Re: Dynamically allocated classes

Thomas Maeder <>
10 May 2006 17:26:17 -0400
Theo Richter <> writes:

[You are using some terms in a (IMHO) confusing way, so please accept

a little pickiness from my part.]

memory can be allocated dynamically at runtime using "new".

Indeed, with a call such as

::operator new(200)

For example, I could define a class "item" to hold information about

an item available in the stockroom of a company

class item {

/* information about the item and functions to access these data */


and use

item* pencil = new item;

to allocate memory for a pointer to item holding information about

say a pencil.

The new expression

new item

does *not* allocate memory for a pointer.

The definition

item* pencil;

already does that.

The new expression allocates memory for an object of class item. But

not only that, it also initializes it using class item's default


I would now like to add an interface that the user can add new items

during the program execution until he has finished his job - so the

number is not fixed.

My first idea: I could use a vector of items (vector<item>) to hold

the classes and just append using push_back.

This vector won't hold classes; it will hold objects.

Are there other/better ways?

There are certainly other ways (you mention one further down); whether

there are better ways depends on what you mean by "good".

Can I dynamically create pointers with their names provided by the

user (e.g. the user types in "ruler" and this leads to an execution

like "item* ruler=new item").

No. Not exactly anyway.

First, again, this statement doesn't dynamically create a pointer. It

dynamically creates an object of type item.

Second, variable names are compile-time entities. In a program, you

can't create a variable whose name is a string input by the user to

that very program.

If class item has a constructor that takes a std::string (and a data

member holding the value passed to this constructor), you can do

something like:

std::string word;

if (std::cin >> word)


  item *anotherItem = new item(word);

  // do something with *anotherItem


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