Composition QUESTION

[hooi1997]

Hi, may I know if there is a way to B object without calling A object beforehand?

class A{
   private:
       int attackPower;
   public:
      void setPower(){
           cout<<attackPower;
       }
    
   A(int a)
   {
       attackPower=a;
   }
};

class B{
   private:
      A eob;
   public:
      B(A ee)
      :eob(ee)
   {	
   }
   void show()
      {
         eob.setPower();
      }	
};

int main()
{
      //A obj(1); What if I never declare this first?
      B no(obj); //Is there a way to call B object(which has A object as 
                     //private variable)
      no.show();
}

[MikeyBoy]

If you want to be able to create an object of type B without passing in an object of type A, then you need to define a default constructor.

Also, I strongly encourage you to adopt a sensible indentation style. It will make it much easier to see the structure of your code, and will help you spot errors more easily.

[hooi1997]

Thanks for your reply and sorry for not having indentation for the codes(I haf fixed it). May I know how to define a default constructor?Thank you.

[MikeyBoy]

A default constructor is just a constructor that doesn't take any arguments. You define it the same way as you'd define any other constructor.

[hooi1997]

I don't quite understand it.Could you rewrite my code with default constructor? I would really appreciate it! Thanks!

[JLBorges]

#include <iostream>

class A {

   private: int attackPower;

   public:

       explicit A( int ap ) : attackPower(ap) {}

       void show() const {

           std::cout << "A { attackPower == " << attackPower << " }\n" ;
        }

       // ...
};

class B {

   private: A eob;

   public:

      explicit B( A a ) : eob(a) {} // copy construct eob

      explicit B( int ap ) : eob(ap) {} // construct eob with attack power ap

      // ...

      void show_eob() const {

          std::cout << "B::eob: " ;
          eob.show() ;
      }
};

int main() {

    const B bb(23) ;
    bb.show_eob() ;
}

http://coliru.stacked-crooked.com/a/628cae50f62368a9

[MikeyBoy]

I'm not going to write your code for you.

You already know how to write a constructor that takes arguments, because you've already done that in both your classes.

You already know how to write a function that takes no arguments, because you've already done that in both your classes.

What is confusing you about writing a constructor that takes no arguments?