将array转换为interface {}进行切片，但结果不能使用len（）和其他方法

Question

I tried :

var a [100]int
func fun1(src interface{}) interface{} {
    src, _ = src.([100]int) // changed []int to [100]int
    fmt.Println(reflect.TypeOf(src)) // result: []int
    dest := make([]int, len(src))
    return dest
}

there is an error:

message: 'invalid argument src (type interface {}) for len'

But if I redefine a variable:

var a [100]int
func fun1(src interface{}) interface{} {
    slice_src, _ := src.([100]int) //changed []int to [100]int
    fmt.Println(reflect.TypeOf(slice_src)) // result: []int
    dest := make([]int, len(slice_src))
    return dest
}

it will be ok.

why reflect.TypeOf(src) will print []int after I used src.([]int) but error shows src is still interface{} ? I have checked this convert interface{} to int, but I still don't understand how to use correct conversion.

There is another question:

I changed the []int to [100]int since the type assertion before will return [] and false.

But if I don't know the type of a, how can I use type assertion to transfer an array (like[99]int) as a interface{} to function and return slice ([]int)?

Answer 1

when you first declare src, in fun1(src interface{}) you are making a variable of type interface. Which, of course cannot have len called on it.

The reason reflect.TypeOf says []int is due to how TypeOf works. It takes an interface{} and tells you the type of the thing in the interface{}

so, in the first example, you already had an interface and in the second example, go automatically created an interface{} instance to hold your []int slice.