C++ passing “default” struct to function

Question

I'm trying to figure out how I can "pass default struct values" to a function without initializing the struct, this is what I have now:

struct Color3i
{
    Color3i(): r(255), g(255), b(255) { }
    int r, g, b;
};

void CCore::Color(Color3i color)
{
    double red, green, blue;
    red = color.r / 255.0f;
    green = color.g / 255.0f;
    blue = color.b / 255.0f;

    glColor3f(red,green,blue);
}

Color3i defaultColor;
Core.Color(defaultColor);

What I'm trying to do would look like this but this clearly doesn't work:

Core.Color(Color3i defaultColor);

How would I pass the struct to the function without initializing it with Color3i defaultColor; is this possible?

Sorry if this has been asked before but I tried searching the interwebs but I couldn't find anything (maybe I'm using the wrong keywords to search)

Answer 1

You should just be able to do this:

Core.Color(Color3i());

That is, call the default constructor to initialize a new instance, and pass it.

Answer 2

There are a few ways to pass an information that you want the color to be default. The simplest one is given in the comments already:

Core.Color(Color3i())

If you want the code to be shorter, you can set the default value in the function parameter you invoke:

void CCore::Color(Color3i color = Color3i()) { ... }

....
Core.Color(); //no argument = default

If you want to be more descriptive instead, you can create a static function acting as a constructor in the Color :

struct Color3i {
    Color3i(): r(255), g(255), b(255) { }
    int r, g, b;
    static Colo3i defaultColor() { return Color3i(); }
};
...
Core.Color(Color3i::defaultColor());

Finally, if you want to control the context where the default color can be used, you can create a new dummy enum type and overload the function(s) to accept the default version explicitly:

enum DefaultColorEnum {
    DefaultColor
};

void CCore::Color(Color3i color) {
    ... //normal code
}

void CCore::Color(DefaultColorEnum) { //ignore the parameter value, the type is what matters
    Color(Color3i()); //invoke the generic version with default value
}

...

Core.Color(DefaultColor);

It all depends on what you actually wants to achieve...