Hi All, Andrey Semashev

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-t...

Le 02/04/13 19:42, Evgeny Panasyuk a écrit :

02.04.2013 21:16, Vicente J. Botet Escriba:

...

Is this a know bug on boost::result_of or gcc compiler?

Try http://liveworkspace.org/code/J6bEg$0 :

#define BOOST_THREAD_PROVIDES_FUTURE #define BOOST_RESULT_OF_USE_DECLTYPE #include

int main() { boost::future<int> f1 = boost::async( []() -> int {return 123;} ); } // ___________________________ //

For additional info refer http://www.boost.org/users/news/a_special_note_for_boost_1_52_0_and_higher.h... , http://www.boost.org/doc/libs/1_53_0/libs/utility/utility.htm#result_of

Thanks, this works. Should then the user of Boost.Thread define BOOST_RESULT_OF_USE_DECLTYPE explicitly? Best, Vicente

2013/4/2 Evgeny Panasyuk:

As Andrey Semashev pointed out, looks like current GCC implementation of decltype is not mature enough to be used by default within boost::result_of.

Why? Have any proofs? -- Regards, niXman ___________________________________________________ Dual-target(32 & 64-bit) MinGW compilers for 32 and 64-bit Windows: http://sourceforge.net/projects/mingwbuilds/ ___________________________________________________ Another online IDE: http://liveworkspace.org/

On 2 April 2013 20:44, niXman wrote:

2013/4/2 Evgeny Panasyuk:

...

As Andrey Semashev pointed out, looks like current GCC implementation of decltype is not mature enough to be used by default within boost::result_of.

Why? Have any proofs?

See http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52748 The support is complete, but only in 4.8.1 and later.

2013/4/3 niXman:

I.e. in gcc-4_8-branch N3276 is already implemented?

I readed bug 52748. I have no more questions. -- Regards, niXman ___________________________________________________ Dual-target(32 & 64-bit) MinGW compilers for 32 and 64-bit Windows: http://sourceforge.net/projects/mingwbuilds/ ___________________________________________________ Another online IDE: http://liveworkspace.org/

Le 02/04/13 20:46, Vicente J. Botet Escriba a écrit :

Le 02/04/13 19:42, Evgeny Panasyuk a écrit :

...

02.04.2013 21:16, Vicente J. Botet Escriba:

...

Is this a know bug on boost::result_of or gcc compiler?

Try http://liveworkspace.org/code/J6bEg$0 :

#define BOOST_THREAD_PROVIDES_FUTURE #define BOOST_RESULT_OF_USE_DECLTYPE #include

int main() { boost::future<int> f1 = boost::async( []() -> int {return 123;} ); } // ___________________________ //

For additional info refer http://www.boost.org/users/news/a_special_note_for_boost_1_52_0_and_higher.h... , http://www.boost.org/doc/libs/1_53_0/libs/utility/utility.htm#result_of

Thanks, this works.

Should then the user of Boost.Thread define BOOST_RESULT_OF_USE_DECLTYPE explicitly?

BTW, the definition of BOOST_RESULT_OF_USE_DECLTYPE can be done only if BOOST_NO_CXX11_DECLTYPE because otherwise the inclusion of of result_of.hpp generates some compiler errors on compilers not supporting decltype. #include #if ! defined BOOST_NO_CXX11_DECLTYPE #define BOOST_RESULT_OF_USE_DECLTYPE #endif #include Could result_of.hpp undefine BOOST_RESULT_OF_USE_DECLTYPE if BOOST_NO_CXX11_DECLTYPE is defined? Vicente

Le 02/04/13 19:42, Evgeny Panasyuk a écrit :

...

02.04.2013 21:16, Vicente J. Botet Escriba:

...

Is this a know bug on boost::result_of or gcc compiler?

Try http://liveworkspace.org/code/J6bEg$0 :

#define BOOST_THREAD_PROVIDES_FUTURE #define BOOST_RESULT_OF_USE_DECLTYPE #include

int main() { boost::future<int> f1 = boost::async( []() -> int {return 123;} ); } // ___________________________ //

For additional info refer http://www.boost.org/users/news/a_special_note_for_boost_1_52_0_and_higher.h... , http://www.boost.org/doc/libs/1_53_0/libs/utility/utility.htm#result_of

Thanks, this works.

Should then the user of Boost.Thread define BOOST_RESULT_OF_USE_DECLTYPE explicitly?

I define it whenever I use lambdas with any Boost library, whether it be Range, Thread, or something else. Regards, Nate