C++ - Have a type be able to hold it's children

Question

I have a linked list of type Device:

Node<Device> list = new Node<device>(device);

And Device have derived classes:

And I need my list to be able to hold every derived class of Device.

How can I do it?

Note: If I send an Aerobic_Device to the list, it just dynamically converts to Device, which loses data.

EDIT:

I changed all the logic of my program to work with pointers, and now the list is: Node<Device*> - Which still converts everything to Device type. Now, as many suggested, I switched it to Node<std::unique_ptr<Device>>, but this just introduced many many many errors.

You need a list of pointers. If this is not a homework, please use a standard container. — n. m. could be an AI, Dec 28 '15 at 08:57
You can store an ID for each type, and define in Device: int getType(); to get that ID. — Pierre Emmanuel Lallemant, Dec 28 '15 at 08:58
I _think_ that your problem is two things: 1. You need to store a pointer to `Device` in the list. 2. You will need a common interface [or use `dynamic_cast`] using virtual functions. — Mats Petersson, Dec 28 '15 at 09:08
This question is now closed. If you have a question about storing raw pointers or unique_ptrs in a list, ask a new question. I recommend you do that, because your remarks suggest that your understanding of polymorphism in C++ is lacking. — n. m. could be an AI, Dec 28 '15 at 09:58

score 2 · Answer 1 · answered Dec 28 '15 at 08:58

2

You cannot store your objects by value and have polymorphic behavior at the same time. You can store your objects by (preferably smart) pointer:

Node<std::unique_ptr<Device>> list;

answered Dec 28 '15 at 08:58

el.pescado - нет войне

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Ok, why not just regular pointers? – Amit Dec 28 '15 at 09:02
@Amit Because smart pointers will do the memory management for you, and that's what you should use. – Angew is no longer proud of SO Dec 28 '15 at 09:03
You can use regular pointer, since there is no multi inheritance. – Pierre Emmanuel Lallemant Dec 28 '15 at 09:03
@Amit If somebody else owns the objects, it is OK to use plain pointers. But it the list owns them, or shares the ownership, then you need a smart pointer that expresses that ownership model. – juanchopanza Dec 28 '15 at 09:05

CinCout · Accepted Answer · 2015-12-28T09:04:08.873

1

Make the list to be of type Device* (note the pointer).

Node<Device*> list;

Then, declare objects of derived types (PowerDevice, AerobicDevice, PowerAerobicDevice) and put them in the list.

This way you can leverage the power of runtime polymorphism.

Alternatively, you can (and should) use smart pointers instead of raw pointers to automatically handle the memory:

std::unique_ptr<Device> pDevice= std::make_unique<PowerDevice>(...);
// put into list
std::unique_ptr<Device> aDevice= std::make_unique<AerobicDevice>(...);
// put into list
std::unique_ptr<Device> paDevice= std::make_unique<PowerAerobicDevice>(...);
// put into list

edited Dec 28 '15 at 09:04

answered Dec 28 '15 at 08:57

CinCout

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This doesn't answer to him. What he wants is to identify which sub class is used, to cast them to retrieve the attributes. He already does what you say. – Pierre Emmanuel Lallemant Dec 28 '15 at 08:59
3

@PierreEmmanuelLallemant Can you read OP's code? Because I can't see that in the question. – juanchopanza Dec 28 '15 at 09:00
juanchopanza: re-read the question. – Pierre Emmanuel Lallemant Dec 28 '15 at 09:03

C++ - Have a type be able to hold it's children

2 Answers2