Copy Inherited Objects

[timid]

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;

// Prints "Apple" but should print "Orange"
std::cout << "myfruit3 = " << myfruit3->get_name() << std::endl;

// Wait for a keypress
std::cin.get();

delete myfruit1;
delete myfruit2;
delete myfruit3;
}

 

[Christian Hackl]

#include <iostream>

class Fruit
{
public:
virtual const char* get_name() {return "(nothing)";}

Use std::string rather than char*.

Also, a function such as "get_name" should usually be declared const.

};

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();

[...]

// The following does not work
std::cout << "Now change myfruit3 from Apple to Orange..." <<
std::endl;
*myfruit3 = *myfruit2;

* derefences the pointer, yielding the object it's pointing to.
Consequently, in your example you don't assign the _pointers_, you
assign the _objects_ they are pointing to. That's a huge difference.
Assigning an Orange to an Apple obviously does not do anything because
you did not specify anything to happen in case of such an assignment.
The invisible assignment operator automatically generated by the
compiler is empty. In other words, the Apple is completely unaffected by
the operation.


If you want to assign a pointer to another pointer, you have to write:

myfruit3 = myfruit2;

However, in your program this would have disastrous results because you
would lose the old value of myfruit3, which means you cannot delete your
second Apple anymore. (You'd have to save the old value somewhere, for
example in a temporary pointer variable.)

What do you try to achieve by having myfruit3 point to a different object?

 

[timid]

#include <iostream>

class Fruit
{
public:
virtual const char* get_name() {return "(nothing)";}

Use std::string rather than char*.

Also, a function such as "get_name" should usually be declared const.

};

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();

[...]

// The following does not work
std::cout << "Now change myfruit3 from Apple to Orange..." <<
std::endl;
*myfruit3 = *myfruit2;

* derefences the pointer, yielding the object it's pointing to.
Consequently, in your example you don't assign the _pointers_, you
assign the _objects_ they are pointing to. That's a huge difference.
Assigning an Orange to an Apple obviously does not do anything because
you did not specify anything to happen in case of such an assignment.
The invisible assignment operator automatically generated by the
compiler is empty. In other words, the Apple is completely unaffected by
the operation.

If you want to assign a pointer to another pointer, you have to write:

myfruit3 = myfruit2;

However, in your program this would have disastrous results because you
would lose the old value of myfruit3, which means you cannot delete your
second Apple anymore. (You'd have to save the old value somewhere, for
example in a temporary pointer variable.)

What do you try to achieve by having myfruit3 point to a different object?

Thank's for the help so far, it would appear that I need to learn a
lot more.

I'm going to try & rewrite this program so it'll work.

 

[Marcel Müller]

Hi!

Use std::string rather than char*.

Normally I would agree, but in case of /const/ char* there is usually no
problem. Only the use of char* for strings in C++ applications is nearly
always a risk. On the other hand, using std::string for this (and
similar) purposes is a significant runtime overhead. Only if you already
have a std::string it is more advisable to return std::string, or if
this causes no threading or aliasing problem const std::string&.

Also, a function such as "get_name" should usually be declared const.

That's obviously true.

If you want to assign a pointer to another pointer, you have to write:

myfruit3 = myfruit2;

However, in your program this would have disastrous results because you
would lose the old value of myfruit3, which means you cannot delete your
second Apple anymore. (You'd have to save the old value somewhere, for
example in a temporary pointer variable.)

What do you try to achieve by having myfruit3 point to a different object?

I think the OP is seeking for something like a clone method.


Marcel

 

[James Kanze]

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]

I think the OP is seeking for something like a clone method.

Or the letter/envelope idiom. Or perhaps just simply some
information on OO design; he hasn't really expressed a concrete
need for assignment.