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

From: Edward Diener (eldiener_at_[hidden])
Date: 2008-05-06 23:15:00

Steven Watanabe wrote:
> 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.

Ok, understood.

> 
>> 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.

Your firsat sentence above I understand. The ssecond sentence above 
eludes me.

> 
>>>  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/
>     // /.../
> };

OK, I think I get it. If it is described as a metafunction, with its 
nested type, then '::type' is always used.