FIX: C2784 Instantiating STL Objects with a UDT Parameter (168078)

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 that this is 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;
   }