Re: Confused about a thread-safe singleton example.

On Dec 3, 6:47 am, Alan Johnson <aw...@yahoo.com> wrote:


GoF


I can not agree more.

If the singleton is not going to be used before or after main(), then
the following is enough:

    // header.h start
    class X { /* ... */ };
    extern X global_x;
    // header.h end

If the singleton should be usable before and after main(), then there
is Schwarz Counter idiom to initialise your global variables, just the
same way as std::cout, std::cin, ... globals get initialised:

    // header.h start
    class X { /* ... */ };
    extern X* global_x;

    struct X_init
    {
        X_init();
        ~X_init();
    }
    // Put a copy of x_init (const === static)
    // in every translation unit that includes header.h
    const x_init;
    // header.h end

    // source.cc start
    // These PODs are zero-initialised before
    // any constructors are invoked.
    static int counter;
    X* global_x;

    X_init::X_init()
    {
        if(!counter++)
            global_x = new X;
    }

    X_init::~X_init()
    {
        if(!--counter)
            delete global_x;
    }
    // source.cc end

--
Max