Re: [boost] [mpl] Using apply with gcc error

Subject: Re: [boost] [mpl] Using apply with gcc error
From: Edward Diener (eldiener_at_[hidden])
Date: 2011-02-03 18:41:48

On 2/3/2011 6:02 PM, Steven Watanabe wrote:
> AMDG
>
> On 2/3/2011 2:42 PM, Edward Diener wrote:
>> Compiling this code with gcc:
>>
>> #include <boost/config.hpp>
>> #include <boost/mpl/apply.hpp>
>>
>> template
>> <
>> class MF,
>> class T
>> >
>> struct myMF :
>> boost::mpl::apply<MF,T>
>> {
>> BOOST_STATIC_CONSTANT(bool,value=type::value);
>> };
>>
>> int main()
>> {
>> return 0;
>> }
>>
>> Gives:
>>
>> "gcc.compile.c++
>> ..\..\..\bin.v2\libs\tti\test\TestMFHasTypeMFC.test\gcc-mingw-4.5.2\debug\TestMFHasTypeMFC.o
>>
>>
>> TestMFHasTypeMFC.cpp:18:5: error: 'type' has not been declared
>>
>
> gcc is correct. Members of a dependent base class are
> not visible in the derived class. (This is necessary
> to allow the template to be parsed before the template
> arguments are known.).

You are correct.

>
> Add
> using typename boost::mpl::apply<MF,T>::type;

This should work but gcc does not like it and gives:

error: 'type' is not a class or namespace.

Simply forgetting about metafunction forwarding and using the tried and 
true:

typedef typename boost::mpl::apply<MF,T>::type type;
BOOST_STATIC_CONSTANT(bool,value=type::value);

works everywhere.