double add_physics_vector(vector<double> v1, vector<double> v2)
{
    double horizontal_1 = v1[1] * cos(v1[0]);
    double vertical_1 = v1[1] * sin(v1[0]);
    double horizontal_2 = v2[1] * cos(v2[0]);
    double vertical_2 = v2[1] * sin(v2[0]);

    double horizontal_total = horizontal_1 + horizontal_2;
    double vertical_total = vertical_1 + vertical_2;
    double final_magnitude = sqrt(pow(horizontal_total, 2) + pow(vertical_total, 2));
    double final_direction = atan2(vertical_total, horizontal_total);
    double final_set[2] = {final_direction, final_magnitude};
    return final_set;
}

error: cannot convert ‘double*’ to ‘double’ in return
warning: control reaches end of non-void function [-Wreturn-type]

vector<double> final_set = {final_direction, final_magnitude};
return final_set;

vector<double> add_physics_vector(vector<double> v1, vector<double> v2);

pair<double, double> add_physics_vector(vector<double> v1, vector<double> v2)
{
    // ...as before
    return make_pair(final_direction, final_magitude);
}

double * make_array()
{
    double * array = malloc(sizeof(double) * 2);
    if (array == NULL) { /* .. error handling */ }
    return array;
}

struct Vector
{
    double direction;
    double magnitude;
}

Vector make_vector()
{
    vector = (Vector){ 1, 2 };
    return vector;
}