Difference between include directive and forward declaration in C++

Question

I must refactor an old code. One of its problems is that it exceeds on useless 'includes'. In the same project, I've seen the following syntax:

#include <AnyClass> //A system header
#include "AnotherAnyClass" //A application header

class AnotherClass;

class Class : public OneMoreClass
{

public:
    explicit Class();
    ~Class();

private:
    AnotherClass *m_anotherClass;

}

I'd like to figure out:

1. What are the differences between a 'include "Class"' and a 'class Class'?
2. When should the second method be used and how?

Answer 1

Declaring a class like this:

class AnotherClass;

is called a Forward Declaration. It lets the compiler know about the existence of the class, in situations where knowning the details about the class aren't needed, like in the scenario you posted.

Generally if you aren't going to be using an instance of the class, you're just dealing with a pointer and not calling any methods on it, you don't need to know anything more than the class name. This can speed up compilation in large projects.