Re: c++/6327: g++ problems where partial specializing nested templates

Re: c++/6327: g++ problems where partial specializing nested templates

[rodrigc]

Synopsis: g++ problems where partial specializing nested templates

State-Changed-From-To: open->closed
State-Changed-By: rodrigc
State-Changed-When: Tue Apr 16 18:51:13 2002
State-Changed-Why:
    Compiles OK with gcc 3.0.4.
    
    With gcc 3.1 (prerelease) there are some warnings, but
    it still compiles:
    a.cc:18: warning: `typename Array_traits<T>::Rebind<U>::Type' is implicitly a
       typename
    a.cc:18: warning: implicit typename is deprecated, please see the documentation
       for details
    

http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=6327

c++/6327: g++ problems where partial specializing nested templates

[cuni]

>Number:         6327
>Category:       c++
>Synopsis:       g++ problems where partial specializing nested templates
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Tue Apr 16 10:16:01 PDT 2002
>Closed-Date:
>Last-Modified:
>Originator:     g++
>Release:        Thread model: single - gcc version 3.0
>Organization:
>Environment:
Mandrake 8.0 on AMD Athlon 800
>Description:
It seems that the compiler doesn't use correctly partial specialization of a nested template, even if it produce correct code when template are not nested.
>How-To-Repeat:
#include <typeinfo>
#include <iostream>

// generic template
template<class T>
struct Array_traits
{
  template<class U>
  struct Rebind
  {
    typedef U Type;
  };
};

// partial specialization for arrays
template<class T, int N>
struct Array_traits<T[N]>
{
  template<class U>
  struct Rebind
  {
    typedef Array_traits<T>::Rebind<U>::Type Type[N];
  };
};

int main()
{
  typedef int array_of_int[10][20][30];
  Array_traits<array_of_int>::Rebind<float>::Type array_of_float;

  // it prints out A10_A20_A30_i
  std::cout << typeid(array_of_int).name() << std::endl;

  // it should print A10_A20_A30_f, but it prints A10_f
  std::cout << typeid(array_of_float).name() << std::endl;
}
>Fix:
Here is a work-around that doesn't use nested templates:

// generic template
template<class T, class U> 
struct Array_rebind
{
  typedef U Type;
};

// partial specialization
template<class T, int N, class U>
struct Array_rebind<T[N], U>
{
  typedef Array_rebind<T,U>::Type Type[N];
};
>Release-Note:
>Audit-Trail:
>Unformatted: