[Boost-users] Re: Re: Re: result_of tutorial

From: Ian McCulloch (ianmcc_at_[hidden])
Date: 2005-03-16 10:36:57

Peter Dimov wrote:

> Ian McCulloch wrote:
>>
>> Yes, if you want to forward to a function but you don't know if it
>> takes arguments by value or const reference.
> 
> It doesn't matter. When you forward, you pass to result_of the exact types
> of the arguments you are supplying in the function (object) call, with a
> reference when the argument is an lvalue.
> 
> int x;
> long y;
> int const z;
> int g();
> int & h();
> 
> f(x, y); // result_of<F(int&, long&)>
> f(1, x); // result_of<F(int, int&)>
> f(z, 4); // result_of<F(int const&, int)>
> f( g(), h() ); // result_of<F(int, int&)>
> 
> What is 'f' and how it takes its arguments in the cases above does not
> affect how result_of is used.

But what happens if F doesn't have an operator() overload that takes 
F(int const&, int) but only has F(int, int) ?  It seems you need to know
this in advance.  I tried detecting it with SFINAE but that doesn't work; I
would have expected that if result<F(...)> has no member type then
result_of<F(...)> would simply be an empty struct, but instead the program
is ill-formed.

What I am trying to do is forward calls to an object with an overloaded
operator() and nested struct result defined appropriately, but I don't know
whether the operator() wants pass-by-value or pass-by-const-reference.

Cheers,
Ian