struct


1/4 test "dot product"...OK
2/4 test "struct namespaced variable"...OK
3/4 test "field parent pointer"...OK
4/4 test "linked list"...OK
All 4 tests passed.

Each struct field may have an expression indicating the default field value. Such expressions are executed at comptime, and allow the field to be omitted in a struct literal expression:

test.zig

const Foo = struct {
    a: i32 = 1234,
    b: i32,
};
test "default struct initialization fields" {
    const x = Foo{
        .b = 5,
    };
    if (x.a + x.b != 1239) {
        @compileError("it's even comptime known!");
    }
}

$ zig test test.zig
1/1 test "default struct initialization fields"...OK
All 1 tests passed.

An extern struct has in-memory layout guaranteed to match the C ABI for the target.

This kind of struct should only be used for compatibility with the C ABI. Every other use case should be solved with or normal struct.

See also:

extern enum

Unlike normal structs, packed structs have guaranteed in-memory layout:

Fields remain in the order declared.
There is no padding between fields.
Zig supports arbitrary width and although normally, integers with fewer than 8 bits will still use 1 byte of memory, in packed structs, they use exactly their bit width.
bool fields use exactly 1 bit.
A packed enum field uses exactly the bit width of its integer tag type.
A field uses exactly the bit width of the union field with the largest bit width.
Non-ABI-aligned fields are packed into the smallest possible ABI-aligned integers in accordance with the target endianness.

This means that a packed struct can participate in a @bitCast or a to reinterpret memory. This even works at comptime:

test.zig

const std = @import("std");
const builtin = std.builtin;
const assert = std.debug.assert;
const Full = packed struct {
    number: u16,
};
const Divided = packed struct {
    half1: u8,
    quarter3: u4,
    quarter4: u4,
};
test "@bitCast between packed structs" {
    doTheTest();
    comptime doTheTest();
}
fn doTheTest() void {
    assert(@sizeOf(Full) == 2);
    assert(@sizeOf(Divided) == 2);
    var full = Full{ .number = 0x1234 };
    var divided = @bitCast(Divided, full);
    switch (builtin.endian) {
        .Big => {
            assert(divided.half1 == 0x12);
            assert(divided.quarter3 == 0x3);
            assert(divided.quarter4 == 0x4);
        },
        .Little => {
            assert(divided.half1 == 0x34);
            assert(divided.quarter3 == 0x2);
            assert(divided.quarter4 == 0x1);
        },
    }
}

$ zig test test.zig
1/1 test "@bitCast between packed structs"...OK
All 1 tests passed.

Zig allows the address to be taken of a non-byte-aligned field:

const assert = std.debug.assert;
const BitField = packed struct {
    a: u3,
    b: u3,
    c: u2,
var foo = BitField{
    .a = 1,
    .b = 2,
    .c = 3,
};
test "pointer to non-byte-aligned field" {
    const ptr = &foo.b;
    assert(ptr.* == 2);
}

However, the pointer to a non-byte-aligned field has special properties and cannot be passed when a normal pointer is expected:

test.zig

const std = @import("std");
const assert = std.debug.assert;
const BitField = packed struct {
    a: u3,
    b: u3,
    c: u2,
};
var bit_field = BitField{
    .a = 1,
    .b = 2,
    .c = 3,
};
test "pointer to non-bit-aligned field" {
    assert(bar(&bit_field.b) == 2);
}
fn bar(x: *const u3) u3 {
    return x.*;
}

$ zig test test.zig
./docgen_tmp/test.zig:17:26: error: expected type '*const u3', found '*align(:3:1) u3'
    assert(bar(&bit_field.b) == 2);
                         ^

In this case, the function bar cannot be called becuse the pointer to the non-ABI-aligned field mentions the bit offset, but the function expects an ABI-aligned pointer.

Pointers to non-ABI-aligned fields share the same address as the other fields within their host integer:

test.zig

const std = @import("std");
const assert = std.debug.assert;
const BitField = packed struct {
    a: u3,
    b: u3,
    c: u2,
};
var bit_field = BitField{
    .a = 1,
    .b = 2,
    .c = 3,
};
test "pointer to non-bit-aligned field" {
    assert(@ptrToInt(&bit_field.a) == @ptrToInt(&bit_field.b));
    assert(@ptrToInt(&bit_field.a) == @ptrToInt(&bit_field.c));
}

$ zig test test.zig
1/1 test "pointer to non-bit-aligned field"...OK
All 1 tests passed.

This can be observed with and byteOffsetOf:

test.zig

const std = @import("std");
const assert = std.debug.assert;
const BitField = packed struct {
    a: u3,
    b: u3,
    c: u2,
};
    comptime {
        assert(@bitOffsetOf(BitField, "b") == 3);
        assert(@bitOffsetOf(BitField, "c") == 6);
        assert(@byteOffsetOf(BitField, "a") == 0);
        assert(@byteOffsetOf(BitField, "b") == 0);
        assert(@byteOffsetOf(BitField, "c") == 0);
    }
}

$ zig test test.zig
1/1 test "pointer to non-bit-aligned field"...OK
All 1 tests passed.

Packed structs have 1-byte alignment. However if you have an overaligned pointer to a packed struct, Zig should correctly understand the alignment of fields. However there is :

test.zig

$ zig test test.zig
./docgen_tmp/test.zig:8:32: error: expected type '*u32', found '*align(1) u32'
    const ptr_to_b: *u32 = &ptr.b;
                               ^

It's also planned to be able to set alignment of struct fields.

Using packed structs with is problematic, and may be a compile error in the future. For details on this subscribe to this issue. TODO update these docs with a recommendation on how to use packed structs with MMIO (the use case for volatile packed structs) once this issue is resolved. Don't worry, there will be a good solution for this use case in zig.

Since all structs are anonymous, Zig infers the type name based on a few rules.

If the struct is in the initialization expression of a variable, it gets named after that variable.
If the struct is in the return expression, it gets named after the function it is returning from, with the parameter values serialized.
Otherwise, the struct gets a name such as (anonymous struct at file.zig:7:38).

struct_name.zig

const std = @import("std");
pub fn main() void {
    const Foo = struct {};
    std.debug.warn("variable: {}\n", .{@typeName(Foo)});
    std.debug.warn("anonymous: {}\n", .{@typeName(struct {})});
    std.debug.warn("function: {}\n", .{@typeName(List(i32))});
}
fn List(comptime T: type) type {
    return struct {
        x: T,
    };
}

$ zig build-exe struct_name.zig
$ ./struct_name
variable: Foo
anonymous: struct:6:51
function: List(i32)

Zig allows omitting the struct type of a literal. When the result is , the struct literal will directly instantiate the result location, with no copy:

struct_result.zig

const std = @import("std");
const assert = std.debug.assert;
const Point = struct {x: i32, y: i32};
test "anonymous struct literal" {
    var pt: Point = .{
        .x = 13,
        .y = 67,
    };
    assert(pt.x == 13);
    assert(pt.y == 67);
}

$ zig test struct_result.zig
1/1 test "anonymous struct literal"...OK
All 1 tests passed.

The struct type can be inferred. Here the result location does not include a type, and so Zig infers the type:

struct_anon.zig

const std = @import("std");
const assert = std.debug.assert;
test "fully anonymous struct" {
    dump(.{
        .int = @as(u32, 1234),
        .float = @as(f64, 12.34),
        .b = true,
        .s = "hi",
    });
}
fn dump(args: var) void {
    assert(args.int == 1234);
    assert(args.float == 12.34);
    assert(args.b);
    assert(args.s[0] == 'h');
    assert(args.s[1] == 'i');
}