PRB: LNK2001 on Template Member Functions

• Microsoft Visual C++, 32-bit Editions, versions 5.0, 6.0

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
Version : winnt:5.0,6.0
Platform : winnt
Issue type : kbprb