为什么 [capacity]string assert to []string 在 Golang 中会失败？

Question

I am using Golang1.14.

Here is the test code.

package main

import "time"

func checkData(data interface{}) {
    if _, ok := data.([]string); ok {
        println("Assert true.")
    } else {
        println("Assert false.")
    }
}

func main() {
    var years [20]string
    for i := 0; i < 20; i++ {
        years[i] = string(time.Now().Year() - 10 + i)
    }
    checkData(years)

    foods := []string{"Fruit", "Grass", "Fish", "Meat"}
    checkData(foods)
}

The output is:

Assert false.
Assert true.

I am new to Golang and really confusing that [20]string is not a []string .Can someone tell me why?Thanks.

Answer 1

[20]string is an array. It is a type that contains 20 strings, and if you pass it as an interface{}, you can recover it using intf.([20]string) .

[]string is a slice. It has a backing array, but it is essentially a view over an array. You assertion checks if the interface is a slice, so this one works.

Arrays and slices are different things in Go. An array is a data type with a fixed size. For instance:

func f(arr [10]int) {...}

You can only call f with an int array of size 10. When you do call it, the array will be passes as value, so the function will get a copy of the array, all 10 members of it. But:

func f(arr []int) {...}

You can call f with any size of slice. A slice contains a reference to its underlying array, so an array copy will not take place here. You cannot call this f` with an array.