Day 11: Many item pointers

The 11th day of my 1000 days of Zig was spent on the Zig guide, reviewing some of the articles on the page.

Many item pointers

Most programs in Zig need to keep track of buffers which don’t have compile-time known lengths. Many-item pointers are used for these. These act similarly to their single-item counterparts, using the syntax [*]T instead of *T. Many-item pointers differ from single-item pointers in the following major ways:

1. Single-item pointers are dereferenceable, e.g., ptr.*, while many-item pointers are not.
2. Many-item pointers are indexable, unlike single-item pointers. With many-item pointers, we can access elements using ptr[0].
3. Single-item pointers support arithmetic, e.g., ptr + 1 or ptr - 1, while many-item pointers do not.
4. Single-item pointers allow for any size, including unknown, while many-item pointer sizes must be known.
5. Lastly, while many-item pointers allow for coercion from an array pointer, single-item pointers do not.

In this example code, we’ve written a function that can take in a buffer of any length. Notice how a single-item pointer to an array of bytes coerces into a many-item pointer of bytes.

fn doubleAllManypointer(buffer: [*]u8, byte_count: usize) void {
    var i: usize = 0;
    while (i < byte_count) : (i += 1) buffer[i] *= 2;
}

test "many-item pointers" {
    var buffer: [100]u8 = [_]u8{1} ** 100;
    const buffer_ptr: *[100]u8 = &buffer;

    const buffer_many_ptr: [*]u8 = buffer_ptr;
    doubleAllManypointer(buffer_many_ptr, buffer.len);
    for (buffer) |byte| try expect(byte == 2);

    const first_elem_ptr: *u8 = &buffer_many_ptr[0];
    const first_elem_ptr_2: *u8 = @ptrCast(buffer_many_ptr);
    try expect(first_elem_ptr == first_elem_ptr_2);
}

We can convert from a many-item pointer to a single-item pointer by either indexing an element and dereferencing that, or by using @ptrCast to cast the pointer type. This is only valid when the buffer has a length of at least 1.

What I Learned Today

• many-item pointers in Zig

Looking Ahead

Tomorrow I will continue with Ziglings, getting deeper into it, and learning more new things about Zig.

References

1. Ziglings
2. Many item Pointers