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

19 Mar 2005


      Ian McCulloch wrote:
...
Peter Dimov wrote:
...
Peter Dimov wrote:
...
Ian McCulloch wrote:
...
Peter Dimov wrote:
[...]
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I don't get it: can't you detect whether
nested::result::type exists?  As long as the primary
template of nested::result is defined as an empty struct this
should work fine(?).
No, consider what happens when 'nested' is int (*) (), for example.
Hm, it can't be int (*) (), because result_of will not attempt to
look for a nested result, but it can be an user-defined type with no
nested result, or a member named result.
Wouldn't both those cases (no nested result, or a member named
result) cause a substitution failure?
Allowed but not required by 14.8.2/2, I think.
...
template <typename T>
typename T::result::type
func(T const& x)
{
  std::cout << "func(T const&)\n";
  return typename T::result::type();
}
There should be a "template" before "result".
...
This compiles cleanly with g++ 3.3.4
Yes, it also works on MSVC 7.1 and (with "template" added before "result") 
on Comeau 4.3.0.1 strict, so it seems that this is a "supported" case of 
substitution failure. However tr1::result_of implementations aren't required 
to use this trick (which, to the best of my knowledge, hasn't been 
publicized anywhere so far) to not generate errors when 
::result::type is not defined, and boost::result_of doesn't, 
either.