union

test.zig

$ zig test test.zig
Test [1/1] test "simple union"... 
thread 11537 panic: access of inactive union field
/home/andy/Downloads/zig/docgen_tmp/test.zig:8:12: 0x20677b in test "simple union" (test)
    payload.float = 12.34;
           ^
/home/andy/Downloads/zig/lib/std/special/test_runner.zig:76:28: 0x22d05a in std.special.main (test)
        } else test_fn.func();
                           ^
/home/andy/Downloads/zig/lib/std/start.zig:471:37: 0x2255ba in std.start.callMain (test)
            const result = root.main() catch |err| {
                                    ^
/home/andy/Downloads/zig/lib/std/start.zig:413:12: 0x20943e in std.start.callMainWithArgs (test)
    return @call(.{ .modifier = .always_inline }, callMain, .{});
           ^
/home/andy/Downloads/zig/lib/std/start.zig:332:17: 0x208466 in std.start.posixCallMainAndExit (test)
    std.os.exit(@call(.{ .modifier = .always_inline }, callMainWithArgs, .{ argc, argv, envp }));
                ^
/home/andy/Downloads/zig/lib/std/start.zig:245:5: 0x208272 in std.start._start (test)
    @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
    ^
error: the following test command crashed:
docgen_tmp/zig-cache/o/333a8741816702dafd0ff99af380ebb1/test /home/andy/Downloads/zig/build-release/zig

You can activate another field by assigning the entire union:

test.zig

const std = @import("std");
const expect = std.testing.expect;
const Payload = union {
    int: i64,
    float: f64,
};
test "simple union" {
    var payload = Payload{ .int = 1234 };
    payload = Payload{ .float = 12.34 };
    try expect(payload.float == 12.34);
}

$ zig test test.zig
Test [1/1] test "simple union"... 
All 1 tests passed.

In order to use with a union, it must be a Tagged union.

To initialize a union when the tag is a -known name, see @unionInit.

test.zig

$ zig test test.zig
Test [1/3] test "switch on tagged union"... 
Test [2/3] test "get tag type"... 
Test [3/3] test "coerce to enum"... 
All 3 tests passed.

In order to modify the payload of a tagged union in a switch expression, place a * before the variable name to make it a pointer:

test.zig

const std = @import("std");
const expect = std.testing.expect;
const ComplexTypeTag = enum {
    ok,
    not_ok,
};
const ComplexType = union(ComplexTypeTag) {
    ok: u8,
    not_ok: void,
};
test "modify tagged union in switch" {
    var c = ComplexType{ .ok = 42 };
    try expect(@as(ComplexTypeTag, c) == ComplexTypeTag.ok);
        ComplexTypeTag.ok => |*value| value.* += 1,
        ComplexTypeTag.not_ok => unreachable,
    try expect(c.ok == 43);
}

$ zig test test.zig
Test [1/1] test "modify tagged union in switch"... 
All 1 tests passed.

Unions can be made to infer the enum tag type. Further, unions can have methods just like structs and enums.

test.zig

$ zig test test.zig
Test [1/1] test "union method"... 
All 1 tests passed.

test.zig

const std = @import("std");
const expect = std.testing.expect;
const Small2 = union(enum) {
    a: i32,
    b: bool,
    c: u8,
};
test "@tagName" {
    try expect(std.mem.eql(u8, @tagName(Small2.a), "a"));
}

$ zig test test.zig
Test [1/1] test "@tagName"... 
All 1 tests passed.

An extern union has memory layout guaranteed to be compatible with the target C ABI.