How to set the right method

[Zunneh]

Hello guys , i have a question and this is an exemple to understand my question

class A {
 A();
 ~A();
 virtual void test();

;}

class B :public A{
 B();
 ~B();
 virtual void test();
 ;}

 class C:public A {
 C();
 ~C();
 virtual void test();

 ;}

as you see , i m using Heritage her , for exemple

B.test() 

will call the method in class B ,
Now if i write Alone

           test(); // 

Which method test() he will call ? the A,B or C?

[mrjj]

Hi
since its a member function, you need a class instance to call it.
so
test(); //
wont work.

[JonB]

@Zunneh
As @mrjj has said, you will need to create instance, e.g. one of new A(), new B(), new C(), before you can call test() on that instance. And it will call the test() of whichever class you created the instance of, e.g.

B *b = new B();
b->test();
// or
B b;
b.test();

will call B's test().

[Zunneh]

@JonB an if i delete the object , for exemple

B *b=new B();
delete b;
test(); // this will work ?

[mrjj]

@Zunneh
hi
Nope. There is no way to call a normal member function without the object.
Why do you want to do this ?
you could just have a normal non class function then.

[Pl45m4]

@mrjj

This works, if he calls test() inside the B class (e.g. constructor).

@Zunneh
Where do you want to call the test()-function?

Back to your question: B inherits from A, C inherits from A. Because "test" is a virtual function in A, it will call the test()-function in B, but of course your function call wont work :)

[jsulm]

@Pl45m4 said in How to set the right method:

This works, if he calls test() inside the B class

No, it doesn't. Constructor is a method of a class also. So, if you call a method inside the constructor you're calling it with a class instance already (not necessarily completely constructed one though).
This does not make any sense in C++ (if test() is a method of B or a base class of B), not even for static methods:

B *b=new B();
delete b;
test();

@Zunneh You simply can't call a non-static method without a class instance. You can have a static method, then you don't need an instance:

class B
{
public:
    static void test() {}
};
B::test();

[Pl45m4]

@jsulm

I meant if he calls it in constructor, he doesn't have to "B->test ()".
But yeah, to call the constructor, you'll need an object instance first :)

[KillerSmath]

@Zunneh
To understand what the virtual term, i suggest you to read this IBM article.

Static vs Dynamic Binding
https://www.ibm.com/support/knowledgecenter/en/ssw_ibm_i_72/rzarg/cplr139.htm

Virtual functions are dynamically bound, meaning that the particular
function called is determined by the dynamic type of the object
through which it’s invoked (Mayer S. Effective C++: 55 Specific Ways to Improve Your Programs and Designs. Third edition. p. 181)

A *a = new A();
B *b = new B();

a->test() // call A::test()
b->test() // call B::test()

a = b;

a->test() // call B::test() because A::test() is virtual

delete a;
delete b;

[Pl45m4]

@KillerSmath

Why you mentioned me?! I know what virtual functions are :)
I guess you meant @Zunneh

[KillerSmath]

@Pl45m4
Just a little mistake xD.