Zig interface example (Allocator)

interface_example_allocators.zig

const Ally = struct {
    data: ?*anyopaque,
    proc: *const fn(
        ally_data: ?*anyopaque, 
        mode: Mode, 
        count: usize, 
        alignment: usize, 
        old_memory: []u8,
    ) error{OutOfMemory}![]u8,

    fn alloc(ally: Ally, comptime T: type, count: usize) ![]T {
        const num_bytes = @sizeOf(T) * count;
        if (num_bytes == 0) return &.{};
        const buffer = try ally.proc(ally.data, .alloc, num_bytes, @alignOf(T), &.{});
        return std.mem.bytesAsSlice([]T, buffer);
    }
    fn realloc(ally: Ally, buffer: anytype, new_count: usize) ![]std.meta.Child(@TypeOf(buffer)) {
        const ElemType = std.meta.Child(@TypeOf(buffer));
        const old_bytes = std.mem.sliceAsBytes(buffer);
        const new_buffer = try ally.proc(ally.data, .realloc, new_count, @alignOf(ElemType), old_bytes);
        const new_elems = std.mem.bytesAsSlice([]ElemType, new_buffer);
        @memcpy(new_elems[0..buffer.len], buffer);
        return new_elems;
    }
    fn free(ally: Ally, buffer: anytype) void {
        const ElemType = std.meta.Child(@TypeOf(buffer));
        const old_bytes = std.mem.sliceAsBytes(buffer);
        _ = ally.proc(ally.data, .free, 0, 0, old_bytes) catch return;
    }

    const Mode = enum {
        alloc,
        realloc,
        free,
    };
};

const Arena = struct {
    // fields omitted for brevity
    
    fn ally(a: *Arena) Ally {
        return .{
            .data = a,
            .proc = ally_proc,
        };
    }

    fn ally_proc(ally_data: ?*anyopaque, mode: Mode, count: usize, alignment: usize, old_memory: []u8) error{OutOfMemory}![]u8 {
        const arena: *Arena = @ptrCast(@alignCast(ally_data.?));
        // NOTE: the cast is just in here, instead of making a wrapper proc for it!
      
        switch (mode) {
            .alloc => { ... },
            .realloc => { ... },
            .free => { ... },
        }

        return error.OutOfMemory;
    }
};