[Solved] How to create a class that only can be instantiated from another?

[inejose]

Hi,

I would like to create a class ( ClassA ) with the following property:

ClassA only can be instantiated from ClassB.

For example, if another class called ClassC tries to instantiate ClassA, compiler must launch an error:

@ClassC::someMethod {

ClassA *objectA = new ClassA();

}@

ERROR: ClassC can't instantiate ClassA.

It is possible?

I was thinking about private constructor and friend class (ClassA and ClassB in my example), but constructor can't be private by definition.

Thank you in advance.

[Edit: Moved to C++ Gurus forum, as this is not Qt-related; mlong]

[Zlatomir]

Have you tried the code for what you where thinking?
I don't know about anything that stops you to code a class with private or protected constructor.

And if you give more details about what are you trying to accomplish, maybe some of us come with more appropriate solutions.

[task_struct]

Hi,

If you put definition of class A inside private section of class B, it will be visible only inside calss B.

[Tannin]

I agree with Zlatomir: the constructor CAN be private and there are many reasonable use-cases (i.e. singletons)
Just remember to also make the copy constructor private because otherwise it will be auto-generated as public, so class C could then make a copy of an A that was created in B.

[inejose]

Thanks to Zlatomir, task_struct and Tannin, it works!

I read in a C++ manual that constructors cannot be private, so I didn't try.

Finally, I used private constructors and friends class to achieve my first objective as follow:

Class A definition:

@class ClassA {

public:
friend class ClassB;

private:
ClassA();

//(edited) Copy constructor added
ClassA(const ClassA &classAsource);

};@

Class B definition:

@class ClassB
{

public:
ClassB();

};@

Now, only ClassB can create objects from ClassA class, because only ClassB can access to constructor.

In this way I ensure that nobody will use ClassA on my code except ClassB.

Thank you!

[mlong]

Don't forget what Tannin wrote:

bq. Just remember to also make the copy constructor private because otherwise it will be auto-generated as public, so class C could then make a copy of an A that was created in B.

[inejose]

[quote author="mlong" date="1338822657"]Don't forget what Tannin wrote:

bq. Just remember to also make the copy constructor private because otherwise it will be auto-generated as public, so class C could then make a copy of an A that was created in B.[/quote]

Thanks mlong, I will keep it in mind.

(I have edited my previous code example.)