How to make a class prototype?

Question

OOP

Hello. There are 2 classes.
The first class contains a vector that stores references to objects of the second class type.
And the second class is inherited from the first.
How to implement the definitions of these classes? Because always a class that is defined earlier swears that it does not know the second and the program fails to compile. How to be?

Answer 1

class Class2;

class Class1 {
std::vector<Class2*> v;
}

class Class2 : public Class1{
//...
}

Answer 2

https://en.wikipedia.org/wiki/%D0%9F%D1%80%D0%B5%D...
en.stackoverflow.com/questions/41844/%D0%97%D0%B0%.. .

Answer 3

Write like this for one of the classes.
And then the full definition.

Answer 4

If you didn’t know how to do it, how could you indicate the “class prototype” in the question?