CODEWITHSUNDEEP
c++polymorphism
nburk
nburk

Reputation: 22731

Why does this C++ polymorphism not work?

I don't understand why this inheritance doesn't work. I have the following setup:

struct Shape{}
struct Stain : Shape {}

Now, why can't I do the following:

vector<Shape> shapes;
Stain stain();
shapes.push_back(stain);

I would this expect this to be working since Stain is a subclass of Shape, so I should be able to put a Stain into a vector<Shape>? Or is this even conceptually wrong and what I am trying to do is indeed not possible?

Upvotes: 2

Views: 289

Answers (1)

Bartek Banachewicz
Bartek Banachewicz

Reputation: 39370

For polymorphism in C++ you need reference semantics. The easiest way to achieve that would be std::vector<std::unique_ptr<Shape>>.

Alternatively, if you want to be able to copy your shapes, look at value_ptr concept. A lot of implementations also allow COW (Copy-on-Write). Essentially for nearly all purposes it works just like a value, e.g. a copy of it makes a copy of the value it holds and allocates it.

The obvious change is that you can't treat your vector as a POD memory block anymore.

Upvotes: 10

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