[Boost-users] [thread] Static mutex members

From: David A. Greene (greened_at_[hidden])
Date: 2006-01-27 21:40:38

The following single-file testcase fails to link:

#include <boost/thread/thread.hpp>
#include <boost/thread/recursive_mutex.hpp>

#include <iostream>

template<typename Type>
class singleton {
public:
   typedef boost::recursive_mutex mutex_type;
   typedef Type * handle;

private:
   typedef mutex_type::scoped_lock lock_type;

   static mutex_type mutex;

   static handle inst;

public:
   static Type &instance(void) {
     lock_type lock(mutex);

     if (!inst) {
       inst = new Type;
     }
     return(*inst);
   };
};

class tester {
public:
   void test(void) { std::cerr << "Test" << std::endl; };
};

template<> singleton<tester>::mutex_type singleton<tester>::mutex;
template<> singleton<tester>::handle     singleton<tester>::inst = 0;

int main(void)
{
   singleton<tester>::instance().test();

   return(0);
}

thread.o: In function 
`singleton<tester>::instance()':/home/greened/projects/apsim/current/apsim/msim/test/thread.cc:21: 
undefined reference to `singleton<tester>::mutex'
collect2: ld returned 1 exit status

I was getting undefined references to "inst" before I added the
initializer.  Is this a new adherance to the standard by g++ 4.0.2?
I didn't used to have to add the initializer to get it to link.

This is troublesome with boost::recursive_mutex because it is
noncopyable.  How do I specify an initializer for it?  Is it
just not possible to have a mutex static member?

                             -Dave