Thread: How come this simple 10-line code can work ?! (very interesting)

FileA.h

Code:

#include <FileB.h>
#include <FileC.h>
class A{
typedef B<C> Engine;
std::auto_ptr<Engine> engine;
...
};

==================

FileB.h

Code:

class A; //forward declaration
template <typename handle>
class B{
B(A& a){...}
};

=================

FileC.h

Code:

#include<FileA.h>
class C{
C(A& a) {...}
};

This code can pass compiling. But FileA includes FileC while FileC also includes FileA. How come that does NOT cause a problem?!