Resizing array of pointers on objects with the same base class

Question

I have a problem in C++ and i can't find answer for it. I have some classes:

class Xyz{
   //...
};
class MyClass1: public Xyz{
   //...
};
class MyClass2: public Xyz{
   //...
};
class MyClass3: public Xyz{
   //...
};

As you can see MyClass1, MyClass2 and MyClass3 have the same base class. Now i want to define some objects:

MyClass1 *object1 = new MyClass1();
MyClass2 *object2 = new MyClass2();
MyClass3 *object3 = new MyClass3();

After that i want to create an dynamic array with pointers at those objects (as i know it's possible) and an integer which save the number of elements in that array. I named that array someName and in future it should store object1, object2 and object3.

Xyz **someName;
int numberOfElements = 0;

Problems start here. someName is empty so i want to add something to it. But i don't know how to make an new array with given number of places for pointers to copy old element to it. This don't work ofcourse:

Xyz **someNameTemp = new Xyz[numberOfElements+1];

Any idea how the resizing function should look? Thanks for help.

¤ You don't have an array. The statement "someName is empty" is false. So it's meaningless to talk about resizing. This declares and initializes a suitable array: Xyz* array[] = {object1, object2, object3};. It cannot be resized. By the way, whoever is declaring the standard library as off-limits to you, is not a good teacher of this subject. Consider studying on your own. Cheers & hth., — Cheers and hth. - Alf
– Cheers and hth. - Alf, Commented Dec 18, 2011 at 0:18
But how about creating a new temporary array, rewriting all objects and then switching pointers? — Elektryk
– Elektryk, Commented Dec 18, 2011 at 0:22
@AlfP.Steinbach, I disagree - it's important to understand both the basics as well as the STL. Learning only one or the other isn't a good idea, but you have to learn one before the other in the end, and learning the fundamentals first is a perfectly good idea. — bdonlan
– bdonlan, Commented Dec 18, 2011 at 0:46
@bdonlan: well you have a lot to learn. there's nothing inherently wrong in your statement that learning the fundamentals first is a good idea, but that statement is not in conflict with learning at a reasonable pace and without unreasonable struggles. not using the standard library is however in conflict with that, as you may learn. anyway, don't take my word for it. get yourself a good book (that doesn't make you needlessly struggle with details), such e.g. Koenig & Moo's "Accelerated C++", or see the C++ book list FAQ here on SO. cheers & hth., — Cheers and hth. - Alf
– Cheers and hth. - Alf, Commented Dec 18, 2011 at 8:36

Oliver Charlesworth · Accepted Answer · 2011-12-18 00:12:03Z

2

You could avoid all the hassle, and just use a std::vector:

std::vector<Xyz *> pointers;

pointers.push_back(new MyClass1());
pointers.push_back(new MyClass2());
// etc.

[Side-note: You should strongly consider not using raw pointers; instead use smart pointers, as it makes resource-management far simpler.]

answered Dec 18, 2011 at 0:12

Oliver Charlesworth

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Benjamin Lindley · Accepted Answer · 2011-12-18 00:42:38Z

0

Xyz **someNameTemp = new Xyz*[numberOfElements+1];
for (int i=0; i<numberOfElements; ++i)
    someNameTemp[i] = someName[i];
delete [] someName;
someName = someNameTemp;
++numberOfElements

// someName[numberOfElements - 1] uninitialized

Also, when you first create the pointer, be sure to initialize it. Either to NULL, or an allocated array.

edited Dec 18, 2011 at 0:42

answered Dec 18, 2011 at 0:37

Benjamin Lindley

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1 Comment

Cheers and hth. - Alf Over a year ago

proceeding in this fashion (to add new elements) yields square time, O(n^2). it's imho not a good idea to teach beginners such impractical techniques. it's like teaching a novice driver to refuel the car by demounting the fuel tank: yes it's technically possible, but.

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