Q130276: FIX: C2065 Error on Functions Returns Template Class By Value

Article: Q130276
Product(s): Microsoft C Compiler
Version(s): winnt:
Operating System(s): 
Keyword(s): kbCompiler kbCPPonly kbVCkbbuglist kbfixlist
Last Modified: 23-JUL-2001

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The information in this article applies to:

- The C/C++ Compiler (CL.EXE), included with:
   - *EDITOR Please do not choose this product*Microsoft Visual C++ 32-bit Edition* use 241, 265, 225, versions 2.0, 2.1 
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SYMPTOMS
========

Any reference to a template function that returns a template class by value
generates these errors:

  error C2065: 'function name' : undeclared identifier

  error C2064: term does not evaluate to a function C2065

RESOLUTION
==========

Modify the function to return a reference to the template class instead of
returning by value. In the "Sample Code" section of this article, function 'f1'
returns a template class by value and fails. Function 'f2' returns a template
class by reference and succeeds.

STATUS
======

Microsoft has confirmed this to be a bug in the Microsoft products listed at the
beginning of this article. This problem was fixed in Microsoft Visual C++,
32-bit Edition, version 4.0.

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

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

  /* Compile options required: none
  */ 
  template<class T>
  class M {
  public:
     T m;
  };

  template<class T>
  M<T> f1(const M<T>& x) {return x;}

  template<class T>
  M<T>& f2(const M<T>& x) {return x;}

  void test(void)
  {
     M<int> x;
     M<int> y = f1(x);  // errors C2065 & C2064
     M<int> z = f2(x);  // no error
  }

NOTE: Although this has been fixed under Visual C++ 4.0, the definition of
function f2:

     template<class T>
     M<T>& f2(const M<T>& x) {return x;}

causes the following compilation errors:

  error C2446: [*] 'return' : no conversion from 'const class M<int>' to
  'class M<int> &' (new behavior; please see help)

  error C2561: 'f2' : function must return a value

To avoid this, the return value of that statement should be casted appropriately,
as follows:

     M<T>& f2(const M<T>& x) {return (M<T>&)x;}

Additional query words: 2.00 2.10 9.00 9.10

======================================================================
Keywords          : kbCompiler kbCPPonly kbVC kbbuglist kbfixlist
Technology        : kbVCsearch kbAudDeveloper kbCVCComp
Version           : winnt:
Issue type        : kbbug
Solution Type     : kbfix

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