switch

switch

switch.zig

const std = @import("std");
const expect = std.testing.expect;

test "switch simple" {
    const a: u64 = 10;
    const zz: u64 = 103;

    // All branches of a switch expression must be able to be coerced to a
    // common type.
    //
    // Branches cannot fallthrough. If fallthrough behavior is desired, combine
    // the cases and use an if.
    const b = switch (a) {
        // Multiple cases can be combined via a ','
        1, 2, 3 => 0,

        // Ranges can be specified using the ... syntax. These are inclusive
        // both ends.
        5...100 => 1,

        // Branches can be arbitrarily complex.
        101 => blk: {
            const c: u64 = 5;
            break :blk c * 2 + 1;
        },

        // Switching on arbitrary expressions is allowed as long as the
        // expression is known at compile-time.
        zz => zz,
        comptime blk: {
            const d: u32 = 5;
            const e: u32 = 100;
            break :blk d + e;
        } => 107,

        // The else branch catches everything not already captured.
        // Else branches are mandatory unless the entire range of values
        // is handled.
        else => 9,
    };

    expect(b == 1);
}

// Switch expressions can be used outside a function:
const os_msg = switch (std.Target.current.os.tag) {
    .linux => "we found a linux user",
    else => "not a linux user",
};

// Inside a function, switch statements implicitly are compile-time
// evaluated if the target expression is compile-time known.
test "switch inside function" {
    switch (std.Target.current.os.tag) {
        .fuchsia => {
            // On an OS other than fuchsia, block is not even analyzed,
            // so this compile error is not triggered.
            // On fuchsia this compile error would be triggered.
            @compileError("fuchsia not supported");
        },
        else => {},
    }
}
``` ```
$ zig test switch.zig
1/2 test "switch simple"... OK
2/2 test "switch inside function"... OK
All 2 tests passed.

switch can be used to capture the field values of a Tagged union. Modifications to the field values can be done by placing a * before the capture variable name, turning it into a pointer.

test.zig

const expect = @import("std").testing.expect;

test "switch on tagged union" {
    const Point = struct {
        x: u8,
        y: u8,
    };
    const Item = union(enum) {
        a: u32,
        c: Point,
        d,
        e: u32,
    };

    var a = Item{ .c = Point{ .x = 1, .y = 2 } };

    // Switching on more complex enums is allowed.
    const b = switch (a) {
        // A capture group is allowed on a match, and will return the enum
        // value matched. If the payload types of both cases are the same
        // they can be put into the same switch prong.
        Item.a, Item.e => |item| item,

        // A reference to the matched value can be obtained using `*` syntax.
        Item.c => |*item| blk: {
            item.*.x += 1;
            break :blk 6;
        },

        // No else is required if the types cases was exhaustively handled
        Item.d => 8,
    };

    expect(b == 6);
    expect(a.c.x == 2);
}
``` ```
$ zig test test.zig
1/1 test "switch on tagged union"... OK
All 1 tests passed.

Exhaustive Switching

When a switch expression does not have an else clause, it must exhaustively list all the possible values. Failure to do so is a compile error:

test.zig

const Color = enum {
    auto,
    off,
    on,
};

test "exhaustive switching" {
    const color = Color.off;
    switch (color) {
        Color.auto => {},
        Color.on => {},
    }
}
``` ```
$ zig test test.zig
./docgen_tmp/test.zig:9:5: error: enumeration value 'Color.off' not handled in switch
    switch (color) {
    ^
./docgen_tmp/test.zig:7:29: note: referenced here
test "exhaustive switching" {
                            ^

Switching with Enum Literals

Enum Literals can be useful to use with switch to avoid repetitively specifying enum or union types:

test.zig

const std = @import("std");
const expect = std.testing.expect;

const Color = enum {
    auto,
    off,
    on,
};

test "enum literals with switch" {
    const color = Color.off;
    const result = switch (color) {
        .auto => false,
        .on => false,
        .off => true,
    };
    expect(result);
}
``` ```
$ zig test test.zig
1/1 test "enum literals with switch"... OK
All 1 tests passed.