How to protect object passed by const reference from destruction ?

[Brendan Cullen]

Hi folks,

In the following code, I have passed an object by reference to a function. I have attempted to protect the object by using the const parameter. But I can still destroy the object (through a pointer to it). How do I protect against this ? I know the code is strange - but it is valid for the vehicle class to wish to have a data member (ptrperson) pointing to the incoming object.

class person
{
public :
int age;
};

class vehicle
{
public :
person *ptrperson;
void destroy(const person & refperson);

};

void vehicle::destroy(const person& refperson)
{
*ptrperson = refperson;
ptrperson->age++; // I don't like this - but it's legal !
delete ptrperson; // I like this evne less - but it too is legal !
}

void main (void)
{
person me;
vehicle car;
car.destroy(me);
}

[Burlacu Ovidiu]

I have 2 things unclear:
1)Why you must destry the parameter?
2)When you run the code the program doesn't crash when you delete the pointer?

Answer me maybe I can help you

Regards,
Ovidiu

[Brendan Cullen]

Hi Ovidiu,

1 : I do not need to destory the object. But I do need a private data member of my class (vehicle) to be able to point to the object. I am aware that, unfortunately, a (carelsss) programmer can :

delete ptrperson;



2 : Yes - the program crashes. But I'd prefer if the program could not compile.

Regards,

Brendan

[Burlacu Ovidiu]

Why don't declare a member variable like this:

Person& refperson;



?

[Tomaz Stih]

Ave,

the code you posted doesn't make sense (perhaps a typo?) A decent operating system should raise exception when you do *ptrperson=refperson; and that is calling the copy constructor on *ptrperson while ptrperson points to unknown.

Note that if ptrperson was initialized you would not actually delete your reference but instead you would do a shallow copy of the refperson object.

So if your main body would be something like

person me;
vehicle car;
car.ptrperson=new person; // now we have space!
car.destroy(me);



it would work.

On the other side if you want compiler protection and that:

void vehicle::destroy(const person& refperson)
{
ptrperson = &refperson;
ptrperson->age++; // I don't like this - but it's legal !
delete ptrperson; // I like this evne less - but it too is legal !
}



was what you originally intended to do the compiler will protect your const by dropping an error (unable to cast from const type to type)

Regards,
Tomaz

---------------------------------------------
Tomaz Stih, B.Sc.CS tomaz@nameco.com
Ob sotoccju 10 Nameco Group
SI-1000 Ljubljana http://www.nameco.com
Europe

[Brendan Cullen]

(I used 'arefperson' instead of 'refperson' to distinguish the data member from the parameter name.)

If I do as you suggest :

class vehicle
{
public :
vehicle(void);
person& arefperson;
person* ptrperson;
void destroy(const person & refperson);
};

vehicle::vehicle(void)
{
}



then I receive a compiler error (VC++ 5.0) : "error C2758: 'arefperson' : must be initialized in constructor base/member initializer list".

MS Help says : "If a const or reference member variable is not given a value when it is initialized, it must be given a value in the object constructor."

I'd be okay if it was the constructor of vehicle that receives the object by reference. But it isn't. It is destroy().

Regards,

Brendan

[Brendan Cullen]

Hi Tomaz,

Yes - the code doesn't make sense. It includes the use of an uninitialised pointer. I understand that. But yes - what I was really looking for was compiler protection. So your idea of
ptrperson = &refperson;


instead of

ptrperson = &refperson;


is very good. Indeed, I can go further and make ptrperson a pointer to a const object.
class vehicle
{
public :
const person* ptrperson;
void destroy(const person & refperson);
};


This still provides compiler protection.

Thanks,

Brendan

[Tomaz Stih]

Akhmmm,

well,

Brendan,

did you perhaps forget to rate my post... ;-)

Tomaz

---------------------------------------------
Tomaz Stih, B.Sc.CS tomaz@nameco.com
Ob sotoccju 10 Nameco Group
SI-1000 Ljubljana http://www.nameco.com
Europe