Re: [boost] detecting a constructor with a specific signature

Subject: Re: [boost] detecting a constructor with a specific signature
From: Vicente Botet Escriba (vicente.botet_at_[hidden])
Date: 2010-01-27 10:29:50

Daniel Frey-3 wrote:
> 
> 
> On 26.01.2010, at 23:36, Kenny Riddile wrote:
> 
>> I'm trying to create a type trait to detect the presence of a constructor
>> with a very specific signature.
> 
> You might want to have a look into is_constructible. It's a new type trait
> from the upcoming standard
> <http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2009/n3000.pdf>, and a
> partial implementation is possible. Here's a version for GCC 4.4+
> (requires -std=c++0x):
> 
> #include <type_traits>
> 
> namespace boost
> {
>   namespace is_constructible_impl
>   {
>     template< typename T >
>     typename std::add_rvalue_reference< T >::type declval();
> 
>     template< typename T >
>     decltype( T(), std::true_type() ) select( int );
> 
>     template< typename T, typename A >
>     decltype( static_cast< T >( declval< A >() ), std::true_type() )
> select( int );
> 
>     template< typename T, typename A1, typename A2 >
>     decltype( T( declval< A1 >(), declval< A2 >() ), std::true_type() )
> select( int );
> 
>     template< typename T, typename A1, typename A2, typename A3 >
>     decltype( T( declval< A1 >(), declval< A2 >(), declval< A3 >() ),
> std::true_type() ) select( int );
> 
>     template< typename T, typename A1, typename A2, typename A3, typename
> A4 >
>     decltype( T( declval< A1 >(), declval< A2 >(), declval< A3 >(),
> declval< A4 >() ), std::true_type() ) select( int );
> 
>     // A variadic version of the above is not possible,
>     // GCC 4.4 crashes with an internal compiler error.
>     // GCC 4.5 will likely provide is_constructible itself
> 
>     template< typename, typename... >
>     std::false_type select( ... );
>   }
> 
>   template< typename T, typename... As >
>   struct is_constructible
>     : std::is_same< decltype( is_constructible_impl::select< T, As... >( 0
> ) ), std::true_type >
>   {
>   };
> }
> 
> HTH.
> 
> Regards, Daniel
> 

Hi, glad to hear that a partial solution exists.

Can this be emulated with Boost.Typeof on C++98 compilers? 

Best,
Vicente
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