foo.h:

struct Bar;

struct Foo { Bar *bar; }

bar.h:
struct Bar
{
  void baz();
};

bar.cpp:
#include "bar.h"

void Bar::baz() {}

main.cpp:
#include "foo.h"
#include "bar.h"

int main()
{
  Foo foo;
  foo.bar->baz();
  return 0;
}