如何在 Zig 中的堆上创建 HashMap？

Question

The conventional way to make an object on the heap is to make a create fn:

const Something = struct {
   a:Allocator,
   b:[1000]u8,
   pub fn create(a:Allocator) !*Something {
      var mem = try a.create(Something)
      mem.* = {
       .a =a,
       .b = undefined
      };
      return mem;
   }
}

But what about if I want to put a std lib ArrayHashMap on the heap? For example:

const StringStringArrayHashMap = std.StringArrayHashMap([]const u8);

fn makeMap(a:Allocator) StringStringArrayHashMap {
   return StringStringArrayHashMap.init(a);
}

const WithMap = struct {
   map:StringStringHashMap
};

fn fillMap(a:Allocator) !WithMap {
   const map = makeMap(a);
   try map.put("a", "hello");
   try map.put("b", "world");
   return WithMap { .map = map };
}
fn badMemory(a:Allocator) !void {
   const with_map = fillMap(a);
   _ = with_map;
}

badMemory will receive a WithMap but it's internal map , having been made on the stack in fillMap will be freed at the end of fillMap and consequently unsade in badMemory .

I can't see any way to make a valid HashMap without somehow hacking the internals of the zig stdlib.

Answer 1

You can put the map on the heap the same way you did with Something type:

var map: *StringStringArrayHashMap = try allocator.create(StringStringArrayHashMap);
map.* = StringStringArrayHashMap.init(allocator);

badMemory will receive a WithMap but it's internal map , having been made on the stack in fillMap will be freed at the end of fillMap and consequently unsafe in badMemory .

This is false. The map is safe to use in badMemory as it's copied to the stack in badMemory . (Or maybe the compiler could decide to pass it as a pointer, I'm not sure if parameter's pass-by-value rule applies to the return value. Doesn't matter here.) But you should probably be careful when copying the map or you might step into the same issue as this question .