How to initialize an object with another object of different class?

Applications Programming
1

How to initialize an object of class say "A", with an object of type say "B".
The following code give the error message "error: conversion from �A� to non-scalar type �B� requested"

#include <iostream>
using namespace std;

class B;
class A{
 public:
  A() { cout <<"\nA()" << endl; }

  A(A& rhs) { cout <<"\nCopyCtr:A&" << endl; }

  A& operator=(A& rhs){ cout <<"\nAssignment: A&" << endl; }

  operator B(A &rhs){
    B smplb;
    return smplb;
  }

};

class B{
 public:
  B() { cout <<"\nB()" << endl; }

  B(B& rhs) { cout <<"\nCopyCtrB&" << endl; }

  B& operator=(B& rhs) { cout <<"\nAssignment: B&" << endl; }
};

int main(){
  A a;
  B b = a;
  return 0;
}
2

Your 'operator B' doesn't quite work, so it can't convert.

Making an operator there is tricky, because A doesn't know what the contents of B are yet... So you just have to say that the function exists inside the class, then put the function definition after B is defined.

#include <iostream>
using namespace std;

class B;
class A{
 public:
  A() { cout <<"\nA()" << endl; }

  A(A& rhs) { cout <<"\nCopyCtr:A&" << endl; }

  A& operator=(A& rhs){ cout <<"\nAssignment: A&" << endl; }

  operator B();
};

class B{
 public:
  B() { cout <<"\nB()" << endl; }

  B(const B& rhs) { cout <<"\nCopyCtrB&" << endl; }

  B& operator=(B& rhs) { cout <<"\nAssignment: B&" << endl; }
};

A::operator B() {
        B smplb;
        return(smplb);
}

int main(){
  A a;
  B b = a;
  return 0;
}
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