Re: [boost] boost::result_of and C++03

From: Eric Niebler (eric_at_[hidden])
Date: 2008-04-05 21:26:17

Peter Dimov wrote:
> Eric Niebler:
>> make_plus is trickier. It must be optimal and safe. When passed lvalues,
>> it should return a new node that holds the children by reference. When
>> passed rvalues, it should return a new node that hold the children by 
>> value.
>>
>> == Problem ==
>>
>> Here's the thing ... I have no idea how to implement make_plus as a TR1
>> function object.
> 
> I think that you are right; you can't implement make_plus as a C++03 
> function object. result_of is a red herring. The missing part is &&, not 
> decltype (which result_of is supposed to approximate).

Right!

> You can do this:
> 
>> struct make_plus
>> {
>>   template<typename Sig> struct impl;
>>
>>   template<typename This, typename Left, typename Right>
>>   struct impl< This(Left, Right) >
>>   {
>>     // Left and Right are allowed to be reference types
>>     typedef plus<Left, Right>::type result_type;
>>
>>     result_type operator()(
>>       typename add_const_ref<Left>::type left
>>      ,typename add_const_ref<Right>::type right
>>     ) const
>>     {
>>       return result_type(left, right);
>>     }
>>   };
>> };
> 
> but it's not a function object.

Correct. However, I can do this:

struct make_plus : function_adaptor<make_plus>
{
   // same as before
};

where function_adaptor implements the TR1 function object protocol and 
makes some guess about const-ref arguments -- probably that they are 
rvalues. It could even implement the reference_wrapper gunk to treat 
reference_wrapper<X> arguments as if they were X&.

This is the route Proto is taking. Proto can query a function object F 
if it uses this protocol and use it if it's available, getting correct 
lvalue/rvalue handling. If it's not available, it uses result_of, 
stripping references from argument types first.

I think I got the idea from reading through Egg's documentation. It 
seems like if this sort of thing doesn't exist there, it probably 
should. Shunsuke?

-- 
Eric Niebler
Boost Consulting
www.boost-consulting.com