virtual copy constructors

Author

122

January 04, 2003 01:03 AM

just when i thought i understood..........

  


#include <iostream>
using namespace std;

class Mammal
{
public:
	Mammal():itsAge(0), itsWeight(0) {}
	virtual ~Mammal() {}
	Mammal(const Mammal &rhs);
	virtual Mammal * Clone()
		{return new Mammal(*this);}
	int GetAge()const
		{return itsAge;}
	void SetAge(int age)
		{itsAge = age;}
	int GetWeight()const
		{return itsWeight;}
	void SetWeight(int weight)
		{itsWeight = weight;}
protected:
	int itsAge;
	int itsWeight;
};

Mammal::Mammal (const Mammal & rhs):
itsAge(rhs.GetAge()),
itsWeight(rhs.GetWeight())
{
}

class Dog : public Mammal
{
public:
	Dog() {}
	virtual ~Dog() {}
	Dog (const Dog & rhs);
	virtual Mammal * Clone()
		{return new Dog(*this);}
};

Dog::Dog(const Dog & rhs):Mammal(rhs)
{
}

//-------------------------

int main()
{
	Dog *Lucky = new Dog;
	Mammal *Meg = Lucky->Clone(); // this is fine

	//Dog *Meg = Lucky->Clone();  // cannot convert from ''class Mammal *'' to ''class Dog *''


	return 0;
}

can someone explain what is preventing Dog *Meg = Lucky->Clone(); from working ? and yes lucky and meg are my dogs

Arild Fines

968

January 04, 2003 01:50 AM

Clone is declared as returning a Mammal*, so even though you know that it in this case returns a Dog*, you need to manually cast to the actual type, like this:

  Dog* meg = static_cast<Dog*>(Lucky.Clone())

However, IIRC, C++ does support covariant returns, so you can do this in the Dog class:

  virtual Dog* Clone(){...}

I''m 95% sure that covariant returns are in the standard - whether it is supported by compilers is another matter - I have never tried it.

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deal

Author

122

January 04, 2003 02:24 AM

  Dog* meg = static_cast<Dog*>(Lucky.Clone())

wrong operator ?

  Dog* meg = static_cast<Dog*>(Lucky->Clone())

thanx problem solved.

btw Visual C++ 6 does not support covariant returns.

which sucks because that makes my code look so bloody ugly

[edited by - deal on January 4, 2003 3:28:49 AM]

LordByron

122

January 04, 2003 07:20 AM

Well you shouldn't use casting since that isn't very good form. You can try this example I made for you that uses mammals and dogs.

#include < iostream >

using namespace std;

class Mammal
{
public:
Mammal();
virtual ~Mammal(){};
void setAgeAndWeight(int age,int weight)
{
itsAge=age;
itsWeight=weight;
}

int getAge() {return itsAge;}
int getWeight() {return itsWeight;}
virtual Mammal *clone()
{
return new Mammal(*this);
}
private:
int itsAge;
int itsWeight;
};

Mammal::Mammal():
itsAge(0),itsWeight(0)
{
}

class Dog: public Mammal
{
public:
Dog():Mammal(){};
~Dog(){};
virtual Mammal *clone()
{
Dog *pDog=new Dog;
*pDog=*this;
return pDog;
}
};

int main()
{
Mammal *pLucky=new Dog;

pLucky->setAgeAndWeight(5,5);

Mammal *Megs;

Megs=pLucky->clone();

return 0;
}

[edited by - LordByron on January 4, 2003 8:22:22 AM]

[edited by - LordByron on January 4, 2003 8:23:17 AM]

[edited by - LordByron on January 4, 2003 8:24:24 AM]

[edited by - LordByron on January 4, 2003 8:26:18 AM]

virtual copy constructors

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