Re: Help with std::bind2nd() and GCC 3.3

[Claudio Bley <bley at cs dot uni-magdeburg dot de>]

Hi.

On Tue, Jul 15, 2003 at 11:09:26AM -0400, Paul Dubuc wrote:
> Here is a simplified example of some code that compiles and works with g++
> 2.95.3 but gives the following compiler error with g++ 3.3.  Can anyone 
> tell me
> what is wrong with this code and how to get it to work with the new compiler
> (other than rewriting the loop without find_if and bind2nd)?  Help would be 
> very
> much appreciated.  Thank you.
> 
> #include <iostream>
> #include <list>
> #include <functional>
> #include <algorithm>
> 
> class FPB {
>      private:
>          long key;
>      public:
>          long getkey() const { return key; }
> };
> 
> struct compareKey :
>      std::binary_function<const FPB, const long*, bool>
> {
>      bool operator()(const FPB& pb, const long* key_ptr) const
>      {
>          return pb.getkey() == *key_ptr;
>      }
> };
> 
> main()
> {
>      std::list<FPB> lst;
>      long key = 1;
> 
>      std::list<FPB>::iterator iter(
>          std::find_if(
>              lst.begin(),
>              lst.end(),
>              std::bind2nd(compareKey(), &key)
>          )
>      );
>      if (iter != lst.end()) std::cout << "found" << std::endl;
>      else std::cout << "not found" << std::endl;
> } 

[snip]

> /vol/gnu/gcc/include/c++/3.3/bits/stl_function.h: In instantiation of 
> `std::bind
> er2nd<compareKey>':
> b2nd.cc:34:   instantiated from here
> /vol/gnu/gcc/include/c++/3.3/bits/stl_function.h:401: error: `typename
>     _Operation::result_type std::binder2nd<_Operation>::operator()(typename
>     _Operation::first_argument_type&) const [with _Operation = compareKey]' 
>     and
>     `typename _Operation::result_type
>     std::binder2nd<_Operation>::operator()(typename
>     _Operation::first_argument_type&) const [with _Operation = compareKey]'
>     cannot be overloaded

If you look at the class declaration of binder2nd in
/vol/gnu/gcc/include/c++/3.3/bits/stl_function.h around line 401
you could see the problem.

The problem is that (if _GLIBCPP_RESOLVE_LIB_DEFECTS is defined) the
() operator of binder2nd is overloaded. These are the (simplified)
prototypes:

op::result_type operator() (const op::first_argument_type&) const ;
op::result_type operator() (      op::first_argument_type&) const ;

If you fill in the templatized typenames of your compareKey struct
viz. "const FPB" for "op::first_argument_type" you'd see that the
operator's prototypes are essentially the same which is forbidden, of
course.

If you declare

struct compareKey : std::binary_function<FPB, const long*, bool>

the error is gone. I suppose you'd want to remove the const
modifier on "long*" also, then. Or you could try to undef'ine
_GLIBCPP_RESOLVE_LIB_DEFECTS before bits/stl_function.h gets
included... but that would be kinda wacky, YMMV. :-)

HTH
-- 
Claudio Bley                                 ASCII ribbon campaign (")
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