Re: problem with partial ordering of classes

From:

=?ISO-8859-1?Q?Daniel_Kr=FCgler?= <daniel.kruegler@googlemail.com>

Newsgroups:

comp.lang.c++.moderated

Date:

Mon, 26 Jul 2010 08:13:08 CST

Message-ID:

<0bb5851a-be96-47f9-a522-09bae3948d43@l14g2000yql.googlegroups.com>

On 26 Jul., 01:56, Daniel Kr?gler <daniel.krueg...@googlemail.com>
wrote:

On 25 Jul., 17:18, Rodolfo Lima <rodo...@rodsoft.org> wrote:

Hi, I'm having some problems with the following code, gcc-4.5 refuses
to compile it:

#include <iostream>

template <bool B, class V=void> struct enable_if {};
template <class V> struct enable_if<true,V> { typedef V type; };

template <class T>
struct bar {};

template <class T, int ID, class V=void>
struct foo;

template<template<class> class V, class T, int ID>
struct foo<V<T>, ID>
{
   static const int value = 1;
};

template<class T, int ID>
struct foo<bar<T>,ID, typename enable_if<ID!=0>::type>
{
   static const int value = 2;
};

int main()
{
   std::cout << foo<bar<int>,1>::value << std::endl;
}

//------------------------------------------

The error message:
teste.cpp: In function ?int main()?:
teste.cpp:26:30: error: ambiguous class template instantiation for
?struct foo<bar<int>, 1>?
teste.cpp:14:1: error: candidates are: struct foo<V<T>, ID>
teste.cpp:20:1: error: struct foo<bar<T>, ID, typename
enable_if<(ID != 0)>::type>
teste.cpp:26:15: error: incomplete type ?foo<bar<int>, 1>? used in
nested name specifier

According to partial ordering rules, struct foo<bar<T>,ID, typename
enable_if<ID!=0>::type> is more specialized than struct foo<V<T>, ID>,
isn't it? So it should be used, instead of giving an ambiguity error.

Am I wrong or is the compiler?

This time it's you ;-)

The problem is that in the attempt to define the partial
specialization

template<class T, int ID>
struct foo<bar<T>,ID, typename enable_if<ID!=0>::type>;

you are running into a non-deducible context for the last
argument. Unfortunately the standard does not require this
attempt to be ill-formed. There is a core issue concerning
this situation, see:

http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#549

I was a bit too quick here: My analysis was wrong, because
ID has already been deduced. So let's declare the two test
overloads to verify partial ordering:

template<template<class> class V, class T, int ID>
void test(foo<V<T>, ID>);

template<class T, int ID>
void test(foo<bar<T>,ID, typename enable_if<ID!=0>::type>);

I agree that the ambiguity is astonishing. I have the feeling you
are running into core issue

http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#455

which describes problems with partial ordering in the presence
of non-deduced arguments, but I'm not 100% sure.

HTH & Greetings from Bremen,

Daniel Kr?gler

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