Go: How to check if a struct property was explicitly set to a zero value?

Question

type Animal struct {
    Name string
    LegCount int
}

snake := Animal{Name: "snake", LegCount: 0}
worm := Animal{Name: "worm"}

Question: How can I check snake and worm after they've been set, to tell that:

snake was explicitly set with a LegCount of 0.
The worm's LegCount was not explicitly set (and therefore based off of its default value)?

You can't with what you've shown. What is setting the values? Is it up to the programmer or via a package like json, database/sql? — JimB, Mar 15 '16 at 14:40
@JimB: Thank you. I'm going to be processing data from an API. I'm assuming there's a way to do this with Unmarshal but I didn't want to make the scope of the question too large. — Nathan Lippi, Mar 15 '16 at 14:42

Grzegorz Żur · Accepted Answer · 2016-03-15T14:46:48.950

12

It is simply impossible to distinguish.

If you are unmarshalling data from XML or JSON, use pointers.

type Animal struct {
    Name *string
    LegCount *int
}

You will get nil values for absent fields.

You can use the same convention in your case.

edited Mar 15 '16 at 14:46

answered Mar 15 '16 at 14:40

Grzegorz Żur

1 Answers1