带参数的 C++ 转换运算符

Question

Is there any way to define a conversion operator that takes a parameter?

Here is my use case:

class RGBColor
{
    operator RGBAColor (const float alpha = 1.0) const noexcept;
}

I have conversion operators to/from HSB and RGB colors, and RGBA to RGB (by dropping the alpha), but I'm having difficulty converting from RGB to RGBA since I need to the ability to supply the alpha as a parameter (rather than always defaulting to one).

I assume that I'm going to have to fall back to something like:

RGBAColor ToRGBAColor (const float alpha = 1.0) const noexcept;

However, I would prefer to use standard C++ conversion, so I'm just wondering if there's any way to use a conversion operator that takes a parameter.

Answer 1

This is literally what constructors are for.

Declare and define one.

Answer 2

There's no way to pass additional parameters to a cast operator. The syntax doesn't allow that.

As mentioned in comments and in the other answer, provide an appropriate constructor instead:

struct RGB {
    float r_;
    float g_;
    float b_;
};

struct RGBA : RGB {
    float alpha_;

    RGBA(const RGB& rgb) : RGB(rgb), alpha_(1.0) {}
    RGBA(const RGB& rgb, float alpha) : RGB(rgb), alpha_(alpha) {} // <<<<
    RGBA& operator=(const RGB& rgb) {
        *static_cast<RGB*>(this) = rgb;
        alpha_ = 1.0;
        return *this;
    }
};