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