The zero value of a slice is not nil

Question

I was following the example https://tour.golang.org/moretypes/10 I modified the code expecting to get the same result. I did not. Is this a bug, or a documentation error? The to

Answer 1

The doc you referenced states that a nil slice has a length and capacity of 0, but not that every slice of length and capacity of zero is a nil slice. The specification only says that the value of an uninitialized slice is nil.

This is a convenience to support len and cap on slices which are uninitialised (nil). Otherwise we would need to check for non-nil first in order to avoid panic. (This also holds for other in-built types like maps or channels.)

In terms of the fmt.Print output, the difference in behaviour is similar to printing an uninitialised (nil) pointer vs pointer to an empty structure:

var s *struct{} // uninitialised pointer
fmt.Println(s)  // <nil>

s = &struct{}{} // pointer to an empty structure
fmt.Println(s)  // &{}