How to create an array variable and allocate it later in Zig?

I'm trying to allocate some number of bytes to an uninitialized var of type u8 in a different scope, the array can not be const. The compiler keeps complaining about unmatching type:

error: expected type '*const[0:0]u8', found '[]u8'

What am I doing wrong, how can we do this?

Create an empty array, here one of type u8 (a string)
```
var my_array = "";
```

In another scope, allocate it to match some parameter

var arena = std.heap.ArenaAllocator.init(std.heap.page_allocator);
defer arena.deinit();
const allocator = arena.allocator();

if (true) {
  my_array = try allocator.alloc(u8, 4);
  defer allocator.free(my_array);
  std.mem.copyForwards(u8, my_array, "okok");
}

When building you get a type error:

error: expected type '*const[0:0]u8', found '[]u8'

However, it works when calling the allocator.alloc on a const array. But then I can not access it in the greater scope.

const my_array = try allocator.alloc(u8, 4);

Solution

The type of "" is *const[0:0]u8 (which is just a pointer) but allocator.alloc(u8, 4) returns []u8 (which consists of a pointer and a length), so you get a type mismatch.

To create a variable of type []u8 and leave it uninitialized, you can use

var my_array: []u8 = undefined;

This means you must not access my_array at all (e.g. use my_array.len) until you have assigned a valid slice value to it.

If you cannot guarantee this, you can initialize it as a valid, but empty, slice like this:

var my_array: []u8 = &.{};