Const-correctness with getters of vector of non-pointers [duplicate]

Question

I have a quick question regarding const-correctness for getters of vector of non-pointers.

Basically, I have this class with a non-const getter and a const getter for m_Vertices:

class Mesh
{
public:
    std::vector<Vertex>& GetVertices() { return m_Vertices; }

    const std::vector<Vertex>& GetVertices() const { return m_Vertices; }

private:
    std::vector<Vertex> m_Vertices;
}

The non-const getter makes sense; the getter is non-const, we return a non-const reference to a vector of non-const Vertex instances.

The const getter in the other hand doesn't makes sense to me. I'm questioning the const-correctness and whether I should just return a copy of the vector. The problem I see is that we return a const reference to a vector of non-const Vertex instances. So, the callee will have a const reference to m_Vertices, but will still be able to modify the Vertex instances, which feels wrong to me in terms of const-correctness.

In that case, should I simply return a copy of m_Vertices?

"The callee will still be able to modify the Vertex instances" Will it? std::vector doesn't work the way you think — Nathan Pierson
– Nathan Pierson, Commented Sep 2, 2023 at 16:24
const std::vector does not allow to modify its elements so your assumption is incorrect. — Quimby
– Quimby, Commented Sep 2, 2023 at 16:26
I don't like this pattern, and try to avoid it in my own code. (Although sometimes it is the right pattern for the problem at hand.) It tightly couples the private member variable to the public API. If there were any invariants imposed upon m_Vertices, those invariants would be at the mercy of whatever manipulations the users of the GetVertices do to the member variable... why even bother to make it private: in the first place? Perhaps it has no preconditions/postconditions, and the class has no invariants...? — Eljay
– Eljay, Commented Sep 2, 2023 at 17:02
@Eljay It's for a game engine, which have to render multiple meshes every frame. I believe returning a copy of the vector each time GetVertices is called would create a big overhead in big scenes. Also, I haven't identified pre-conditions/post-conditions nor invariants for the class. Truth be told, I'm not doing contract programming anyway. — Anthony Blanchette-Potvin
– Anthony Blanchette-Potvin, Commented Sep 2, 2023 at 17:18

jwezorek · Accepted Answer · 2023-09-02 20:19:45Z

First of all, understand that this is an opinion-based question. There are no right or wrong answers when you are designing a class regarding the usage of const.

However, that said, in this case your const declarations are reasonable. In particular your statement "the callee will have a const reference to m_Vertices, but will still be able to modify the Vertex instances" is not true -- because of the decisions that were made in the design of std::vector regarding const-ness.

There are three main ways someone might want to modify the vector and its elements. The callee may want to add/delete items to/from the vector, may want to change the value of a field of the elements, or may want to replace one of the elements. A reference to a const vector will make all three of those actions errors:

#include <iostream>
#include <vector>
#include <span>

struct point {
    double x;
    double y;
};

class vertices {
    std::vector<point> points_;
public:
    vertices(std::span<const point> pts) :
        points_{pts.begin(), pts.end()}
    {}

    const std::vector<point>& get() const {
        return points_;
    }

    std::vector<point>& get() {
        return points_;
    }
};

int main()
{
    vertices verts({{ {1.0,2.0}, {3.0,4.0}, {5.0,6.0} }});
    const auto& const_verts = verts;

    verts.get().push_back({ 7.0,8.0 }); // <= yes.
    //const_verts.get().push_back({ 7.0,8.0 }); <= no.

    verts.get().front().x += 1; // <= yes.
    //const_verts.front().x += 1; // <= also no.

    verts.get()[2] = { 0.0,0.0 }; // <= yes.
    //const_verts.get()[2] = { 0.0,0.0 }; <= no.
}

Collectives™ on Stack Overflow

Const-correctness with getters of vector of non-pointers [duplicate]

1 Answer 1

Comments

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Linked

Related