Go中恒定的全局用户类型值

Question

I want to arrange to compute a value that will not change after initialization.

I'd use const, but Go restricts consts to built-in types, IIUC.

So I figured I'd use var, and compute their initial values in init()

var (
    // ScreenBounds is the visible screen
    ScreenBounds types.Rectangle

    // BoardBounds is the total board space
    BoardBounds  types.Rectangle
)

func init() {
    ScreenBounds := types.RectFromPointSize(
        types.Pt(-ScreenWidth/2, 0),
        types.Pt(ScreenWidth, ScreenHeight))

    BoardBounds := ScreenBounds
    BoardBounds.Max.Y += TankSpeed * TotalFrames
}

Which is pretty good - but is there a way to "lock" the values once computed, other than to change the vars to unexported names, and then uses function-accessors to return their values?

Answer 1

No, there is not. Variables are called that because their values can be changed. In Go there is no "final" or similar modifier. Simplicity of the language.

Only way to guard a variable from being changed from the outside is to make it unexported, and yes, then you need exported functions to get their values.

A workaround could be to not use variables but constants. Yes, you can't have struct constants, but if the structs are small, you may use its fields as separate constants, e.g.:

const (
    ScreenMinX = ScreenWidth / 2
    ScreenMinY = ScreenHeight / 2
    ScreenMaxX = ScreenWidth
    ScreenMaxY = ScreenHeight
)