Its not OO without inheritance is a widely used idea. Is-a relationship is a much stronger relationship than has-a or use-a. For the management of dependences composition/membership is much better than inheritance.

Use as strong relationship as neccessary but no stronger.

// x.h: sample header
// version 3

#include <iosfwd>
#include "a.h"      // class A

class C;
class E;

class X : public A
{
public:
        X( const C&);
    B   f(int, char*);
    C   f(int, C);
    C&  g(B);
    E   h(E);
    virtual std::ostream& print(std::ostream&) const;
private:
    // opaque pointer to forward-declared class   
    class XImpl *pimpl_;
};

inline std::ostream& operator<<( std::ostream& os, const X& x)
{
    return x.print(os);
}

// file x.cpp

#include "x.h"
#include "b.h"      // class B
#include "c.h"      // class C
#include "d.h"      // class D

struct XImpl
{
    B               b_;
    std::list<C>    clist_;
    D               d_;
};