From: Lyle Johnson <lyle@...>
Date: 2002-06-11T22:59:50+09:00
Subject: Re: SWIG, C++ and static member variables

Phil Tomson wrote:
> OK, my terminology may not be correct in C++ terms (I've been doing a lot 
> of Ruby and not a lot of C++ until the last few weeks), but, say I've got 
> a C++ class with a class variable (statically scoped), like:
> 
> //Person.h
> class Person {
> 	static unsigned numPeople;
> 	const char* name;
> 	unsigned   age;
> 	
> public:
>         Person(const char* nm, unsigned age=0);
>         Person(const char* nm, int age=0);
>         
> 	static unsigned numberOfPeople() { return numPeople; }
>         void identify() const;
> 
> };
> 
> unsigned Person::numPeople = 0;
> //end of Person.h
> 
> I did: 
>  swig -c++ -ruby Person.i
>  g++ -c Person.cpp
>  g++ -c Person.cc
>  g++ -c Person_wrap.cxx -I /usr/lib/ruby/1.6/i686-linux-gnu
>  <all ok, till here:>
>  g++ -shared Person.o Person_wrap.o -o Person.so
> 
> Trying to compile that shared lib gave me:
> Person_wrap.o(.data+0x20): multiple definition of `Person::numPeople'
> Person.o(.data+0x0): first defined here
> collect2: ld returned 1 exit status
> 
> So SWIG interpretted numPeople as a static member function instead of as a 
> static variable (class variable).

Ummm, no. The problem is that (as the error message states) you have 
multiple definitions of Person::numPeople, because it's declared in the 
Person.h header file and that file gets included in more than one C++ 
source file.

You should remove the definition of Person::numPeople, i.e. the line:

     unsigned Person::numPeople = 0;

out into the implementation file (I guess Person.cpp?) and this should 
fix things.

I'm also a little concerned that your trace of commands shows you 
compiling both a "Person.cpp" and a "Person.cc" (in addition to the 
SWIG-generated "Person_wrap.cxx"). Compiling Person.cpp would result in 
a Person.o object file, and following that with a compile of Person.cc 
would do the same, overwriting the previous Person.o file.