Re: unable to resolve template parameter

From:

James Kanze <james.kanze@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Thu, 3 Dec 2009 01:04:47 -0800 (PST)

Message-ID:

<ff96f74b-3359-4e0f-b82f-89499ea31ae0@a32g2000yqm.googlegroups.com>

On Dec 2, 10:28 pm, Noah Roberts <d...@reply.com> wrote:

In article <597a2b29-13b0-4eea-bd9b-9d7ad6836284
@b2g2000yqi.googlegroups.com>, gert-jan.de....@onsneteindhoven.nl
says...

On Dec 2, 7:02 pm, Noah Roberts <d...@reply.com> wrote:

What is wrong with the following code because I just can't
see it:

#include <string>
#include <iostream>

namespace something {

  template < typename R, typename T >
  inline R f(T const& x)
  {
    return R(x);
  }
}

template < typename T >
struct test
{
  test(double d = something::f<double>(5))
    : test_double(d) {}

  double test_double;

};

int main()
{
  test<int> t;

  std::cout << t.test_double;

  std::cin.get();
}

The compiler complains that it cannot resolve R in the
call to something::f<double>() in the test constructor.

Comeau C++ compiles it so it is probably a compiler bug.

G++ does as well, and I can't think of anything in the standard
which would make this a special case.

That's really where I'm leaning too. It's just such a basic,
everyday use that I'm pretty blown away that it made it
through QA. I guess that's just MS for you though.

IIRC, you said that it only occurs in default arguments, and
only if there is a namespace qualifier. Which means that it is
a result of some faulty interaction between different features.
Off hand, it makes me wonder about the design of the compiler
(no encapsulation?), but it doesn't surprise me too much that it
slipped through quality control: given the number of features of
C++, you can't really test all possible combinations.

--
James Kanze