Basic polymorphism

Question

I have two structs:

struct X { double x; };

struct Y : X { double y; };

I want a vector filled with basic objects (of type X) which possibly can be extended (to objects of type Y):

    std::vector<X*> vec;

    if(condition)
    {
        for(size_t i=0; i<n; ++i)
        {
            vec.push_back(new Y);
            vec[i]->x = ...;
            vec[i]->y = ...; // error
        }
    }
    else
    {
        for(size_t i=0; i<n; ++i)
        {
            vec.push_back(new X);
            vec[i]->x = ...;
        }
    }

This gives the error " no member named 'y' in 'X' ". Do you know how I could achieve what I wish?

Polymorphism is for behaviour, usually. Looks like you need composition, maybe? — cigien, Apr 05 '20 at 21:54
@ Thomas Sablik That's exactly what I want to avoid: using members which are possibly not used. — PolarBear, Apr 05 '20 at 21:57
The immediate problem is that the code stores an `X*`. It can’t access `Y` members through that pointer. So keep the actual type: `Y* ptr = new Y; ptr->y = ...; vex.push_back(ptr);`. — Pete Becker, Apr 05 '20 at 22:01
I'm not sure what the purpose of inheritance here. It depends on your problem, but would `struct Y { X x_; double y; };` work? — cigien, Apr 05 '20 at 22:02
@AdrianMole — `dynamic_cast` won’t work unless the base type has one or more virtual functions. — Pete Becker, Apr 05 '20 at 22:03
@PeteBecker Fair comment! Would `static_cast` do the trick in this case? I see your answer [here](https://stackoverflow.com/a/42011553/10871073) addresses this issue. — Adrian Mole, Apr 05 '20 at 22:03
Dou you want to put either X or Y in the vector or can the vector also be filled with a mixture? — Sebastian, Apr 05 '20 at 22:06
@AdrianMole — yes `static_cast` would work **if you know** that the `X*` in fact points at an object of type `Y`. — Pete Becker, Apr 05 '20 at 22:09

Sebastian · Accepted Answer · 2020-04-05T22:44:24.073

Quick solution for the code you posted so far. Change

vec.push_back(new Y);
vec[i]->x = ...;
vec[i]->y = ...; // error

to

Y* newy = new Y;
vec.push_back(newy);
newy->x = ...;
newy->y = ...; // no error

But then you will still need a cast of X* to Y* when reading the object back to access the member variables only contained in Y.

if(condition)
    cout << static_cast<Y*>(vec[i])->y;

Instead of casts you could also create separate vectors for X and Y (do not need to be pointers, if they contain only the one type, but could also be the pointers) and just fill and access one of them.

vector<X> vecX;
vector<Y> vecY;
bool condition;

Basic polymorphism

1 Answers1