static class members

[space_otter]

This is probably a noob question, but...

When I create a static class member of an object type, for example:

class A {
static const A fixed = A();
// should really be immutable, but const not necessary
};

class A {
static const A fixed = A();
// should really be immutable, but const not necessary
};

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It produces an error "invalid in-class initialization of static data member of non-integral type" so i leave it as:

class A {
static A mFoo;
// should really be immutable, but const not necessary
};

class A {
static A mFoo;
// should really be immutable, but const not necessary
};

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hoping that A will be defined, but apparently it's not. If I leave it like this and try to define it in a function it complains "undefined reference to `A::mFoo'". Examples on the subject use integral data types so it works. I want it to be a fixed object. So how am I supposed to define it?? If I use a pointer and allocate memory the situation doesn't change.

[Lykurg]

If I use a pointer and allocate memory the situation doesn't change.

It will change, since that is possible (because the compilers does not have to know the size of A). A common case for that is the singleton pattern.
Short and "bad":

class Singleton
{
   public:
       static Singleton* getInstance( );
       ~Singleton( );
   private:
       Singleton( ) {instance = new Singleton();}
       static Singleton* instance;
};

class Singleton
{
   public:
       static Singleton* getInstance( );
       ~Singleton( );
   private:
       Singleton( ) {instance = new Singleton();}
       static Singleton* instance;
};

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[lyuts]

Maybe this will help?
In your cpp, outside any function

A* A::mFoo=NULL;

A* A::mFoo=NULL;

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in header

class A;
 
class A
{
    ///...
    static A* mFoo;
    ///...
};

class A;

class A
{
    ///...
    static A* mFoo;
    ///...
};

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[space_otter]

Ok! I got this.

class A {
public:
    static const A* Foo()
    {
	if(mFoo == 0)
	{
	    mFoo = new A();
	    mFoo->clear();
	}
 
	return mFoo;
    }
private:
    static A*mFoo;
};
 
// in cpp
A *A::mFoo= 0;

class A {
public:
    static const A* Foo()
    {
	if(mFoo == 0)
	{
	    mFoo = new A();
	    mFoo->clear();
	}

	return mFoo;
    }
private:
    static A*mFoo;
};

// in cpp
A *A::mFoo= 0;

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works.

Attempting to use

A *A::mFoo = new A();

A *A::mFoo = new A();

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to simplify the code caused the most random errors I've seen.