import std.stdio;

//==============================================================================
/*
    This implementation of the Singleton pattern instantiates the object the
    first time the instance method is called. This is the implementation
    described by the Gang of Four.
*/
//==============================================================================
class MySingleton
{
public:
    static MySingleton instance()
    {
        if(_instance is null) _instance = new MySingleton;
        return _instance;
    }

    char[] toString()
    {
        return "Hello MySingleton";
    }

private:
    this() {}

    static MySingleton _instance;
}

//==============================================================================
/*
    This is a templated version of MySingleton.
*/
//==============================================================================
class Singleton(T)
{
public:
    static T instance()
    {
        if(_instance is null) _instance = new T;
        return _instance;
    }

private:
    this() {}

    static T _instance;
}

class TMySingleton : Singleton!(TMySingleton)
{
public:
    char[] toString()
    {
        return "Hello TMySingleton";
    }
}

//==============================================================================
/*
    This implementation of the Singleton pattern instantiates the object
    automatically during program startup, when D runs static class and module
    constructors.
*/
//==============================================================================
class MySingleton2
{
public:
    static MySingleton2 instance() { return _instance; }

    char[] toString()
    {
        return "Hello MySingleton2";
    }
private:
    this() {}

    static this() { _instance = new MySingleton2; }

    static const MySingleton2 _instance;
}

//==============================================================================
/*
    This is a templated version of MySingleton2.
*/
//==============================================================================
class Singleton2(T)
{
public:
    static T instance() { return _instance; }

private:
    this() {}

    static this() { _instance = new T; }

    static const T _instance;
}

class TMySingleton2 : Singleton!(TMySingleton2)
{
public:
    char[] toString()
    {
        return "Hello TMySingleton2";
    }
}

//==============================================================================
void main()
{
    writefln(MySingleton.instance.toString());
    writefln(TMySingleton.instance.toString());
    writefln(MySingleton2.instance.toString());
    writefln(TMySingleton2.instance.toString());
}