union

A bare union defines a set of possible types that a value can be as a list of fields. Only one field can be active at a time. The in-memory representation of bare unions is not guaranteed. Bare unions cannot be used to reinterpret memory. For that, use @ptrCast, or use an extern union or a packed union which have guaranteed in-memory layout. Accessing the non-active field is safety-checked Undefined Behavior:

test.zig

  1. const Payload = union {
  2. int: i64,
  3. float: f64,
  4. boolean: bool,
  5. };
  6. test "simple union" {
  7. var payload = Payload{ .int = 1234 };
  8. payload.float = 12.34;
  9. }
  1. $ zig test test.zig
  2. 1/1 test "simple union"... access of inactive union field
  3. /home/andy/Downloads/zig/docgen_tmp/test.zig:8:12: 0x2059db in test "simple union" (test)
  4. payload.float = 12.34;
  5. ^
  6. /home/andy/Downloads/zig/lib/std/special/test_runner.zig:61:28: 0x22daf1 in std.special.main (test)
  7. } else test_fn.func();
  8. ^
  9. /home/andy/Downloads/zig/lib/std/start.zig:334:37: 0x2072dd in std.start.posixCallMainAndExit (test)
  10. const result = root.main() catch |err| {
  11. ^
  12. /home/andy/Downloads/zig/lib/std/start.zig:162:5: 0x207012 in std.start._start (test)
  13. @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
  14. ^
  15. error: the following test command crashed:
  16. docgen_tmp/zig-cache/o/8fe80128a94c1613e48317295fc76cfc/test

You can activate another field by assigning the entire union:

test.zig

  1. const std = @import("std");
  2. const expect = std.testing.expect;
  3. const Payload = union {
  4. int: i64,
  5. float: f64,
  6. boolean: bool,
  7. };
  8. test "simple union" {
  9. var payload = Payload{ .int = 1234 };
  10. expect(payload.int == 1234);
  11. payload = Payload{ .float = 12.34 };
  12. expect(payload.float == 12.34);
  13. }
  1. $ zig test test.zig
  2. 1/1 test "simple union"... OK
  3. All 1 tests passed.

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

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

Tagged union

Unions can be declared with an enum tag type. This turns the union into a tagged union, which makes it eligible to use with switch expressions. One can use @TagType to obtain the enum type from the union type. Tagged unions coerce to their tag type: Type Coercion: unions and enums.

test.zig

  1. const std = @import("std");
  2. const expect = std.testing.expect;
  3. const ComplexTypeTag = enum {
  4. ok,
  5. not_ok,
  6. };
  7. const ComplexType = union(ComplexTypeTag) {
  8. ok: u8,
  9. not_ok: void,
  10. };
  11. test "switch on tagged union" {
  12. const c = ComplexType{ .ok = 42 };
  13. expect(@as(ComplexTypeTag, c) == ComplexTypeTag.ok);
  14. switch (c) {
  15. ComplexTypeTag.ok => |value| expect(value == 42),
  16. ComplexTypeTag.not_ok => unreachable,
  17. }
  18. }
  19. test "@TagType" {
  20. expect(@TagType(ComplexType) == ComplexTypeTag);
  21. }
  22. test "coerce to enum" {
  23. const c1 = ComplexType{ .ok = 42 };
  24. const c2 = ComplexType.not_ok;
  25. expect(c1 == .ok);
  26. expect(c2 == .not_ok);
  27. }
  1. $ zig test test.zig
  2. 1/3 test "switch on tagged union"... OK
  3. 2/3 test "@TagType"... OK
  4. 3/3 test "coerce to enum"... OK
  5. 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

  1. const std = @import("std");
  2. const expect = std.testing.expect;
  3. const ComplexTypeTag = enum {
  4. ok,
  5. not_ok,
  6. };
  7. const ComplexType = union(ComplexTypeTag) {
  8. ok: u8,
  9. not_ok: void,
  10. };
  11. test "modify tagged union in switch" {
  12. var c = ComplexType{ .ok = 42 };
  13. expect(@as(ComplexTypeTag, c) == ComplexTypeTag.ok);
  14. switch (c) {
  15. ComplexTypeTag.ok => |*value| value.* += 1,
  16. ComplexTypeTag.not_ok => unreachable,
  17. }
  18. expect(c.ok == 43);
  19. }
  1. $ zig test test.zig
  2. 1/1 test "modify tagged union in switch"... OK
  3. 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

  1. const std = @import("std");
  2. const expect = std.testing.expect;
  3. const Variant = union(enum) {
  4. int: i32,
  5. boolean: bool,
  6. // void can be omitted when inferring enum tag type.
  7. none,
  8. fn truthy(self: Variant) bool {
  9. return switch (self) {
  10. Variant.int => |x_int| x_int != 0,
  11. Variant.boolean => |x_bool| x_bool,
  12. Variant.none => false,
  13. };
  14. }
  15. };
  16. test "union method" {
  17. var v1 = Variant{ .int = 1 };
  18. var v2 = Variant{ .boolean = false };
  19. expect(v1.truthy());
  20. expect(!v2.truthy());
  21. }
  1. $ zig test test.zig
  2. 1/1 test "union method"... OK
  3. All 1 tests passed.

@tagName can be used to return a comptime []const u8 value representing the field name:

test.zig

  1. const std = @import("std");
  2. const expect = std.testing.expect;
  3. const Small2 = union(enum) {
  4. a: i32,
  5. b: bool,
  6. c: u8,
  7. };
  8. test "@tagName" {
  9. expect(std.mem.eql(u8, @tagName(Small2.a), "a"));
  10. }
  1. $ zig test test.zig
  2. 1/1 test "@tagName"... OK
  3. All 1 tests passed.

extern union

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

See also:

packed union

A packed union has well-defined in-memory layout and is eligible to be in a packed struct.

Anonymous Union Literals

Anonymous Struct Literals syntax can be used to initialize unions without specifying the type:

anon_union.zig

  1. const std = @import("std");
  2. const expect = std.testing.expect;
  3. const Number = union {
  4. int: i32,
  5. float: f64,
  6. };
  7. test "anonymous union literal syntax" {
  8. var i: Number = .{.int = 42};
  9. var f = makeNumber();
  10. expect(i.int == 42);
  11. expect(f.float == 12.34);
  12. }
  13. fn makeNumber() Number {
  14. return .{.float = 12.34};
  15. }
  1. $ zig test anon_union.zig
  2. 1/1 test "anonymous union literal syntax"... OK
  3. All 1 tests passed.