reflect: reflect.Value.IsZero

[lukescott]

Many encoding packages have a custom internal "isEmptyValue" function. The
functionality is pretty much identical. This expands to other packages that use reflect,
such as an ORM (is primary key set?).

It would be useful to have a standard "IsZero" method on reflect.Value,
something like this (copied from encoding/xml):

func (v Value) IsZero() bool {
    switch v.Kind() {
    case reflect.Array, reflect.Map, reflect.Slice, reflect.String:
        return v.Len() == 0
    case reflect.Bool:
        return !v.Bool()
    case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
        return v.Int() == 0
    case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
        return v.Uint() == 0
    case reflect.Float32, reflect.Float64:
        return v.Float() == 0
    case reflect.Interface, reflect.Ptr:
        return v.IsNil()
    }
    return false
}

[bradfitz]

Comment 1:

At least for Map and Slice, though, there's a difference between nil and Len 0.

[robpike]

Comment 2:

I believe this is correct:
func (v Value) IsZero() bool {
    switch v.Kind() {
    case reflect.Array, reflect.String:
        return v.Len() == 0
    case reflect.Bool:
        return !v.Bool()
    case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
        return v.Int() == 0
    case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
        return v.Uint() == 0
    case reflect.Float32, reflect.Float64:
        return v.Float() == 0
    case reflect.Interface, reflect.Map, reflect.Ptr, reflect.Slice:
        return v.IsNil()
    }
    return false
}
it seems a reasonable feature request.

Labels changed: added release-go1.4, repo-main.

Status changed to Accepted.

[extemporalgenome]

Comment 3:

r's suggested revision still slightly deviates from the language defined notion of a
type's zero value. Arrays and structs are zero if all their members have respective zero
values. Yes, zero-size types (such as with the above array length check) are guaranteed
to be their own zero value, but that's not a particularly common scenario.
Zero length slices and maps would only be zero if they're also nil.

[rsc]

Comment 4:

We could define IsZero but it won't be usable in any of the packages that define their
own 'emptiness' predicates.

[lukescott]

Comment 5:

Perhaps there could be an "IsZero" and "IsEmpty" method. "IsEmpty" would be the same as
"IsZero", but also include zero length slices and maps.
Including structs would be nice, although the current omitempty implementations in
encoding/* don't. However, that doesn't mean those implementations have to use "IsEmpty"
for Go 1. Third party / internal encoding libraries may choose to use the method though,
which still makes it useful to implement.

[rsc]

Comment 6:

Maybe for 1.5.

Labels changed: added release-go1.5, removed release-go1.4.