Question

Yesterday i tried to use std::unordered_map and this code confused me how much memory it used.

typedef list<string> entityId_list;
struct tile_content {
   char cost;
   entityId_list entities;
};
unordered_map<int, tile_content> hash_map;

for (size_t i = 0; i < 19200; i++) {
   tile_content t;
   t.cost = 1;
   map[i] = t;
}

All this parts of code was compiled in MS VS2010 in debug mode.
What I’ve been seen in my task manager was about 1200 kb of “clean” process, but after filling hash_map it uses 8124 kb of memory. Is it normal behavior of unordered_map? Why so much memory used?

Answer 1

The unordered_map structure is designed to hold large numbers of objects in a way that makes adds, deletes, lookups, and orderless traverses efficient. It’s not meant to be memory-efficient for small data structures. To avoid the penalties associated with resizing, it allocates many hash chain heads when it’s first created.