Reflect Type.Field() order

Question

I can't seem to find it in documentation, is there any guarantee that the order from the fields will match the order declared in the struct? I know it seems like it would logically (due to memory layout),and it seems to perform this way too, but just making sure. I don't want code to break later on if this isn't a guarantee.

For example, if I had

type Foo struct {
    bar    string `tag:"bar"`
    baz    string `tag:"baz"`
    barbaz string `tag:"barbaz"`
}

and I ran this code:

var c Foo
t := reflect.TypeOf(c)
nf := t.NumField()

tags := make([]string, nf)

for f := 0; f < nf; f++ {
    tags[f] = t.Field(f).Tag.Get("tag")
}

Would tags be guaranteed to be ["bar", "baz", "barbaz"]?

Answer 1

I asked on golang-nuts about this, and got an answer from Ian Lance Taylor confirming it's declaration order, and will not change.

It's the order in which the fields appear in the struct declaration. It's not going to change. If you find a case where it is not the order in the declaration, please file a bug. Thanks.

Answer 2

Even though GC (the standard Go compiler) and GCCGO don't reoder struct fields today, I wouldn't rely on any ordering. There are no express guarantees in the documentation. This might be done in a future version of either compiler.

Field reordering is a technique used to memory-align fields inside of a struct without resorting to padding (unnecessarily inflating the struct's memory representation). You can read about it in the following question:

Why can't C compilers rearrange struct members to eliminate alignment padding?