Constructor with references not properly assigning??

Question

I'm trying to write a simple color class that's supposed to be as versatile as possible. Here's what it looks like:

class MyColor {
private:
uint8 v[4];
public:
uint8 &r, &g, &b, &a;

MyColor() : r(v[0]),  g(v[1]), b(v[2]), a(v[3]) {}
MyColor(uint8 red, uint8 green, uint8 blue, uint8 alpha = 255) : r(v[0]),  g(v[1]), b(v[2]), a(v[3]) {
    printf("%d, %d, %d, %d\n", red, green, blue, alpha);
    r = red;
    g = green;
    b = blue;
    a = alpha;
}
MyColor(uint8 vec[]) : r(v[0]),  g(v[1]), b(v[2]), a(v[3]) {
    MyColor(vec[0], vec[1], vec[2], vec[3]);
}
uint8 operator [](int i) {
    return v[i];
}
operator const GLubyte*() {
    return v;
}
};

And here's the code I'm trying:

uint8 tmp[] = {1,2,3,4};
MyColor c(tmp);
printf("%d, %d, %d, %d\n", c.r, c.g, c.b, c.a);

(I would have liked it if I could have done MyColor c = {1,2,3,4} but I'm not sure that's possible in the current spec?)

Anyway, it outputs this:

1, 2, 3, 4
112, 22, 104, 89

So the values it gets in the 2nd constructor are correct, but when it returns... those values are random??

r = red should set both r and v[0] to red shouldn't it? Since r is just a reference to v[0] they are actually share the same value, no? I'm not doing some weird reassigning of the reference to somewhere in space am I?

Answer 1

Unfortunately, you can't do constructor forwarding at the moment in C++. The issue is here:

MyColor(uint8 vec[]) : r(v[0]),  g(v[1]), b(v[2]), a(v[3]) {
    MyColor(vec[0], vec[1], vec[2], vec[3]);
}

What this actually does is to bind the references to the member vector v and then in the body of the constructor create a temporary MyColor value which is then thrown away.

The second line in your output is printing the garbage initial values of the member vector v of the constructed MyColor.

I'd recommend breaking out the value assign part of the constructor taking 4 uint8s and calling that from both constructors.

void AssignColorValues( uint8 red, uint8 green, uint8 blue, uint8 alpha)
{
    printf("%d, %d, %d, %d\n", red, green, blue, alpha);
    r = red;
    g = green;
    b = blue;
    a = alpha;
}

MyColor(uint8 red, uint8 green, uint8 blue, uint8 alpha = 255) : r(v[0]),  g(v[1]),     b(v[2]), a(v[3])
{
    AssignColorValues( red, green, blue, alpha );
}

MyColor(uint8 vec[]) : r(v[0]),  g(v[1]), b(v[2]), a(v[3])
{
    AssignColorValues(vec[0], vec[1], vec[2], vec[3]);
}

Answer 2

Actually, I believe that you can do most of this today. boost::array can be initialized from a constant expression, so you should be able to as well. I believe that you will have to get rid of the other constructors, however. You can keep assignment operators, if that helps.