Array of objects question

**tezcatlipooca** · 01-02-2007

Hi,
Suppose i have a class A and which has a subclass B is it possible to create an array which elements are objects of either class A or B?
If so, how can this be done?
Thanks..

**Mario F.** · 01-02-2007

If by subclass you mean B derives from A, then yes. Your array can be an array of pointers to A.
An array of pointers to a base class can store derived objects.

Note that you can also assign derived objects to an array of A. It will not be flagged as an error. But on this case, since the array is not storing pointers to the base class, but the base class itself, if you assign to one of its elements a derived object, only the base part of that object will be stored. It's called slicing...

**tezcatlipooca** · 01-02-2007

Thanks!
But now it seems that i have a problem with the array's elements' initialization..

**Mario F.** · 01-02-2007

Can you show the code?

**tezcatlipooca** · 01-02-2007

Sorry, it's quite large.
I'm just confused about the way i should call the constructor for each element after the array declaration.
If you could show me a small example i would really be grateful

**whiteflags** · 01-02-2007

> But now it seems that i have a problem with the array's elements' initialization..

Such an array would be dynamically sized with operator new or built out of existing instances. You could try to hack it I suppose, but in it's most glaring form, you end up taking the address of a temporary.

For instance, this is very wrong:

Let foo be a class whose constructor takes an integer, and bar a derivative class whose constructer has an additional char parameter.

Code:

foo *array[] = { &foo(123), &bar(234, 'a'), &bar(567, 'b'), &foo(0) };

**Daved** · 01-02-2007

>> I'm just confused about the way i should call the constructor for each element after the array declaration.

You would use new to allocate space for the objects and store the pointer in the array. You would then have to remember to delete the pointers when you are done with them. A boost::ptr_vector is probably a better solution than an array because it handles the deletes for you.

You also have to make sure that class A has a virtual destructor.

**tezcatlipooca** · 01-02-2007

I understand. But the problem is that i do not know how to store the pointer in the array.
It seems that i can't just use the assignment operator "="..

**Daved** · 01-02-2007

Show the code where you attempt to use the assignment operator, along with the declaration of the array.

**tezcatlipooca** · 01-03-2007

Ok, i tried the following but nothing worked:

Code:

class_a *table=new class_a[5];
class_a *node=new class_a("Jack");
table[0]=node;

Code:

vector <class_a*> vec(5); 
class_a *node=new class_a("Jack");
vec.push_back(node);

I really appreciate your concern..
Thanks.

**vart** · 01-03-2007

try this

Code:

class_a **table=new class_a*[5];
class_a *node=new class_a("Jack");
table[0]=node;

**tezcatlipooca** · 01-03-2007

Didn't work either, i got the same errors:

expected constructor, destructor or type conversion before '=' token
expected ',' or ';' before '=' token

**vart** · 01-03-2007

This Is working:

Code:

#include <iostream>
class class_a
{
	int x;
};
int main()
{
class_a **table=new class_a*[5];
class_a *node=new class_a();
table[0]=node;
}

So do you have a class definition?

**tezcatlipooca** · 01-03-2007

Yeap, ok what you say is true indeed. I was just trying to add the pointer to the array outside a function (outside main to be honest). Thank you..
Could you possibly explain to me why

Code:

class_a **table=new class_a*[5];

is needed? I thought that an array of pointers would be ok..

**vart** · 01-03-2007

it IS an array of pointers
new class_a*[5]; asks to allocate memory for 5 elements of the type
class_a*

class_a* - IS pointer to class

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