Type converting slices of interfaces
Solution 1:
In Go, there is a general rule that syntax should not hide complex/costly operations. Converting a string
to an interface{}
is done in O(1) time. Converting a []string
to an interface{}
is also done in O(1) time since a slice is still one value. However, converting a []string
to an []interface{}
is O(n) time because each element of the slice must be converted to an interface{}
.
The one exception to this rule is converting strings. When converting a string
to and from a []byte
or a []rune
, Go does O(n) work even though conversions are "syntax".
There is no standard library function that will do this conversion for you. You could make one with reflect, but it would be slower than the three line option.
Example with reflection:
func InterfaceSlice(slice interface{}) []interface{} {
s := reflect.ValueOf(slice)
if s.Kind() != reflect.Slice {
panic("InterfaceSlice() given a non-slice type")
}
// Keep the distinction between nil and empty slice input
if s.IsNil() {
return nil
}
ret := make([]interface{}, s.Len())
for i:=0; i<s.Len(); i++ {
ret[i] = s.Index(i).Interface()
}
return ret
}
Your best option though is just to use the lines of code you gave in your question:
b := make([]interface{}, len(a))
for i := range a {
b[i] = a[i]
}
Solution 2:
The thing you are missing is that T
and interface{}
which holds a value of T
have different representations in memory so can't be trivially converted.
A variable of type T
is just its value in memory. There is no associated type information (in Go every variable has a single type known at compile time not at run time). It is represented in memory like this:
- value
An interface{}
holding a variable of type T
is represented in memory like this
- pointer to type
T
- value
So coming back to your original question: why go does't implicitly convert []T
to []interface{}
?
Converting []T
to []interface{}
would involve creating a new slice of interface {}
values which is a non-trivial operation since the in-memory layout is completely different.