implicit/explicit? conversion of classes?

this is bothering me for a while...

what i want to achieve is that the same instance of the inherited class will have the same value of the parent class like so...

Code:

class A{
  //ctor
  //dtor
  //cctor
  //ator
   ...
};

class B : public A{
  //ctor
  //dtor
  //cctor
  //ator
  ...
};

class C : public A{
  //ctor
  //dtor
  //cctor
  //ator
  
  B * b;

  func(){
     //OK... i achieve to have (explicitly?) convert C to A 
     A * a = new C(*this);
     b = new B();
     //Error... how would i copy? A to B
     *b = (B)*a;

  }


};

I would like to have class B to have the same values of class A as of those with class C. But i can't copy A to B, it says it could not find a match for B(A). how could i achieve this?

sorry if my terminologies maybe incorrect about explicit or implicit conversion... i'm still having trouble knowing which is which (as well as object slicing).

Thanks.

EDIT: forgot something

typecast B to A instead of A to B?

sorry, but what do you mean?

1)

I would like to have class B to have the same values of class A

Classes do not have values. Objects of a class have members that have values.

Code:

*b = (B)*a;

What the??!! Can you explain what that is supposed to mean?

I would like to have class B to have the same values of class A as of those with class C.

Totally incomprehensible!

ok...

let's add something on the sample code:

Code:

class A{
  //ctor
  //dtor
  //cctor
  //ator
  int myvar; //assume myvar is actually public for simplicity
};

class B : public A{
  //ctor
  //dtor
  //cctor
  //ator
  ...
};

class C : public A{
  //ctor
  //dtor
  //cctor
  //ator
  
  B * b;

  func(){
    //i assigned myvar to 1
    myvar = 1; 
    //now what i want is that class B has the same myvar value as it was on class C
    //which is 1.
     
     //this is my try but it didn't work.
     //OK... i achieve to have (explicitly?) convert C to A 
     A * a = new C(*this);
     b = new B();
     //Error... how would i copy? A to B
     *b = (B)*a;

  }

};

sorry if i caused you trouble.

I would like to have the objects of class B that is derived from class A has the same values as the objects of class C that is derived from class A.

i want objects of class A for both class B and C to have the same values.

about this

Code:

*b = (B) *a;

i tried to copy the value of a to b. Hoping that typecasting a to class B will solve incompatibilty problems... but it didn't work...

thanks!

The answer is really staring you in the face

//cctor

Create a copy constuctor for B that takes an A object(or pointer)

Code:

#include <iostream>
using namespace std;

class A{
  //ctor
  //dtor
  //cctor
  //ator
public:
  int myvar; //assume myvar is actually public for simplicity
};

class B : public A{
  //ctor
  //dtor
  //cctor
public:
	B(){}
	B(A* a):A(*a)
	{}
  //ator
};

class C : public A{
  //ctor
  //dtor
  //cctor
  //ator
  
};

int main () 
{    
	
	A * a = new C();
	a->myvar = 1;

	B* b = new B(a);

	cout << "a->myvar = " << a->myvar << "\n";
	cout << "b->myvar = " << b->myvar << "\n";
}

This way you have more control over the copy process.

Hoping that typecasting a to class B will solve incompatibilty problems

In C++, type casts are written in such a way that it is clear exactly what you are doing:

Code:

double num = 10.5;
int a = static_cast<int>(num);

In addition, you can't cast across a hierarchy like this:

Code:

    base A
     /   \
    /     \
   B------>C

Ok thank you. i'll try adding a copy constructor that takes A object as an argument...