Re: Copy Inherited Objects
On 17 mai, 23:11, timid <visicalcena...@googlemail.com> wrote:
I'm attempting to learn how to use inheritance in C++, the
following program should print the word 'Orange' on the screen
but it prints 'Apple' instead. Can someone explain what is
wrong?
#include <iostream>
class Fruit
{
public:
virtual const char* get_name() {return "(nothing)";}
};
class Apple: public Fruit
{
public:
const char* get_name() {return "Apple";}
};
class Orange: public Fruit
{
const char* get_name() {return "Orange";}
};
int main()
{
std::cout << "Test inheritation" << std::endl;
Fruit *myfruit1 = new Apple();
Fruit *myfruit2 = new Orange();
Fruit *myfruit3 = new Apple();
std::cout << "myfruit1 = " << myfruit1->get_name() <<
std::endl; // Prints "Apple"
std::cout << "myfruit2 = " << myfruit2->get_name() <<
std::endl; // Prints "Orange"
std::cout << "myfruit3 = " << myfruit3->get_name() <<
std::endl; // Prints "Apple"
// The following does not work
std::cout << "Now change myfruit3 from Apple to Orange..." <<
std::endl;
*myfruit3 = *myfruit2;
And what do you expect this to do? You cannot change the type
of an existing object. In general, when inheritence is
involved, you don't support assignment and copy.
// Prints "Apple" but should print "Orange"
std::cout << "myfruit3 = " << myfruit3->get_name() << std::endl;
No. Should print "Apple". But a lot of typical implementations
of the assignment operator (for more complicated types) will
result in undefined behavior. The base class for an inheritence
hierarchy should normally forbid assignment, so that the
situation can't arrive, even accidentally.
--
James Kanze (GABI Software) email:james.kanze@gmail.com
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