Inheritance: "abstract objects"?

[J.M.]

I have a question concerning inheritance: can an abstract (parent) class
have an abstract object? I would like to make a concrete child inherit from
this class by inheriting from this object. Let me try to make this clear by
example: The following should be "abstract".

"class motor" is abstract because it has a virtual member
"virtual int calculate_horsepower() = 0"

"class car" should have
"motor M;"
"void print_info(){... M.calculate_horsepower(); ..... };"

The following should be concrete:

"class diesel_motor : class motor" and should implement
"virtual int calculate_horsepower(){ calculate hp for diesel };"

"class diesel_car : class car"

For this class, we should have an object "diesel_motor M;" as a child of
"motor M;". In other words, in diesel_car I need an object M as a
diesel_motor, but in the class car, I only need those attributes of M
associated with a motor.

It seems something of this sort is not possible, so maybe there is a
different solution. Let me indicate what I need:

Every car has a motor, either of diesel or gasoline type. Calculating
horsepower is different for each of these, but I want to implement this
method only once, once for each type of motor. For every car, I want to
print the same standardised specifications, so I want one method to do
that. This method only uses methods in available for all motors. In making
a class diesel_car, I need to have a diesel_motor, which is a child of
motor.

Obviously, I could just not have a motor in car, but I need this to define
print_info() - and I do not want to define this in every child....

I hope it is clear, what I need. Thanks for any help in advance.

Jan

 

[Rolf Magnus]

I have a question concerning inheritance: can an abstract (parent) class
have an abstract object?

No. The whole idea of an abstract class is that it can't be instantiated.

I would like to make a concrete child inherit from this class by
inheriting from this object. Let me try to make this clear by example: The
following should be "abstract".

"class motor" is abstract because it has a virtual member
"virtual int calculate_horsepower() = 0"

It is abstract because it has a _pure_ virtual member. Make it just virtual,
and the class won't be abstract anymore. But I guess what you really want
is not store a motor in your car, but rather a pointer to motor, like the
following simplified example:

#include <iostream>

class motor
{
public:
virtual int calculate_horsepower() = 0;
virtual ~motor() {}
};

class diesel_motor : public motor
{
public:
virtual int calculate_horsepower()
{
std::cout << "lots of horses in that diesel\n";
return 300;
}
};

class car
{
public:
car(motor* M)
:M_(M)
{}

void print_info()
{
std::cout << M_->calculate_horsepower() << "HP\n";
}
~car() { delete M_; }
private:
motor* M_;
};

int main()
{
car mercedes(new diesel_motor);
mercedes.print_info();
}

"class car" should have
"motor M;"
"void print_info(){... M.calculate_horsepower(); ..... };"

The following should be concrete:

"class diesel_motor : class motor" and should implement
"virtual int calculate_horsepower(){ calculate hp for diesel };"

"class diesel_car : class car"

For this class, we should have an object "diesel_motor M;" as a child of
"motor M;". In other words, in diesel_car I need an object M as a
diesel_motor, but in the class car, I only need those attributes of M
associated with a motor.

Well, if you want to instantiate motor, simply don't make it abstract.

It seems something of this sort is not possible, so maybe there is a
different solution. Let me indicate what I need:

Every car has a motor, either of diesel or gasoline type. Calculating
horsepower is different for each of these, but I want to implement this
method only once, once for each type of motor. For every car, I want to
print the same standardised specifications, so I want one method to do
that. This method only uses methods in available for all motors. In making
a class diesel_car, I need to have a diesel_motor, which is a child of
motor.

Why exactly do you need a diesel_car? Just let your car have a pointer to
motor, and it can have any motor that you have a class for.

 

[J.M.]

Rolf Magnus schrieb:

Thanks for your response.

No. The whole idea of an abstract class is that it can't be instantiated.

Yup. I figured that much -- as I indicated, I did not expect that to
work ;-)

It is abstract because it has a _pure_ virtual member. Make it just
virtual, and the class won't be abstract anymore.

That would work, but that would require some sort of implementation, would
it not? But without knowing the type of motor, I can't calculate the
horsepower....

But I guess what you
really want is not store a motor in your car, but rather a pointer to
motor,

Actually, I would like to store the motor.. Granted, a pointer would work
too, sort of.. see below.

[zap example]

Well, if you want to instantiate motor, simply don't make it abstract.

Well, I don't "really" want to instantiate it - I want it to be part of an
abstract class

Why exactly do you need a diesel_car? Just let your car have a pointer to
motor, and it can have any motor that you have a class for.

Well, the class diesel_car and gasoline_car would have specific methods that
car (or motor) would not have. For example
"void refuel_octane_98(double volume_purchased);"
might be specific to a gasoline_car. It would not work as a method for car,
because the type of fuel a car needs depends on the motor. On the other
hand, I cannot let this method be associated with motor, because I want to
check that volume_purchased fits into the car - and that depends on the
tank size and the current tank contents. So ideally, when defining the
class "diesel_car" as derived from "car" the motor M in car should become a
diesel_motor.... But I suppose there is no way of doing that.

Jan

 

[Rolf Magnus]

That would work, but that would require some sort of implementation, would
it not? But without knowing the type of motor, I can't calculate the
horsepower....
Right.


Well, I don't "really" want to instantiate it - I want it to be part of an
abstract class

If it is part of a class (be it abstract or not), then this class will
contain an instance of the motor (and exactly that).

Well, the class diesel_car and gasoline_car would have specific methods
that car (or motor) would not have. For example
"void refuel_octane_98(double volume_purchased);"
might be specific to a gasoline_car. It would not work as a method for
car, because the type of fuel a car needs depends on the motor. On the
other hand, I cannot let this method be associated with motor, because I
want to check that volume_purchased fits into the car - and that depends
on the tank size and the current tank contents. So ideally, when defining
the class "diesel_car" as derived from "car" the motor M in car should
become a diesel_motor.... But I suppose there is no way of doing that.

Maybe you could work with templates, like:

template <class T> class car
{
/*...*/
protected:
T M_;
};

class diesel_car: public car<diesel_motor>
{
};

Then you can derive from car, and every derived class will contain the motor
that belongs to it.

 

[J.M.]

Rolf Magnus schrieb:

Right.

I suppose a dummy implementation could work, but I do not really like that.
On the other hand, there is nothing wrong having a dummy implementation for
a dummy motor ;-)))

If it is part of a class (be it abstract or not), then this class will
contain an instance of the motor (and exactly that).

Yes, I know, which is why the really was in quotes ;-)

Maybe you could work with templates, like:

template <class T> class car
{
/*...*/
protected:
T M_;
};

class diesel_car: public car<diesel_motor>
{
};

Then you can derive from car, and every derived class will contain the
motor that belongs to it.

Yes, I think that should work. In fact, that was the solution I was looking
for. Now that you have pointed it out to me, I recall having seen a
solution using templates before, but obviously, I had forgotten it...
Thanks for you help!

Jan