🔒 Locked

Polymorphic array.

Started by Kustaz May 6, 2006 at 1:57 AM 6 replies 1.2k views

Original Post

May 6, 2006 at 1:57 AM

Lets say I have a base class, 'Widget'. What I want to be able to do create several objects of different types, which are all derived from 'Widget', and be able to store them in an array (or other suitable container class). Is this possible, and if not, are there any suitable workarounds? Example:



class Base
{};

class One: public Base
{};

class Two: public Base
{};

One X;
Two Y;

std::vector<Base> Storage;

Storage.push_back(X);
Storage.push_back(Y);

I know its possible to pass a derived class into a function instead of the parent class, I just dont know how to apply this to another situation, such as this one. PS: I apologise if my wording is incorrect.

Mxz

May 6, 2006 at 2:03 AM

It is customary to simply store a base class pointer in the array, and use that polymorphically.

class Base{};class One: public Base{};class Two: public Base{};std::vector<Base*> Storage;Storage.push_back(new One);Storage.push_back(new Two);

Use of a smart pointer would be preferred though.

Fred304

May 6, 2006 at 3:03 AM

The reason why you cannot have a vector of Base and use it polymorphically is that every object of a derived class will take up different amounts of bytes, while Base* is always of constant size (4 bytes on 32 bit machines and 8 bytes of 64 bit I believe).

If you but a derived object in a vector of Base it gets truncated to a Base and is not derived anymore.

Zahlman

May 6, 2006 at 2:30 PM

Indeed. [google] "object slicing C++" for more.

deathkrush

May 6, 2006 at 2:42 PM

Don't store pointers in a std::vector, it's a bad idea. Try Boost Pointer Container, it's made specifically for that task.

Endar

May 6, 2006 at 8:14 PM

Quote:
Original post by deathkrush
Don't store pointers in a std::vector, it's a bad idea. Try Boost Pointer Container, it's made specifically for that task.

Why is it a bad idea? I do it all the time and the only problems I've had is when I've forgotten to do something important.

jdhardy

May 6, 2006 at 10:57 PM

Quote:
Original post by Endar
Quote:
Original post by deathkrush
Don't store pointers in a std::vector, it's a bad idea. Try Boost Pointer Container, it's made specifically for that task.

Why is it a bad idea? I do it all the time and the only problems I've had is when I've forgotten to do something important.

It handles all of the pointer nastiness for you, and is (marginally) more efficient than std::vector< boost::shared_ptr< Base > > >.

MaulingMonkey

May 7, 2006 at 1:39 PM

Quote:
Original post by jdhardy
Quote:
Original post by Endar
Quote:
Original post by deathkrush
Don't store pointers in a std::vector, it's a bad idea. Try Boost Pointer Container, it's made specifically for that task.

Why is it a bad idea? I do it all the time and the only problems I've had is when I've forgotten to do something important.
It handles all of the pointer nastiness for you, and is (marginally) more efficient than std::vector< boost::shared_ptr< Base > > >.

In other words, it takes care of something important (memory deallocation) automatically so you can't froget to do it. Humans are frogetful creatures, anything automatable by the system is a good thing in terms of avoiding bugs :-).

Topic Locked

This topic has been locked by a moderator. New replies are not allowed.