Q168078: FIX: C2784 Instantiating STL Objects with a UDT Parameter

Article: Q168078
Product(s): Microsoft C Compiler
Version(s): winnt:5.0
Operating System(s): 
Keyword(s): kbtool kbVC500bug kbVC600fixkbbuglist
Last Modified: 07-MAY-2001

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The information in this article applies to:

- Microsoft Visual C++, 32-bit Enterprise Edition, version 5.0 
- Microsoft Visual C++, 32-bit Professional Edition, version 5.0 
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SYMPTOMS
========

When you attempt to instantiate an STL component for a class, a C2784 error
occurs, complaining that a relational operator (such as operator!= or
operator>) is not defined. This in spite of including "using namespace
std::rel_ops;", and defining operator< and operator== for the class.

CAUSE
=====

The problem is caused by the nested namespace std::rel_ops. The compiler is
unable to do a proper name lookup for the generic relational operators '!=',
'<=', '>', '>=', which are defined in the namespace std::rel_ops and
are referenced by several STL components.

RESOLUTION
==========

The easiest workaround is to add a using declaration for the specific operator
the compiler is complaining about. See the sections compiled conditionally upon
WORKAROUND in the sample code below.

STATUS
======

Microsoft has confirmed this to be a problem in the Microsoft products that are
listed at the beginning of this article.This problem was corrected in Microsoft
Visual C++ version 6.0.

MORE INFORMATION
================

The following sample demonstrates the problem.

Sample Code
-----------

     /*
     Compile options: /GX
     */ 
     #include <list>

     using namespace std ;
     using namespace std::rel_ops ;

     #ifdef WORKAROUND   // define WORKAROUND to get around the error
     using std::rel_ops::operator!=;
     using std::rel_ops::operator>;
     #endif

     class X {

         int N;
     public:
         X(int n) : N(n) {}
         bool operator==(const X& b) const
         { return N == b.N; }
         bool operator<(const X& b) const
         { return N < b.N; }
     };

     list <X> xList;

     int main()
     {
          return 0;
     }

Additional query words:

======================================================================
Keywords          : kbtool kbVC500bug kbVC600fix kbbuglist
Technology        : kbVCsearch kbAudDeveloper kbVC500 kbVC32bitSearch kbVC500Search
Version           : winnt:5.0
Issue type        : kbbug
Solution Type     : kbfix

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