BUG: Build Errors on Non-Void Return Types for Friend Function Template

ID: Q246891

The information in this article applies to:

Microsoft Visual C++, 32-bit Enterprise Edition, version 6.0

SYMPTOMS

When building an application that has a friend template function in a nontemplated class, you may see one of the following error messages:

error C2440: '=' : cannot convert from 'void' to 'type'
Expressions of type void cannot be converted to other types.

or

error LNK2001: unresolved external symbol "void __cdecl Test(int)" (?Test@@YAXH@Z)

Please refer to the sample in the "More Information" section for details.

CAUSE

The compiler and linker are expecting a void return type.

RESOLUTION

Use one of the following two workarounds:

Include the friend template function implementation either in header file or in main source file.

Change the return type of the friend function template to void. If return type is important, pass it as reference.

STATUS

Microsoft has confirmed this to be a bug in the Microsoft products listed at the beginning of this article.

MORE INFORMATION

There are two scenarios in which this error occurs.

Case 1: When the return type of the friend function template is assigned to a variable, the compiler produces a C2440 error.

Case 2: If the return type of the friend function template is not used, then a LNK2001 error is produced by the linker.

Steps to Reproduce Behavior

The following example demonstrates the error:

Create three files and compile with: cl main.cpp file.cpp

File.h


class A
{
  public :
       template<class T> friend int Test(T);
}; 

template int Test(int); // Instantiate the template friend function explicitly.

File.cpp


#include "file.h"

// Friend function implementation.
template<class T> int  Test(T temp)
{
	return 0;
}

Main.cpp


#include "file.h"

//#define CASE1 
//Uncomment the above line to reproduce Case 1. 
//#define CASE2
//Uncomment the above line to reproduce Case 2.

void main()
{

   int x =1;

#ifdef CASE1
   int n = Test('c'); // Gives a C2440 compile error.
#endif

#ifdef CASE2
   Test(x); // Gives LNK2001 linker error.
#endif

}

Additional query words:

Keywords : kbtemplate kbCompiler kbLangCPP kbLinker kbVC kbVC600 kbVC600bug kbDSupport
Version : WINDOWS:6.0
Platform : WINDOWS
Issue type : kbbug