@TypeOf doesn't give type of struct field

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1 
const A = struct {
	a: u8,
};

then:

_ = @TypeOf(A.a);

results in:

error: struct ‘main.A’ has no member named ‘a’

but it DOES have a member named a! I’m missing something simple, I’m sure.

2

I think you need to use @typeInfo so that you can access the field types.

E.g. You could iterator through the type’s fields:

for (@typeInfo(A).@"struct".fields) |field| {
  const field_type = field.type;
  ...
}
3

A is the struct type, which doesn’t have a .a member, only an instance of type A has a .a member (which is a field of that instance).

You can use:

@FieldType(A, "a")
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4

I didn’t know about @FieldType. Thanks!

1 Like
5

It was added in 0.14.0 to make some code more performant and less awkward Proposal: add `@FieldType` · Issue #21702 · ziglang/zig · GitHub.

3 Likes
6

Ok, thank you both. I’ll concede that an INSTANCE of A would “rightly” have an A.a, but there is a simplicity to notion that @TypeOf(A.a) should indeed give the type of the a field in the A struct, which is known at comptime and plainly declared. I wonder why it’s not a proper use of @TypeOf. I would prefer it to having to put “a” in quotes. Oh well.

7

The @TypeOf evaluates the type of an expression. A.a isn’t a valid expression. There is simplicity in that too.

5 Likes
8

Agreed. Incorrect simplicity is simply incorrect. :slight_smile:

I might have guessed @FieldType to work as @FieldType(A.a) or something similar, but I guess it’s semantics and I DO agree to the need for an @FieldType, here, rather than an incorrect hijack of @TypeOf. Done.

9

I used to do @TypeOf(@as(A, undefined).a) before @FieldType was introduced.

3 Likes
10

Ah, clever! But only slightly less contrived than stringing out a typeInfo() construct. Good thing @FieldType came along, eh? :slight_smile:

I thought it worth mentioning, for posterity, that sometimes a different pattern altogether is worth considering. It’s what I did, in this case. So, the use-case is that: elsewhere in my code, I need a variable that is going to be the same type as that struct field. If that’s likely at all, it’s worth considering:

pub const Magnitude = u64;

Then, inside of the struct, for the field, magnitude: Magnitude, and anywhere else that a user may need that, likewise - import and use it.