Question about singleton

[Jack]

If class A is a singleton, class B inherits class A,

class B : public class A

Is class B also a singleton?

Thanks.

Jack

 

[Kai-Uwe Bux]

If class A is a singleton, class B inherits class A,

class B : public class A

Is class B also a singleton?

Well, if class A is a singleton, it will ensure that only one instance is
created. Since creation of a B instance requires creation of an A instance,
you can only create one. In that sense, class B is a singleton.

However, if I recall correctly, there was a little more to a singleton:
something about a uniform point of access. It is not clear at all, whether
and how class B could inherit that part. In order to support that, class A
would need to be designed appropriately.


Best

Kai-Uwe Bux

 

[Guest]

If class A is a singleton, class B inherits class A,

class B : public class A

Is class B also a singleton?

Usually a singleton have a private constructor, which means that you can
not create an A instance when creating the B instance, in other words
you can not inherit from a singleton.

 

[Uday Bidkar]

If class A is a singleton, class B inherits class A,

class B : public class A

Is class B also a singleton?

Thanks.

Jack

It depends on how singleton pattern is implemented in class A. I guess
if the constructor of A is made private, then you wont be able to
initialize even a single object of B and if its protected, you can
implement B as singleton and non-singleton as well as you would be
able to call A's constructo from that of B.

 

[Jack]

It depends on how singleton pattern is implemented in class A. I guess
if the constructor of A is made private, then you wont be able to
initialize even a single object of B and if its protected, you can
implement B as singleton and non-singleton as well as you would be
able to call A's constructo from that of B.

How about this implementation:

class A{

public:
virtual A* getinstance()
{
if(ptr_A == NULL)
ptr_A = new A;
return ptr_A;
}

protected:
A()tr_A(NULL){}

private:
A* ptr_A;
};

class B : public class A
{
public:
A* getinstance()
{
if(ptr_B == NULL)
ptr_B = new B;
return ptr_B;
}

protected:
B(): ptr_B(NULL),A(NULL){}

private:
B* ptr_B;
};

Thanks.

Jack

 

[Uday Bidkar]

Could you please elaborate on what exactly you want to figure out in
post above?

 

[Gary Coulbourne]

You're probably better off if you can refactor the class so that the
singleton nature can be handled with dependency injection, rather than
making it an explicit singleton.

Peace,
Gary