C++ class properties not updating properly

April 2, 2022

I have a custom galaxy class which has a custom property pos.
pos is an instance of another custom class Vector3D.
A Vector3D has properties x, y, z. The arithmetic operations for Vector3D is basic vector math, the relevant ones below:

Vector3D Vector3D::operator+(const Vector3D& vec) {
    return Vector3D(x + vec.x, y + vec.y, z + vec.z);
}

Vector3D &Vector3D::operator+=(const Vector3D& vec) {
    x += vec.x;
    y += vec.y;
    z += vec.z;
    return *this;
}

Vector3D Vector3D::operator*(double val) {
    return Vector3D(x * val, y * val, z * val);
}

galaxy.cpp has some methods below:

void galaxy::updatePos(double dt) {
    pos += vel * dt;
}

void galaxy::updateVel(Vector3D accel, double dt) {
    vel += accel * dt;
}

void galaxy::updateStateVectors(Vector3D accel, double dt) {
    updateVel(accel, dt);
    updatePos(dt);
}

main.cpp calls method updateStateVectors as follows:

// within some while() loop

for (auto g : galaxies) {
    Vector3D accel = Vector3D();
    for (auto g2 : galaxies) {
        if (g != g2) {
            accel += g.getGravityAccelBy(g2);
        }
    }
    g.updateStateVectors(accel, dt);
}

(galaxies is a std::vector<galaxy>)

The problem is, the changes made to g.pos (or g.vel) within this for loop doesn’t seem to persist. Each iteration on the while loop, I see that g.pos is back to its original value, disregarding the change it previously underwent in the for loop. What am I missing?

This might end up being a very silly mistake but I mainly code in Python and I’ve spent more time on this than I’d like to admit.

>Solution :

auto returns a copy of the object, so the original one is copied, not modified. You’re then altering a copy of the original object.
Using & will make you reference the original one, so that you can alter it.

for (auto & g : galaxies) {
    Vector3D accel = Vector3D();
    for (auto & g2 : galaxies) {
        if (g != g2) {
            accel += g.getGravityAccelBy(g2);
        }
    }
    g.updateStateVectors(accel, dt);
}