Re: Question about a static vector<*A> in class A

From:

Ondra Holub <ondra.holub@post.cz>

Newsgroups:

comp.lang.c++

Date:

Wed, 25 Jul 2007 04:09:33 -0700

Message-ID:

<1185361773.167543.326080@22g2000hsm.googlegroups.com>

But you cannot test on 'vector_ptr' because it may not be initialized,
just like the code I had originally...

Static data (which is not defined in any function) is automatically
initialized to 0, so you can test it for 0 and initialize it. This is
valid only for static data defined in any class/struct or in global
scope.

So code could look like:
-- cut here --
#include <cstdlib>
#include <iostream>

class A
{
public:
    ~A()
    {
        std::clog << "A::~A()\n";
    }

    // Access method for static data
    static A& GetInstance()
    {
        if (instance == 0)
        {
            instance = new A();

            if (atexit(&A::CleanUp) != 0)
            {
                std::cerr << "Cannot register CleanUp function!\n";
                exit(1);
            }
        }

        return *instance;
    }

    void Method()
    {
        std::cout << "A::Method() called\n";
    }

private:
    A()
    {
        std::clog << "A::A()\n";
    }

    static void CleanUp()
    {
        delete instance;
    }

    static A* instance;
};

A* A::instance;

int main()
{
    std::cout << "Start of main\n";

    A::GetInstance().Method();
    A::GetInstance().Method();

    std::cout << "End of main\n";
}
-- cut here --

And will produce following output:
-- cut here --
Start of main
A::A()
A::Method() called
A::Method() called
End of main
A::~A()
-- cut here --