class_forward_decl.h

class Foo;

Foo* newFooSingleton();
Foo* global_foo_ptr_;
const Foo* global_const_foo_ptr_ = newFooSingleton();
const Foo& global_const_foo_ref_ = *global_const_foo_ptr_;

class Bar;
// Class definition here
class Foo {
    public:
        Foo();
        ~Foo();

        Foo& aPubMethod();
    private:
        Bar m_bar; // Bar is declared but not defined
};