Slices
- Sentinel-Terminated Slices

Slices

test.zig

const assert = @import("std").debug.assert;
test "basic slices" {
    var array = [_]i32{ 1, 2, 3, 4 };
    // A slice is a pointer and a length. The difference between an array and
    // a slice is that the array's length is part of the type and known at
    // compile-time, whereas the slice's length is known at runtime.
    // Both can be accessed with the `len` field.
    var known_at_runtime_zero: usize = 0;
    const slice = array[known_at_runtime_zero..array.len];
    assert(&slice[0] == &array[0]);
    assert(slice.len == array.len);
    // Using the address-of operator on a slice gives a pointer to a single
    // item, while using the `ptr` field gives an unknown length pointer.
    assert(@TypeOf(slice.ptr) == [*]i32);
    assert(@TypeOf(&slice[0]) == *i32);
    assert(@ptrToInt(slice.ptr) == @ptrToInt(&slice[0]));
    // Slices have array bounds checking. If you try to access something out
    // of bounds, you'll get a safety check failure:
    slice[10] += 1;
    // Note that `slice.ptr` does not invoke safety checking, while `&slice[0]`
    // asserts that the slice has len >= 1.
}

$ zig test test.zig
1/1 test "basic slices"...index out of bounds
/deps/zig/docgen_tmp/test.zig:22:10: 0x204cc6 in test "basic slices" (test)
    slice[10] += 1;
         ^
/deps/zig/lib/std/special/test_runner.zig:47:28: 0x22bc1e in std.special.main (test)
        } else test_fn.func();
                           ^
/deps/zig/lib/std/start.zig:253:37: 0x20574d in std.start.posixCallMainAndExit (test)
            const result = root.main() catch |err| {
                                    ^
/deps/zig/lib/std/start.zig:123:5: 0x20548f in std.start._start (test)
    @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
    ^
Tests failed. Use the following command to reproduce the failure:
/deps/zig/docgen_tmp/test

This is one reason we prefer slices to pointers.

slices.zig

const std = @import("std");
const assert = std.debug.assert;
const mem = std.mem;
const fmt = std.fmt;
test "using slices for strings" {
    // Zig has no concept of strings. String literals are const pointers to
    // arrays of u8, and by convention parameters that are "strings" are
    // expected to be UTF-8 encoded slices of u8.
    // Here we coerce [5]u8 to []const u8
    const hello: []const u8 = "hello";
    const world: []const u8 = "世界";
    var all_together: [100]u8 = undefined;
    // You can use slice syntax on an array to convert an array into a slice.
    const all_together_slice = all_together[0..];
    // String concatenation example.
    const hello_world = try fmt.bufPrint(all_together_slice, "{} {}", .{ hello, world });
    // Generally, you can use UTF-8 and not worry about whether something is a
    // string. If you don't need to deal with individual characters, no need
    // to decode.
    assert(mem.eql(u8, hello_world, "hello 世界"));
}
test "slice pointer" {
    var array: [10]u8 = undefined;
    const ptr = &array;
    // You can use slicing syntax to convert a pointer into a slice:
    const slice = ptr[0..5];
    slice[2] = 3;
    assert(slice[2] == 3);
    // The slice is mutable because we sliced a mutable pointer.
    // Furthermore, it is actually a pointer to an array, since the start
    // and end indexes were both comptime-known.
    assert(@TypeOf(slice) == *[5]u8);
    // You can also slice a slice:
    const slice2 = slice[2..3];
    assert(slice2.len == 1);
    assert(slice2[0] == 3);
}

$ zig test slices.zig
1/2 test "using slices for strings"...OK
2/2 test "slice pointer"...OK
All 2 tests passed.

Sentinel-Terminated Slices

The syntax [:x]T is a slice which has a runtime known length and also guarantees a sentinel value at the element indexed by the length. The type does not guarantee that there are no sentinel elements before that. Sentinel-terminated slices allow element access to the len index.

null_terminated_slice.zig

const std = @import("std");
const assert = std.debug.assert;
test "null terminated slice" {
    const slice: [:0]const u8 = "hello";
    assert(slice.len == 5);
    assert(slice[5] == 0);
}

$ zig test null_terminated_slice.zig
1/1 test "null terminated slice"...OK
All 1 tests passed.