ME>////////////////////////////////////////
ME>// интерфейсы

ME>struct Alice {};
ME>struct Bob {};
ME>struct Carol {};

ME>struct Trent
ME>{
ME>    virtual Alice& GetAlice() = 0;
ME>    virtual Bob& GetBob() = 0;
ME>    virtual Carol& GetCarol() = 0;
ME>};

ME>////////////////////////////////////////
ME>// реализация

ME>class AliceImpl : public Alice {};
ME>class BobImpl : public Bob {};
ME>class CarolImpl : public Carol {};

ME>class TrentImpl : public Trent
ME>{
ME>public:
ME>    Alice& GetAlice() { return a_; }
ME>    Bob& GetBob() { return b_; }
ME>    Carol& GetCarol() { return c_; }

ME>private:
ME>    AliceImpl a_;
ME>    BobImpl b_;
ME>    CarolImpl c_;
ME>};

ME>

ME>So you couldn't say a Car IS-IMPLEMENTED-IN-TERMS-OF Wheels. A Car HAS Wheels.

ДН>Bill Venners: Please differentiate HAS-A and IS- IMPLEMENTED-IN-TERMS-OF.

ДН>Scott Meyers: When you write software, you deal with two worlds. You deal with the world you want to model, the outside world. You also deal with a world that exists only inside the software, which involves just getting the code to work. HAS-A corresponds to something in the real world. A Car HAS Wheels or a Person HAS Friends. HAS-A corresponds to the application domain. IS-IMPLEMENTED-IN-TERMS-OF never exists in the real world; it is part of the implementation domain. So you couldn't say a Car IS-IMPLEMENTED-IN-TERMS-OF Wheels. A Car HAS Wheels. But you could say the ParkingLot IS-IMPLEMENTED-IN-TERMS-OF a List. There's no List in the real world. The List only exists inside the software. So HAS-A is a relationship between classes that exists in the application domain. IS-IMPLEMENTED-IN-TERMS-OF is a relationship between classes that exists in the implementation domain.

ДН>Bill Venners: That's great. That clarifies it for me.

ДН>Scott Meyers: It took me a few years to figure that out myself, actually.