Doubt Class C++

Question

I would like to understand why some classes in C++ have the following statement:

class Bla;

class Ble
{
 ....
}

My doubt is about the Bla class, it has no meaning as far as I could see.

Answer 1

This is called forward declaration. It is necessary to allow cyclic references between classes. So it is possible to make the class Ble have a pointer to a class object Bla and then do the class Bla have a pointer to a class object Ble:

class Bla;

class Ble
{
    Bla *bla;
};

class Bla
{
    Ble *ble;
};

The first line serves to declare that there will be a class called Bla, but without yet defining its content. This makes it possible to declare pointers and references to objects in this class. At a later time it is necessary to define the content of the class.

In addition to solving the problem of cyclic references, this also avoids the need to give #include to obtain the definition of classes when only pointers or references are to be created. In this case, for example, instead of giving #include "bla.h" it is possible to just do forward declaration class Bla. Doing this can make the build process a little faster.