[c++0x] pair's variadic constructor (N2369, 20.2.3.4)

[CoffeeBuzz]

Howard Hinnant-2 wrote:
> 
> It stems from the recent "emplace" work in the associative containers.
> 

Ah yes. I found the document. N2345 


Howard Hinnant-2 wrote:
> 
>> However, after implementing the full spec, I get some odd deduction  
>> behavior
>> in all the ctor overloads. Take the following mock-pair class:
>>
>> template <typename T1, typename T2>
>> struct P {
>>    P(){}
>>    P(P && p) {}
>>    P(T1 const& t1, T2 const& t2){}
>>
>>    template <typename U1, typename U2>
>>    P(P<U1,U2> const& p) {}
>>
>>    template <typename U1, typename U2>
>>    P(U1&& u1, U2&& u2) {}
>>
>>    template <typename U1, typename U2>
>>    P(P<U1,U2> && p) {}
>>
>>    template <typename U, typename... Args>
>>    P(U && u, Args&&... args) {cout << "why?"; }
>> };
>>
>> When I compile the following code:
>> P<int,int> p1(1,2);
>> P<int,int> p2(p1);
>>
>> It seems that the construction of p2 is not done by P(P<U1,U2>  
>> const& p)  or
>> P(P&& p) as I might have expected. Instead, P(U && u, Args&&...  
>> args) is
>> chosen with U being type P<int, int>& and Args is no type (empty  
>> pack).
>>
>> Now this is an issue because in the pair, this variadic ctor  
>> forwards u to
>> "first" and args to "second" but first is type int, and not  
>> P<int,int>& and
>> thus errors. So, how do you stop this behavior?
> 
> I agree we have a problem here.  I believe the answer is going to be  
> along the lines of:
> 
>      template<class U, class Arg0, class... Args> pair(U&& x, Arg0&&  
> arg0, Args&&... args)
>          : first(std::forward<U>(x)), second(std::forward<Arg0>(arg0),  
> std::forward<Args>(args)...)
> 

This does indeed solve the pair construction issue and doesn't seem to
affect the tuple-pair interface (in my test cases anyway).

Thanks,
Chris

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