BUG: Build Errors on Non-Void Return Types for Friend Function Template (246891)


The information in this article applies to:

  • Microsoft Visual C++, 32-bit Enterprise Edition 6.0
This article was previously published under Q246891

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 that this is a bug in the Microsoft products that are 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
  • File.cpp
  • Main.cpp
class A
{
  public :
       template<class T> friend int Test(T);
}; 

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

#include "file.h"

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

}
Modification Type:MajorLast Reviewed:10/15/2002
Keywords:kbBug kbCompiler kbDSupport kbLangCPP kbtemplate KB246891
©2002 Microsoft Corporation. All rights reserved.