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.

CodePudding user response：

The loop for (auto g : galaxies) { is creating a copy of elements of galaxies to g. Therefore, changes to g are not saved to galaxies.

You should use a reference to the elements like for (auto& g : galaxies) { instead.

disclaimer:

I googled

• site:stackoverflow.com [c ] range for not updated reference
• site:stackoverflow.com [c ] range-based-for not updated reference
• site:stackoverflow.com [c ] range-based-for not updated
• site:stackoverflow.com [c ] range-based change value

but couldn't find a duplicate.

CodePudding user response：

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);
}

CodePudding user response：

Try to use this way to see if it works.

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

bacause there is a temporary object(ex:temp_g) generated in for (auto g : galaxies), all changes are applied to the temporary object.