FIX: C2440 Caused by Template Function with Const Pointer (166282)

The information in this article applies to:

Microsoft Visual C++, 32-bit Enterprise Edition 4.2
Microsoft Visual C++, 32-bit Professional Edition 4.2
Microsoft Visual C++, 32-bit Enterprise Edition 4.2b
Microsoft Visual C++, 32-bit Professional Edition 4.2b
Microsoft Visual C++, 32-bit Enterprise Edition 5.0
Microsoft Visual C++, 32-bit Professional Edition 5.0

This article was previously published under Q166282

SYMPTOMS

The compiler generates the error C2440, when compiling a template function with a const pointer of class T as an argument, where class T is a template parameter.

C2440: 'conversion' : cannot convert from 'type1' to 'type2'

RESOLUTION

Starting with Visual C++ 5.0, explicitly specify the template arguments when calling the template function.

The sample code below demonstrates the problem and the workaround.

STATUS

Microsoft has confirmed this to be a bug in the Microsoft products listed at the beginning of this article. This bug has been corrected in Visual Studio 97 Service Pack 1.

For additional information about the Visual Studio 97 Service Pack 1, please see the following article in the Microsoft Knowledge Base:

170365 INFO: Visual Studio 97 Service Packs - What, Where, and Why

MORE INFORMATION

The following sample code demonstrates the problem.

Sample Code

   /*
   * Compiler Options: None
   */ 

   template <class T> T* func(const T*p)
   {
      return 0;
   }

   int main()
   {
      const int *i=0;
      int*j = func(i);    // erroneous C2440 here

      return 0 ;
   }

The following sample code demonstrates the workaround for Visual C++ 5.0.

Sample Code

   /*
   * Compiler Options: None
   */ 

   template <class T> T* func(const T*p)
   {
      return 0;
   }

   int main()
   {
      const int *i=0;
      int*j = func<int>(i);

      return 0 ;
   }

Modification Type:	Minor	Last Reviewed:	7/5/2005
Keywords:	kbBug kbcode kbCompiler kbCPPonly kbfix kbLangCPP kbProgramming kbVS97sp1fix KB166282