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In contrast to the previous definition of a class such as tt(AirAuto),
situations may arise where the hi(data members: multiply included) double
presence of the members of a base class is appropriate. To illustrate this,
consider the definition of a tt(Truck) from section ref(Truck):
verb(
class Truck: public Auto
{
int d_trailer_weight;
public:
Truck();
Truck(int engine_wt, int sp, char const *nm,
int trailer_wt);
void setWeight(int engine_wt, int trailer_wt);
int weight() const;
};
Truck::Truck(int engine_wt, int sp, char const *nm,
int trailer_wt)
:
Auto(engine_wt, sp, nm)
{
d_trailer_weight = trailer_wt;
}
int Truck::weight() const
{
return // sum of:
Auto::weight() + // engine part plus
trailer_wt; // the trailer
}
)
This definition shows how a tt(Truck) object is constructed to contain two
weight fields: one via its derivation from tt(Auto) and one via its own tt(int
d_trailer_weight) data member. Such a definition is of course valid, but it
could also be rewritten. We could derive a tt(Truck) from an tt(Auto)
em(and) from a tt(Vehicle), thereby explicitly requesting the double presence
of a tt(Vehicle); one for the weight of the engine and cabin, and one for the
weight of the trailer.
A small point of interest here is that a derivation like
verb(
class Truck: public Auto, public Vehicle
)
is not accepted by the bf(C++) compiler: a tt(Vehicle) is already part of
an tt(Auto), and is therefore not needed. An i(intermediate class) solves
the problem: we derive a class tt(TrailerVeh) from tt(Vehicle), and tt(Truck)
from tt(Auto) and from tt(TrailerVeh). All ambiguities concerning the member
functions are then be solved for the class tt(Truck):
verb(
class TrailerVeh: public Vehicle
{
public:
TrailerVeh(int wt);
};
inline TrailerVeh::TrailerVeh(int wt)
:
Vehicle(wt)
{}
class Truck: public Auto, public TrailerVeh
{
public:
Truck();
Truck(int engine_wt, int sp, char const *nm, int trailer_wt);
void setWeight(int engine_wt, int trailer_wt);
int weight() const;
};
inline Truck::Truck(int engine_wt, int sp, char const *nm,
int trailer_wt)
:
Auto(engine_wt, sp, nm),
TrailerVeh(trailer_wt)
{}
inline int Truck::weight() const
{
return // sum of:
Auto::weight() + // engine part plus
TrailerVeh::weight(); // the trailer
}
)
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