How to correctly include classes in c++?

Question

I would like some clarification about including classes in c++. I don't know the good way to do it because i don't know the differences between theme

#pragma once
#include "B.h" // Is it enough to include like this? ? 

class B; // what this line does ? 
class A : public B // what public B does here ?
{
    // .......
};

So how do i know if i should use class b or public B or juste a #include "B.h" ? I hope i was clear enough and thanks for the help

class B; line forward declares it --- just saying there there is some class B. — Ilan Keshet, Dec 30 '21 at 20:48
Code you have would make a linker error, if class B was not actually ever declared. — Ilan Keshet, Dec 30 '21 at 20:48
Look at this link: https://stackoverflow.com/questions/4757565/what-are-forward-declarations-in-c — Ilan Keshet, Dec 30 '21 at 20:49
In short. Forward declaration lets you use a class, before it's been defined. -- Without forward declaration, you would have to put full implementation of class B before class A — Ilan Keshet, Dec 30 '21 at 20:50
... but you can't use forward declared classes as base classes. — πάντα ῥεῖ, Dec 30 '21 at 20:51
After including B.h I'd expect the forward declaration of `B` to be unnecessary. If it is necessary, you probably have a bug or B.h is very poorly named. — user4581301, Dec 30 '21 at 21:02

score 1 · Accepted Answer · answered Dec 30 '21 at 20:51

#include "B.h" // Is it enough to include like this? ?

Yes.

class B; // what this line does ?

This is an alternative to including B's header. Doing both is redundant.

It gives you less freedom. It allows you to create pointers and references to B, but not create instances of it, nor inherit from it, nor access its members.

On the other hand, class B; compiles faster, and can be used to break circular include dependencies.

class A : public B // what public B does here ?

Class A inherits from class B, publicly. Read about inheritance in your favorite C++ book.

Covers the specific question better than the proposed dupe/related. — πάντα ῥεῖ, Dec 30 '21 at 20:54

How to correctly include classes in c++?

1 Answers1