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

Subject: Re: [boost] detecting a constructor with a specific signature
From: vicente.botet (vicente.botet_at_[hidden])
Date: 2010-01-27 12:27:43

----- Original Message ----- 
From: "Kenny Riddile" <kfriddile_at_[hidden]>
To: <boost_at_[hidden]>
Sent: Wednesday, January 27, 2010 6:14 PM
Subject: Re: [boost] detecting a constructor with a specific signature

> 
> Daniel Frey wrote:
>> On 27.01.2010, at 15:52, Kenny Riddile wrote:
>> 
>>> Ya, I was already using the "flag typedef" method, but was just wondering if something non-intrusive was feasible...maybe it isn't.  A compiler with 0x support isn't an option I'm afraid.
>> 
>> No need for C++0x if you only need a special case. Requires a good compiler, though, GCC 4.3 is not good enough, 4.4+ is. The following compiles with GCC 4.4, -ansi -pedantic, replacing decltype with sizeof :)
>> 
>> #include <string>
>> #include <iostream>
>> 
>> struct Foo {
>>   Foo( const std::string& );
>> };
>> 
>> struct Bar {
>>   Bar( const std::string& );
>>   Bar( const std::string&, Foo& );
>> };
>> 
>> template< typename T >
>> T make();
>> 
>> template< int >
>> struct result { typedef double type; };
>> 
>> template< typename T >
>> typename result< sizeof T( make< const std::string& >(), make< Foo& >() ) >::type select( int );
>> 
>> template< typename >
>> char select( ... );
>> 
>> template< typename T >
>> struct has_foo_ctor
>> {
>>   enum { value = sizeof select< T >( 0 ) > 1 };
>> };
>> 
>> int main()
>> {
>>   std::cout << has_foo_ctor< Foo >::value << std::endl;
>>   std::cout << has_foo_ctor< Bar >::value << std::endl;
>> }
>> 
>> 
>> Regards, Daniel
>> 
>> _______________________________________________
>> Unsubscribe & other changes: http://lists.boost.org/mailman/listinfo.cgi/boost
>> 
> 
> Alas, I'm using VC9.  The first declaration of select() gives me the 
> following error:
> 
> error C2564: 'T' : a function-style conversion to a built-in type can 
> only take one argument

Hi,

have you tried to double ()

template< typename T >
 typename result< sizeof T(( make< const std::string& >(), make< Foo& >() )) >::type select( int );

Vicente