Re: [boost] Discovering C++11 lambda argument and result types to construct boost::function

Subject: Re: [boost] Discovering C++11 lambda argument and result types to construct boost::function
From: Frank Birbacher (bloodymir.crap_at_[hidden])
Date: 2012-02-13 14:08:34

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Hi!

Am 12.02.12 17:37, schrieb Mathias Gaunard:
> And that meta-function is just
> 
> template<class F> struct function_type :
> remove_pointer<decltype(&F::operator())> { };

Well, that does not work for the following reason (example code below):

"The same type as T, but with any pointer modifier removed. Note that
pointers to members are left unchanged: removing the pointer
decoration would result in an invalid type."
(see:
http://www.boost.org/doc/libs/1_48_0/libs/type_traits/doc/html/boost_typetraits/reference/remove_pointer.html
)

So this is the point: I need a signature from a member function. There
is no type trait that does this. Any other suggestions?

Frank

//////////// EXAMPLE:
#include <type_traits>
#include <utility>
#include <boost/function.hpp>

using boost::function;

template<typename F>
struct function_type {
        typedef typename std::remove_pointer<
                        decltype(&F::operator())
		>::type signature;
        
        typedef function<signature> type;
};

template<typename F>
typename function_type<F>::type
make_function(F && f)
{
        typedef typename function_type<F>::type result_type;
        return result_type(std::forward<F>(f));
}

int main()
{
        auto a = make_function([](int){ return 4; });
}

//////////// OUTPUT GCC 4.6:
MainTest.cpp:26:45: error: invalid use of incomplete type
'function_type<main()::<lambda(int)> >::type {aka struct
boost::function<int (main()::<lambda(int)>::*)(int)const> }'
function_fwd.hpp:33:38: error: declaration of
'function_type<main()::<lambda(int)> >::type {aka struct
boost::function<int (main()::<lambda(int)>::*)(int)const>}'
MainTest.cpp:26:45: error: unable to deduce 'auto' from '<expression
error>'
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