Re: [boost] [mpl] mpl::c_str fails for mpl::push_back

Subject: Re: [boost] [mpl] mpl::c_str fails for mpl::push_back
From: vicente.botet (vicente.botet_at_[hidden])
Date: 2010-11-08 12:01:58

----- Original Message ----- 
From: "Aleksey Gurtovoy" <agurtovoy_at_[hidden]>
To: <boost_at_[hidden]>
Sent: Monday, November 08, 2010 11:06 AM
Subject: Re: [boost] [mpl] mpl::c_str fails for mpl::push_back

> A more general approach would be something like this (untested):
> 
> 
>     BOOST_MPL_HAS_XXX_TRAIT_DEF( value_type )
> 
>     template< bool, typename X > struct value_type_impl
>     {
>         typedef typename X::value_type type;
>     }
> 
>     template< typename X > struct value_type
>         : value_type_impl< has_value_type<X>::value, X >
>     {
>     };
> 
>     template< typename X > struct value_type_impl<false,X>
>     {
>         typedef typename value_type< typename front<X>::type >::type type;
>     };

Yes this will be more general. I will add that as the sequence can be heterogeneous we would choose as value_type of a Integral Sequece wrapper as the common_type of all the element's value_type. If some of the Seq elements are not integral wrappers or there is not a common type, value_type<Seq> will not compile.

A complementay possibility is to extend the c_str class with a new template parameter ValueType (all the sequence's member should be convertible to it). ValueType could default to Sequence::value_type or value_type<Sequence>.

const char16_t* u16str = c_str<Seq, char16_t>::value;

This will work if each element on Seq is convertible to char16_t. And the following will works as before

const char* str = c_str<String>::value;

>> Is there a possibility to add this trait to Boost.MPL and the  
>> corresponding specialization and move the c_str function to a standalone  
>> file ?
> 
> If it's OK with Eric, I'd be happy to accept the patch.

I'll prepare a patch with the c_str parameter extension in a separated file, as it should not depend any more on mpl::string and could work with other sequences and the value_type trait in another file. Let me know if there is someting wrong.

Best,
Vicente