Difference between "Class X" and "Include <X.h>"

[maksins]

I see in many programs if I have to include a class X, I can do it either by declaring "Class X" or "#include <X.h>" in the beginning of my program.
What is the difference between two and which is the preferred method?

[roberts]

Declaring "Class X" doesn't actually include it if the corresponding header is not included.

Declaring "class X" is a forward declaration. It's often used when you need to need to resolve a reference before it can be defined. However, that doesn't mean it is defined. You still need to include the header file.

[vlad from moscow]

It depends of a project design. If Class X is used in several modules it is better to place its definition in a header file and then include it in each module that uses the class. If only one module uses the class then you can define the class in that module.

[Peter87]

class X; This is a forward declaration. It just says there is a class named X but there is no information on what it contains or how big, so you can't actually create any objects of X or call X's member functions or anything. The only thing you can do is to declare pointers and references to X.

#include <X.h> This includes X.h header file. You would get the same result by just copy paste the content of X.h instead of writing #include <X.h> , but don't do that. X.h probably contains the definition of class X so that will allow you to use class X as usual.

Prefer forward declaration whenever possible. Advantages are faster compilation times and less risk to run into problems with circular dependencies.

[Need4Sleep]

As they have all stated, you'd want to use the line class X; if your current class uses an object of X of a specific part of the class during compile time, so you would want it to make sure it knows what Class X is before it starts looking at the current class.