Re: [boost] [utility]result_of - erroneous result type from a C++11 lambda

Subject: Re: [boost] [utility]result_of - erroneous result type from a C++11 lambda
From: Nathan Crookston (nathan.crookston_at_[hidden])
Date: 2013-04-02 16:45:47

Hi All,

Andrey Semashev <wrote:

> Vicente J. Botet Escriba wrote:
> > Hi,
> >
> > I've started to use C++11 lambda functions with Boost.Thread.
> >
> > The following works with clang-3.2
> >
> > boost::future<int> f1 = boost::async( []() -> int {return 123;} );
> >
> >
> > But it fails with gcc-4.6.2, 4.7.2 and 4.8.0
> >
> > ../example/lambda_future.cpp:25:91: erreur: conversion from
> > ‘boost::future<void>’ to non-scalar type ‘boost::future<int>’ requested
> > boost::future<int> f1 = boost::async( []() -> int {return 123;} );
> >
> > The current implementation of boost::async uses boost::result_of to get
> > the result type of the callable parameter.
> >
> > Is this a know bug on boost::result_of or gcc compiler?
>
> I think this is because result_of is based on decltype for clang and TR1
> protocol for gcc. This is because gcc doesn't support N3276. 4.8.0 release
> notes state that it is supported but I saw someone on the list found some
> bug
> in it so the support is incomplete for now.
>
>
For what it's worth, there's a patch in trac[1] which would allow the
previous to compile without requiring a full decltype implementation and
without causing errors when using tr1::result_of would succeed.

The patch was created after some discussion[2], but I haven't continued to
push for its acceptance -- work happens.  Some may find it useful, however.

Thanks,
Nate

[1] https://svn.boost.org/trac/boost/ticket/7753
[2]
http://boost.2283326.n4.nabble.com/Range-amp-c-0x-Lambdas-Can-this-be-done-td4638947i20.html