Q239436: PRB: LNK2001 on Template Member Functions

Article: Q239436
Product(s): Microsoft C Compiler
Version(s): 5.0,6.0
Operating System(s): 
Keyword(s): kbLangCPP kbLinker kbVC kbVC500 kbVC600 kbDSupport
Last Modified: 02-MAY-2002

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The information in this article applies to:

- Microsoft Visual C++, 32-bit Enterprise Edition, versions 5.0, 6.0 
- Microsoft Visual C++, 32-bit Professional Edition, versions 5.0, 6.0 
- Microsoft Visual C++, 32-bit Learning Edition, version 6.0 
- Microsoft Visual C++.NET (2002) 
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SYMPTOMS
========

You receive a LNK2001 error message on template member functions unless the
member functions are also defined in the same translation unit.

CAUSE
=====

The compiler does not support the use of the "export" keyword as specified in
the C++ standard below:

  

  Section 14:

  Para# 7: Declaring a class template exported is equivalent to declaring all of
  its non-inline function members, static data members, member classes, member
  class templates and non-inline function member templates which are defined in
  that translation unit exported.

  Para# 8: Templates defined in an unnamed namespace shall not be exported. A
  template shall be exported only once in a program. An implementation is not
  required to diagnose a violation of this rule. A non-exported template that
  is neither explicitly specialized nor explicitly instantiated must be defined
  in every translation unit in which it is implicitly instantiated (temp.inst)
  or explicitly instantiated (temp.explicit); no diagnostic is required. An
  exported template need only be declared (and not necessarily defined) in a
  translation unit in which it is instantiated. A template function declared
  both exported and inline is just inline and not exported.

RESOLUTION
==========

You have the following two options:

- Keep the template declaration and definition together in one translation
  unit.

- If you know in advance how your template class will be used in your
  application, you can perform explicit instantiation for the template class.

  For example, for class MyStack declared in Myprog.h

  template <class T> MyStack;

  and implemented in Myprog.cpp, the following explicitly instantiates MyStack
  for int variables:

  template class MyStack<int>;

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

Explicit instantiation allows you to create an instantiation of a template class
or function without actually using it in your code. The idea is to explicitly
instantiate all possible types for the template class in the implementation
file. Thus, when the .cpp file is compiled, the code is generated for these
instantiations, alleviating the LNK2001 error.

Please refer to the Visual C++ Help for more information regarding explicit
instantiation.

REFERENCES
==========

For additional information, click the article number below to view the article
in the Microsoft Knowledge Base:

  Q128789 BUG: LNK2001 on Member Function When Use Nested Class Template

Additional query words:

======================================================================
Keywords          : kbLangCPP kbLinker kbVC kbVC500 kbVC600 kbDSupport 
Technology        : kbVCsearch kbAudDeveloper kbVC500 kbVC600 kbVC32bitSearch kbVCNET kbVC500Search
Version           : :5.0,6.0
Issue type        : kbprb

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