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_horse power() = 0"

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

The following should be concrete:

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

"class diesel_car : class car"

For this class, we should have an object "diesel_mot or 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]

Re: Inheritance: "abstra ct objects"?

J.M. wrote:
[color=blue]
> I have a question concerning inheritance: can an abstract (parent) class
> have an abstract object?[/color]

No. The whole idea of an abstract class is that it can't be instantiated.
[color=blue]
> 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_horse power() = 0"[/color]

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_horse power() = 0;
virtual ~motor() {}
};

class diesel_motor : public motor
{
public:
virtual int calculate_horse power()
{
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_hors epower() << "HP\n";
}
~car() { delete M_; }
private:
motor* M_;
};

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

[color=blue]
> "class car" should have
> "motor M;"
> "void print_info(){.. . M.calculate_hor sepower(); ..... };"
>
> The following should be concrete:
>
> "class diesel_motor : class motor" and should implement
> "virtual int calculate_horse power(){ calculate hp for diesel };"
>
> "class diesel_car : class car"
>
> For this class, we should have an object "diesel_mot or 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.[/color]

Well, if you want to instantiate motor, simply don't make it abstract.
[color=blue]
> 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.[/color]

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.]

Re: Inheritance: &quot;abstra ct objects&quot;?

Rolf Magnus schrieb:

Thanks for your response.

[color=blue]
> J.M. wrote:
>[color=green]
>> I have a question concerning inheritance: can an abstract (parent) class
>> have an abstract object?[/color]
>
> No. The whole idea of an abstract class is that it can't be instantiated.[/color]

Yup. I figured that much -- as I indicated, I did not expect that to
work ;-)[color=blue]
>[color=green]
>> 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_horse power() = 0"[/color]
>
> It is abstract because it has a _pure_ virtual member. Make it just
> virtual, and the class won't be abstract anymore.[/color]

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....
[color=blue]
> But I guess what you
> really want is not store a motor in your car, but rather a pointer to
> motor,[/color]

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

[zap example]
[color=blue]
>[color=green]
>> "class car" should have
>> "motor M;"
>> "void print_info(){.. . M.calculate_hor sepower(); ..... };"
>>
>> The following should be concrete:
>>
>> "class diesel_motor : class motor" and should implement
>> "virtual int calculate_horse power(){ calculate hp for diesel
>> };"
>>
>> "class diesel_car : class car"
>>
>> For this class, we should have an object "diesel_mot or 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.[/color]
>
> Well, if you want to instantiate motor, simply don't make it abstract.[/color]

Well, I don't "really" want to instantiate it - I want it to be part of an
abstract class :)
[color=blue]
>[color=green]
>> 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.[/color]
>
> 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.[/color]

Well, the class diesel_car and gasoline_car would have specific methods that
car (or motor) would not have. For example
"void refuel_octane_9 8(double volume_purchase d);"
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_purchase d 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]

Re: Inheritance: &quot;abstra ct objects&quot;?

J.M. wrote:
[color=blue][color=green][color=darkred]
>>> "class motor" is abstract because it has a virtual member
>>> "virtual int calculate_horse power() = 0"[/color]
>>
>> It is abstract because it has a _pure_ virtual member. Make it just
>> virtual, and the class won't be abstract anymore.[/color]
>
> 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....[/color]

Right.
[color=blue][color=green][color=darkred]
>>> "class car" should have
>>> "motor M;"
>>> "void print_info(){.. . M.calculate_hor sepower(); ..... };"
>>>
>>> The following should be concrete:
>>>
>>> "class diesel_motor : class motor" and should implement
>>> "virtual int calculate_horse power(){ calculate hp for diesel
>>> };"
>>>
>>> "class diesel_car : class car"
>>>
>>> For this class, we should have an object "diesel_mot or 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.[/color]
>>
>> Well, if you want to instantiate motor, simply don't make it abstract.[/color]
>
> Well, I don't "really" want to instantiate it - I want it to be part of an
> abstract class :)[/color]

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).
[color=blue][color=green]
>> 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.[/color]
>
> Well, the class diesel_car and gasoline_car would have specific methods
> that car (or motor) would not have. For example
> "void refuel_octane_9 8(double volume_purchase d);"
> 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_purchase d 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.[/color]

Maybe you could work with templates, like:

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

class diesel_car: public car<diesel_moto r>
{
};

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

[J.M.]

Re: Inheritance: &quot;abstra ct objects&quot;?

Rolf Magnus schrieb:

[color=blue][color=green]
>> 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....[/color]
>
> Right.[/color]

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 ;-)))

[color=blue][color=green][color=darkred]
>>>
>>> Well, if you want to instantiate motor, simply don't make it abstract.[/color]
>>
>> Well, I don't "really" want to instantiate it - I want it to be part of
>> an abstract class :)[/color]
>
> 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).[/color]

Yes, I know, which is why the really was in quotes ;-)
[color=blue]
>[color=green][color=darkred]
>>> 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.[/color]
>>
>> Well, the class diesel_car and gasoline_car would have specific methods
>> that car (or motor) would not have. For example
>> "void refuel_octane_9 8(double volume_purchase d);"
>> 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_purchase d 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.[/color]
>
> Maybe you could work with templates, like:
>
> template <class T> class car
> {
> /*...*/
> protected:
> T M_;
> };
>
> class diesel_car: public car<diesel_moto r>
> {
> };
>
> Then you can derive from car, and every derived class will contain the
> motor that belongs to it.[/color]

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