Q150573: FIX: C2084 When Template Class Constructor Has No Body

Article: Q150573
Product(s): Microsoft C Compiler
Version(s): 4.1 4.2
Operating System(s): 
Keyword(s): kbCompiler kbCPPonly kbVC kbVC500fix
Last Modified: 03-AUG-2001

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The information in this article applies to:

- The C/C++ Compiler (CL.EXE), included with:
   - Microsoft Visual C++, 32-bit Editions, version 4.1 
   - Microsoft Visual C++, 32-bit Enterprise Edition, version 4.2 
   - Microsoft Visual C++, 32-bit Professional Edition, version 4.2 
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SYMPTOMS
========

The following error occurs when a template class is explicitly instantiated and
it has more than one constructor, at least one of which does not have a body
defined and at least one of which is defined outside of the class:

  error C2084: function '<function name>' already has a body

WORKAROUND
==========

This is the preferred workaround. Give every constructor a body. The body is
defined inside or outside the template class.

-or-

- Place all the existing constructor bodies inside the template class
  definition.

  NOTE: You get the following warning associated with the explicit instantiation
  request:

  warning C4661: <symbol> : no suitable definition provided for explicit
  instantiation request

  This warning is issued when a template class does not have all its members
  defined and an explicit instantiation request is made. This warning also
  causes the compiler to ignore the instantiation request.

-or-

- Instantiate the template class implicitly rather than explicitly. In this
  case, the instantiation occurs when declaring an object using the template
  class, but the C2084 does not.

NOTE: the sample code in this article gives examples of explicit versus implicit
template instantiation.

STATUS
======

Microsoft has confirmed this to be a bug in the Microsoft products listed at the
beginning of this article. This bug was corrected in Visual C++ version 5.0.

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

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

     /* Compile options needed: /c
     */ 

     template <class T>
     class A {
     public:
         A();             // give this ctor a body for workaround 1
         A(const T&);
     };

     template <class T> A<T>::A(const T&) {}; // move this ctor body
                                              // inside the class
                                              // definition for
                                              // workaround 2

     template A<int>;     // this is an explicit instantiation -
                          // comment this line out and uncomment the
                          // next line for workaround 3
     // A<int> x = 3;     // this is an implicit instantiation

Additional query words: kbVC400bug

======================================================================
Keywords          : kbCompiler kbCPPonly kbVC kbVC500fix 
Technology        : kbVCsearch kbAudDeveloper kbCVCComp
Version           : 4.1 4.2
Issue type        : kbbug
Solution Type     : kbfix

=============================================================================

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