Re: [Boost-users] Understanding MPL and using '::type'

From: Steven Watanabe (watanabesj_at_[hidden])
Date: 2008-05-06 15:44:48

AMDG

Edward Diener wrote:
> I understand the explanation about how placeholders work. The rule you 
> are specifying appears to be that one never adds the '::type' to the end 
> of a placeholder expression when it is being passed to an mpl construct. 
> Is that correct ?
>   

You should never add ::type to a placeholder expression at all.
Otherwise, the metafunction will probably get very confused.

> I wish I knew what the is an mpl function as opposed to what is not an 
> mpl function. Perhaps the mpl algorithms are the mpl functions and 
> everything which is not an algorithm is not an mpl function.
>   

Inserters, Sequences, and the types listed under "Data Types" are not 
metafunctions.
Everything listed in a section with Metafunctions of Algorithms in its 
name are metafunctions.

>>  If your not 
>> sure, the
>> only thing to do is look it up.
>>     
>
> Look up what ? Where in the reference documentation does it tell me 
> whether I need to add '::type' to the end of an mpl construct ?
>   

Here are two examples from the reference:

template< typename Sequence , typename State , typename ForwardOp >
struct accumulate <./accumulate.html> {
    typedef /unspecified/ type;
};

template< typename Seq >
struct back_inserter <./back-inserter.html> {
    // /unspecified/
    // /.../
};

In Christ,
Steven Watanabe