io_writer.zig

const std = @import("std");

const MyByteList = struct {
    data: [100]u8 = undefined,
    items: []u8 = &[_]u8{},

    const Writer = std.io.Writer(*MyByteList, error{EndOfBuffer}, appendWrite);

    fn appendWrite(self: *MyByteList, data: []const u8) error{EndOfBuffer}!usize {
        if (self.items.len + data.len > self.data.len) return error.EndOfBuffer;
        @memcpy(self.data[self.items.len..][0..data.len], data);
        self.items = self.data[0 .. self.items.len + data.len];
        return data.len;
    }

    fn writer(self: *MyByteList) Writer {
        return .{ .context = self };
    }
};

test "custom writer" {
    var bytes = MyByteList{};
    _ = try bytes.writer().write("Hello");
    _ = try bytes.writer().write(" Writer!");
    try std.testing.expect(std.mem.eql(u8, bytes.items, "Hello Writer!"));
}