a
> Boost library needs to be portable and needs a result_of that works the
> best depending on the compiler and the context used. Maybe we need another
> result_of that has no backward compatibility issues.
>

If I'm understanding correctly, you're suggesting that compatibility
problems are the reason result_of doesn't use the decltype-based
implementation.  If so, I don't think it's quite true.  The reason why
decltype isn't used for more compilers is that deduction fails in some
cases where incomplete types are involved[1].  This isn't so much a
backwards-compatibility issue since we'd like result_of to work in such
cases, even today.

I think individual boost libraries shouldn't need to worry about the
macros.  Users may need to explicitly define one of the macros if they
develop on a compiler which supports decltype-based result_of, but wish to
support compilers without decltype.

In short, I think that (with the decltype fallback) result_of *will* choose
the best implementation, and (if the patch is applied) defining a macro to
explicitly select a back end should be done for cross-platform
compatibility reasons.

Thanks,
Nate

[1] http://lists.boost.org/Archives/boost/2010/04/164714.php -- very
instructive discussion