Enginuity part 2 singleton
Author
Here''s the page I''m referring to:
http://www.gamedev.net/reference/programming/features/enginuity2/page5.asp (about half-way down).
I understand the concept of singletons and I can model my own but I don''t understand one thing about this one. Why bother creating an offset variable and do all that whack casting if the result is always 0. Why not just ms_singleton = (T*)this;?
It''s to do with multiple inheritance.
Consider class Y derived from singleton and X: it could be coded either:
class Y: public X, public Singleton
or
class Y: public Singleton, public X
This would result in the memory being laid out as:
XXX S S YYYY
or
S S XXX YYY
So when you return the pointer to the singleton object in the second case you will return the right address, but in the first case, you will be returning a pointer to half way through the object.
Consider class Y derived from singleton and X: it could be coded either:
class Y: public X, public Singleton
or
class Y: public Singleton, public X
This would result in the memory being laid out as:
XXX S S YYYY
or
S S XXX YYY
So when you return the pointer to the singleton object in the second case you will return the right address, but in the first case, you will be returning a pointer to half way through the object.
Isn't it just possible to instead to make a class a singleton by not inheritance, but ownership? Imagine this...
Alright, begin with the beating and tell me why the above code won't work, heh...
[edited by - biovenger on July 8, 2003 3:13:27 PM]
// singleton.htemplate <typename T> class Singleton{private: static T *self;public: Singleton() { if(!self) self = new T; } GetInterface() { return self; }};template <typename T>bool Singleton<T>::self = NULL;// Other source file with wanted singleton classclass foobar{// Important, singleton class must be able to instantiatefriend class Singleton<foobar>;private: // Variables and whatever you want //... // Important part, the singleton Singleton<foobar> self; // Also important, private ctors for impossible instantation foobar() { /*stuff...*/ } foobar(const foobar &cpy) { /*stuff...*/ } foobar & operator=(const foobar &cpy) { /*stuff...*/ }public: // The method that does it all static foobar * GetInterface() { return self.GetInterface(); } // More methods and variables};// Random cpp filefoobar *foo = foobar::GetInterface(); // Should be a singleton?Alright, begin with the beating and tell me why the above code won't work, heh...
[edited by - biovenger on July 8, 2003 3:13:27 PM]
Author
Out of curiosity, I've toyed with this code a little bit more and I've found out that by simply doing the following in the Singleton constructor suffices:
The address which the above produces is the same as with the code that manually does all the calculations, even if multiple inheritance is involved with the following case:
Or am I wrong because I'm failing to see something?
[edited by - fallenang3l on July 8, 2003 8:24:34 PM]
ms_singleton = static_cast<T*>(this); The address which the above produces is the same as with the code that manually does all the calculations, even if multiple inheritance is involved with the following case:
class Y: public X, public Singleton<Y> Or am I wrong because I'm failing to see something?
[edited by - fallenang3l on July 8, 2003 8:24:34 PM]
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