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

From: Eric Niebler (eric_at_[hidden])
Date: 2008-04-07 01:29:53

shunsuke wrote:
> Eric Niebler wrote:
>> I think that's right so far. Now imagine a simple identity function 
>> object, that just returns its argument unmodified:
>>
>> struct identity
>> {
>>    template<typename Sig>
>>    struct result;
>>
>>    template<typename This, typename Arg>
>>    struct result<This(Arg)>
>>    {
>>      typedef Arg type;
>>    };
>>
>>    template<typename Arg>
>>    Arg const &operator()(Arg const &arg)
>>    {
>>      return arg;
>>    }
>>
>>    // and a non-const overload, too
>> };
> 
> Let me clarify.
> This identity implementation is simply wrong (if you pass an rvalue to identity.)
> because of inconsistency between decltype and result_of.
> Am I missing anything?

I don't think so. If I have an rvalue, I compute the return type and 
invoke the function as follows:

   result_of<identity(int)>::type i = identity()(1);

This is perfectly safe. If I have an lvalue, it looks like this:

   int const i = 0;
   result_of<identity(int const &)>::type j = identity()(i);

Also perfectly safe.

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