Don't including class header when only declaration exists

Question

I have the following case:

class_a.hpp:

#include "class_b.hpp" // is this "include" mandatory?
class class_a{
private:
    class_b b;
};

class_a.cpp:

#include "class_b.hpp"
//code that uses member variables and functions from class_b

class_b.hpp:

class class_b{};

Is it possible to get rid of #include "class_b.hpp" in the class_a.hpp? Since it is just declaration, why I can not just use forward declaration and not including it? (I tried but it was not compiled)

Of Course I have included class_b.hpp in the class_a.cpp.

score 9 · Accepted Answer · answered Aug 18 '16 at 07:46

9

Since class_b needs to be stored inside class_a without any indirection (e.g. a pointer), the size of class_b needs to be known during the declaration of class_a. In order to know the size, the declaration of class_b needs to be available: the #include directive is therefore required.

answered Aug 18 '16 at 07:46

Vittorio Romeo

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Silly me.. this should be obvious. many thanks – Humam Helfawi Aug 18 '16 at 07:47
@HumamHelfawi If this is the answer to your question, you should accept it. – Peter K Aug 18 '16 at 07:54
@PeterK Yes sure :) it needs time before it is allowed to accept it – Humam Helfawi Aug 18 '16 at 08:00

songyuanyao · Answer 2 · 2016-08-18T08:00:54.860

why I can not just use forward declaration and not including it?

With only forward declaration the class type class_b will be incomplete type. But to be declared as a non-static class data member, need it to be complete, the size and layout of it must be know.

Any of the following contexts requires class T to be complete:
...
declaration of a non-static class data member of type `T`;
...
(In general, when the size and layout of T must be known.)

Don't including class header when only declaration exists

2 Answers2