Multiple definitions of my template class

Question

Hi,

In my library, I have several initialize() and cleanup() functions, for different modules it depends on.

To make this part more safe to use, I decided to follow the RAII rule and built up an Initializer template class, that takes two functions as parameters:

// initializer.hpp (include guards omitted)

template <void initialize(), void cleanup()>
class Initializer
{
  // Does something, but that's not relevant
};

Now, in my xml.hpp file I have this:

// xml.hpp (include guards omitted)

namespace xml
{       
  void initialize();
  void cleanup();

  typename ::Initializer<&initialize, &cleanup> Initializer;
}

This compiles fine but fails on linking because several files include xml.hpp and gcc complains that there are multiple definitions for xml::Initializer.

What can I do to solve this ?

Answer 1

include your xml.hpp code inside these macros

#ifndef XML_HPP
#define XML_HPP

// Include code here

#endif

Answer 2

typename ::Initializer<&initialize, &cleanup> Initializer;

This is an explicit instantiation. Please put it in one and only one translation unit

Answer 3

namespace xml
{       
  void initialize();
  void cleanup();

  typename ::Initializer<&initialize, &cleanup> Initializer;
}

typename is illegal here, it is only allowed in template definitions. You meant typedef!

namespace xml
{       
  void initialize();
  void cleanup();

  typedef ::Initializer<&initialize, &cleanup> Initializer;
}