如何在golang中动态更改类型定义？

Question

I have come across a scenario where I have to use a variable to count up to, say, 255 or 65535 (depending on a flag) and overflow to 0. So when I set the flag to say, true, the type definition should be uint8 and when false, the type definition should be uint16. Obviously I can set the master variable as uint16 and perform mathematical manipulations on them, but is there a native solution for this?

Answer 1

Go is a statically typed language, which means types of variables must be known at compile time, and you can't change their type at runtime.

Counter with a struct

Instead create your own counter type with a method responsible to increment its value, which can check if the counter's max value is exceeded, in which case it can reset it.

type Counter struct {
    value, Max int
}

func (c *Counter) Inc() {
    c.value++
    if c.value > c.Max {
        c.value = 0
    }
}

func (c *Counter) Value() int { return c.value }

Example using it:

c := &Counter{Max: 255}

for i := 0; i < 257; i++ {
    c.Inc()
    if i > 252 {
        fmt.Println(c.Value())
    }
}

Output (try it on the Go Playground):

254
255
0
1

So in your case use it like this:

c := &Counter{}
if flagUse8bit {
    c.Max = 0xff
} else {
    c.Max = 0xffff
}

Counter with interfaces

Another nice solution would be to use interfaces, as a variable of interface type can hold a value of any type that implements that interface.

Create a Counter interface type:

type Counter interface {
    Inc()
    Value() int
}

And have different implementations for different types, simply using different underlying types, exploiting the fact that overflow will happen naturally. And you may choose the runtime type to store in the Counter interface variable based on the CLI flag value.

type counter8 uint8

func (c *counter8) Inc()      { *c++ }
func (c counter8) Value() int { return int(c) }

type counter16 uint16

func (c *counter16) Inc()      { *c++ }
func (c counter16) Value() int { return int(c) }

And using it:

var c Counter

c = new(counter8)
for i := 0; i < 257; i++ {
    c.Inc()
    if i > 252 {
        fmt.Println(c.Value())
    }
}

c = new(counter16)
for i := 0; i < 65537; i++ {
    c.Inc()
    if i > 65532 {
        fmt.Println(c.Value())
    }
}

Output (try it on the Go Playground):

254
255
0
1
65534
65535
0
1

So in your case use it like this:

var c Counter
if flagUse8bit {
    c = new(counter8)
} else {
    c = new(counter16)
}