Cannot pass this because wrong conversion of class type

I have defined the two following classes in two different files:

#include "B.h"
class A {
 public:
  A() {}
  ~A() {}
  f() {
   auto b = new B(this);
  }
};

And in another file:

#include "A.h"
class B {
 public:
  B(A* a) {}
  ~B() {}
}

But I don't understand the compile error I get:

B.h: error: ‘A’ has not been declared
A.cpp: error: no matching function for call to ‘B(A&)‘
                                                      *this);
              note: candidate is:
              note: B(int*)
              note: no known conversion for argument 1 from ‘A’ to ‘int*’

Why is my A class has been converted to int ?!