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

Subject: Re: [boost] [mpl] Using apply with gcc error
From: Edward Diener (eldiener_at_[hidden])
Date: 2011-02-04 17:11:09

On 2/4/2011 12:02 PM, Dave Abrahams wrote:
> On Thu, Feb 3, 2011 at 6:02 PM, Steven Watanabe<watanabesj_at_[hidden]>  wrote:
>> n 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.).
>>
>> Add
>> using typename boost::mpl::apply<MF,T>::type;
>
> This is easier, neh?
>
> struct myMF :
> boost::mpl::apply<MF,T>
> {
>      BOOST_STATIC_CONSTANT(bool,value=myMF::type::value);
> };
>
> On the other hand, if this thing is returning an MPL integral
> constant, why wouldn't you simply:
>
> struct myMF :
> boost::mpl::apply<MF,T>::type
> {};
>
> and be done with it?

The last suggestion works well, as does the first.

But I have some confusion about boost::mpl::apply which neither your MPL 
book nor the Boost docs clears up for me. As I understand it 'apply' 
invokes the 'apply' metafunction in a metafunction class and 
apply<...>::type returns the invoked metafunction's type. If I am 
deriving from the metafunction's type, then 'myMF' itself does not have 
a 'type' but only a 'value'. Is this correct or do I have a basic 
misunderstanding of  how boost::mpl::apply works ?