test_nonabstract.cpp

#include <cmath>

class IVec3 {
public:
    virtual ~IVec3() = default;
    virtual float x() const noexcept { return 0; }
    virtual float y() const noexcept { return 0; }
    virtual float z() const noexcept { return 0; }
    virtual void setX(float) {}
    virtual void setY(float) {}
    virtual void setZ(float) {}
    virtual IVec3 add(const IVec3& rhs) const {
        return IVec3(x() + rhs.x(), y() + rhs.y(), z() + rhs.z());
    }
protected:
    IVec3() = default;
    IVec3(float, float, float) {} // dummy for derived class convenience
};

class Vec3 final : public IVec3 {
    float m_x, m_y, m_z;
public:
    Vec3() : m_x(0), m_y(0), m_z(0) {}
    Vec3(float x, float y, float z) : m_x(x), m_y(y), m_z(z) {}
    float x() const noexcept override { return m_x; }
    float y() const noexcept override { return m_y; }
    float z() const noexcept override { return m_z; }
    void setX(float v) override { m_x = v; }
    void setY(float v) override { m_y = v; }
    void setZ(float v) override { m_z = v; }
    Vec3 add(const IVec3& rhs) const override {
        return Vec3(m_x + rhs.x(), m_y + rhs.y(), m_z + rhs.z());
    }
};

int main() {
    Vec3 a(1,2,3), b(4,5,6);
    Vec3 c = a.add(b);
    IVec3* p = &a;
    IVec3 d = p->add(b);  // Returns IVec3 by value through interface pointer
    return 0;
}