Accessing variables between two classes in MFC

You have two possibilities: use an instance, or use a static variable.

When you create a new instance of a class, you can access all the public variables the class contains as long as you have the pointer to that instance: it's like a car in that all cars have a glove box, but each car has a separate glove box. If you put your mobile in the glove box of your car it does not appear in the glove box of my car! To access the glove box content, you need to access the car first - you need a specific instance of the Car class in order to get at your phone.

Static variables are different: there is only one of them and it is shared by all instances of the class. This is similar to the "number of wheels" property of a car - all cars share the same number of wheels (four) because if they had two they would be a motorbike.

Start by looking at your code, and at your classes: if the data may be different between two instances of your class, you need to use the right instance to access it - if it's always the same for all instances, you could use static