Re: Mimicking Javas static class initializer in C++

From:

blargg.h4g@gishpuppy.com (blargg)

Newsgroups:

comp.lang.c++

Date:

Wed, 29 Oct 2008 04:12:15 -0500

Message-ID:

<blargg.h4g-2910080412160001@192.168.1.4>

In article <49062fa0$0$32668$9b4e6d93@newsspool2.arcor-online.net>,
Andreas Wollschlaeger <meister.possel@arcor.de> wrote:

Hi folks,

as the subject says, i'm a poor Java programmer trying to transfer some
of his wisdom into C++ world... here is what im trying to do this evening:

Java has a nifty feature called a static class initializer - something
like this:

public class Foo
{
   private static Vector<Thing> xx;
   static
   {
     xx = new Vector<Thing>(42);
     for (int i=0; i < xx.size(); ++i)
     xx.set(i, new Thing());
   }
}

where the static{} block is called once (when the class is first used),
thus suitable for complex initializations of static members.

Now i would like to have this in C++, can this be done?

Trying to make as literal a translation here. This will initialize things
the first time a Foo object is created; if no Foo is ever created, this
initialization will never run.

    // Foo.hpp
    class Foo {
    public:
        Foo();

    private:
        static vector<Thing> xx;
    }

    // Foo.cpp
    vector<Thing> Foo::xx;

    Foo::Foo()
    {
        if ( xx.empty() )
        {
            xx = new Vector<Thing>(42);
            for (int i=0; i < xx.size(); ++i)
                xx [o] = new Thing();
        }
    }

Here is a more general approach that separates the init code into its own
function:

    // Foo.hpp
    class Foo {
    public:
        Foo();

    private:
        static void first_use();
    }

    // Foo.cpp
    void Foo::first_use()
    {
        // initialize statics
    }

    Foo::Foo()
    {
        static int call_once = (first_use(),0);
    }

You could use a less-hacky approach to having first_use() called only
once, like a helper class, but this is a more concise approach.