CODEWITHSUNDEEP
c++oopclassfunctionfunction-call
user1414276
user1414276

Reputation: 87

How to Call Function of one class on the object of another class?

How can I call one method in one class over using another class ?

I have ;

class A { 
 public :
  foo ( ) ;
};

class B {
 public :
  bar ( ) ;
};

in main :

A data ;          // I am creating instance of class A 

data . bar ( ) ;  //  but, I am calling a method of another class

                  // how can I do that ?

Note : I could not find a appropriate title. If you have one, feel free to share or edit

Upvotes: 4

Views: 22001

Answers (9)

Aimar Hassano
Aimar Hassano

Reputation: 49

Although this question is strange !, but here are some solutions Using inheritance

class A: public B

Type cast

A data;
((B*)&data)->bar();

Or reinterpret cast

B* b = reinterpret_cast <B*> (&data);
b->bar();

If bar() use any member variables of B, then the result is not predictable.

Upvotes: -1

JustThatTypeOfGuy
JustThatTypeOfGuy

Reputation: 21

You can also declare class B as a friend of class A.

I believe the syntax for it is:

class A {
    public:
        foo();
    friend class B;
};
class B {
    public:
        bar();
};

But with this, I believe you can only use functions/variables from A inside B functions. Inheritance will probably be your better approach to it.

Upvotes: 0

aj.toulan
aj.toulan

Reputation: 1431

You can use a function pointer. The only way to make it not static is to use templates.

class A
{ 
  public:
  void setBar(void (*B::func)(void)) { bar = func; };
  void callBar() { bar(); };

  private:
  void(*B::bar)(void);
};

class B
{
  public:
  static void bar() { printf("you called bar!"); };
};

int main() {
  A a;
  a.setBar(B::bar);
  a.callBar();
}

Upvotes: 0

vikramjitSingh
vikramjitSingh

Reputation: 59

I think you should read 1 or 2 c plus plus book(s) and get a fair idea what classes are about and what purpose they are meant to serve.

Some suggestions: The c++ programing by Bjarne Stroustrup or Thinking in c++ by Bruce Eckel or search over net for tutorials.

Upvotes: 0

Horonchik
Horonchik

Reputation: 477

As everyone said in their answers. Its a bad idea and not possible.

You can only use tricks that no one really knows how its gonna behave.

You can get the pointer of an object A and cast it to be poiter of B.

Again the only use of that is to show other what not to do.

A a;
B* b = (B*)&a;
b->bar();

Upvotes: 0

Chip
Chip

Reputation: 3316

It is not clear what you want data.bar() to do.

bar() as no access to A's data, so bar() cannot have anything to do with the variable data. So, I would argue, that data.bar() is unnecessary, you are aiming for just bar(). Presumably, if bar() is just a function, you can declare it static and call B.data()

The other option is that you wanted inheritance which some other people have already written about. Be careful with inheritance, and make sure you inherit A from B only if you there is a is-a relationship, and it satisfies the Liskov Principle. Don't inherit from B just because you have to call bar().

If you want to use B, you can have a instance of B inside A. Read about prefering composition over inheritance

Upvotes: 0

starius
starius

Reputation: 323

Use public inheritance:

class B {
 public:
  void bar();
};

class A : public B
{ };

int main() {
 A a;
 a.bar();
}

Upvotes: 1

Alok Save
Alok Save

Reputation: 206566

Unless the two classes are related(through inheritance) You cannot do that.

A member functions performs some action on the instance of the class to which it belongs.
You created an object of class A so you can only call member functions of A through it.

Grinding an Apple and hoping to get a mango shake, won't really happen right.

Upvotes: 1

Mateusz Rogulski
Mateusz Rogulski

Reputation: 7455

I think if you want use .bar() on A object A must inherit by B.

Upvotes: 0

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