Template type parameters as friends in gcc

From:
Dave Rudolf <dave.rudolf@usask.ca>
Newsgroups:
comp.lang.c++
Date:
Thu, 22 Feb 2007 17:33:36 -0600
Message-ID:
<12ts9tbnmi49r7d@corp.supernews.com>
Hey all,

So I have a template class whose only template parameter is a type that the
class is to manage. It's actually an implementation of the common Singleton
design pattern. The class looks like this:

    template< class Type >
    class Singleton
    {
    public:
            static Type& instance();
    private:
            static Type* _instance;
    };

The instance() method looks like this:

    Type& Singleton::instance()
    {
        if( _instance == NULL )
        {
            _instance = new Type();
        }
        return *_instance;
    }

So, if I wanted to make a singleton class, it would just have to inherit from
this template class like so:

    class Tableau : public Singleton< Tableau >
    {
    private:
        // Make the constructor private so other parts of the
        // code can't create a new instance
        Tableau();

        // Make the Singleton class a friend so that it can
        // create the static instance of this guy.
        friend Singleton< Tableau >;
    };

This all works fine with MS Visual C++ .Net. This also used to work with gcc.
However, I tried to compile this with gcc 3.4.4 for cygwin, and no go. I get
the following error.

tableau.h:35: error: a class-key must be used when declaring a friend

A quick google for that error message and I see that newer versions of gcc do
not allow these so-called "ill-formed type" to be friends. However, I can't
find a fix or a work-around for this.

So, can anyone suggest a way to make make my singleton template more
platform-independent?

Thanks.

Dave

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