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

From: Peter Dimov (pdimov_at_[hidden])
Date: 2008-04-07 07:29:50

> 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.

Yes, it's wrong, result<> and operator() are out of sync. 
result_of<identity(X&)>::type returns X&, whereas identity()(x), where x is 
an lvalue of type X, returns X const&.

    Arg& operator()( Arg& ) const;
    Arg operator()( Arg const& ) const;

maybe? I still don't remember how these overloads are resolved for a const 
lvalue. Either way, this seems the best we can do in C++03.