[C++] Getting one object to access another object's data members

[butterfli]

Homework Statement

see #3

Homework Equations

N/A

The Attempt at a Solution

I'm working on this assignment and I got stuck at this part in programming here's what I need to do:

I need to find a way to have one object access the data members of another object.

A broken down/simplified version of my code looks like this:

class ClassOne 
{
public:
	ClassOne();	
	int m_SomeData;
	string getData();
};

ClassOne::ClassOne()  //setting data to 1
{
	m_SomeData = 1; 
}

string ClassOne::getData() 
{
	return m_SomeData;
}

What I want to do is:

class ClassTwo 
{
public:
	void showData();
};void ClassTwo::showData() 
{
	//pseudocode: cout << m_SomeData from ClassOne; 
	//how do I get this line to work? 
}

I know I have to pass in parameters to showData() and possibly dereference but I don't know what to pass in.

 

[Mark44]

ClassOne is public, and so are its data members. If you create a ClassOne object, any other class has full access to the data and methods on ClassOne. To do what you want to do, I would write showData so that it has a parameter that is a reference to a ClassOne object.

In practice, though what you are trying to do is very seldom done, and is to be avoided. The data members of a class are usually declared using the private or protected access specifier, not public. Making the data private promotes encapsulation via data hiding.

 

[butterfli]

write showData so that it has a parameter that is a reference to a ClassOne object.

Can you show me how that code looks? I know I sound like I'm asking to be spoon-fed but the textbook I have does not have those examples.

 

[Mark44]

void classTwo::showData(ClassOne & classOneobj)
{
   // code for method
}

Inside this method you can call getData or evaluate m_someData directly.

 

[butterfli]

void classTwo::showData(ClassOne & classOneobj)
{
   cout << m_SomeData;
}

My compiler wants me to declare m_SomeData in classTwo because it's giving me an undeclared identifier error. I thought passing in parameters (ClassOne & classOneobj) was suppose to eliminate the need to do that...?

 

[Mark44]

m_SomeData is out of scope, so you can't use it that way. The idea of passing in a parameter to a function is to actually use the parameter, which you haven't done.

If you have an instance of a class, how do you use a method or property on that object?

 

[butterfli]

Oh, duh.

void ClassTwo::showData(ClassOne & classOneobj)
{
   classOneobj.getData();
}

Thank you.

One last question:

int main() {
	ClassTwo dummy;
	dummy.showData();
	return 0;
}

dummy.showData(); wants parameters passed into it. What would they be?

Parameters with data types like strings and ints, I know to pass in strings and ints

void someFunction (string someString)
someObject.someFunction("this is a string");

But what would the parameters for a reference be?

Edit: I tried doing: dummy.showData(ClassOne & dummy); but coming back with an "illegal use of expression" error.

Edit 2: Okay, I got it. It should be

int main() {
	ClassTwo dummy;
	ClassOne dummy1;
	dummy.showData(dummy1);
	return 0;
}

Thank you for everything Mark.