Logic error

[wolfv]

I came across this problem debugging a larger program and distilled it into the following little example.

I was expecting b.print() and a.print() to access the same b.state, and print the same "1", but the output says otherwise.

#include <iostream>
using namespace std;

class B
{
	private:
		int state;
	public:
		void update(int newState) { state = newState; };
		void print() { cout << state << endl; };
};

class A
{
	private:
		B b;
	public:
		A(B& b): b (b) {};
		void print() { b.print(); };
};

B b;
A a(b);

int main()
{
	b.update(1);
			// output:
	b.print();	// 1
	a.print();	// 0	why do a.print() and b.print() not print the same state 1?
	b.print();	// 1
}

output:

1
0
1

Why did a.print() and b.print() not print the same state 1?

Thank you.

[xismn]

I'm surprised that your compiler automatically initializes undefined variables to zero.

Why should they print the same state?
You instanciate an A object, and pass it b, whose state has not been explicitly initialized. a's member b gets set to global b, then you give global b a state.

My guess is that you thought that passing b by reference would somehow establish a relationship between the two objects(?).

*EDIT* this is nit-picky, but the semi-colons after your function bodies are not necessary.

[wolfv]

Thanks xismn. You read my mind.
The following example works as intended.

#include <iostream>
using namespace std;

class B
{
	private:
		int state = 0;
	public:
		void update(int newState) { state = newState; }
		void print() { cout << state << endl; }
};

class A
{
	private:
		B& b;
	public:
		A(B& b): b (b) {};
		void print() { b.print(); }
};

B b;
B &bRef = b;
A a(bRef);

int main()
{			// output:
	b.update(1);
	b.print();	// 1
	a.print();	// 1

	b.update(2);
	b.print();	// 2
	a.print();	// 2
}