c++ - How do I force my std::map to deallocate memory used?

Question

I'm using a std::map, and I can't seem to free the memory back to the OS. It looks like,

int main(){
  aMap m;

  while(keepGoing){
    while(fillUpMap){
       //populate m
    }
    doWhatIwantWithMap(m);
    m.clear();//doesnt free memory back to OS

    //flush some buffered values into map for next iteration
    flushIntoMap(m);
  }
}

Each (fillUpmap) allocates around 1gig, so I'm very much interested in getting this back to my system before it eats up all my memory.

Ive experienced the same with std::vector, but there I could force it to free by doing a swap with an empty std::vector. This doesn't work with map.

When I use valgrind it says that all memory is freed, so its not a problem with a leak, since everything is cleared up nicely after a run.

edit:

The flush has to appear after the clear.

Answer 1

m.clear() releases memory back to the heap, but it's common for heap implementations to not release that back to the OS (even when they can, issues such as fragmentation make it hard).

This is how the default allocator works, if you've specified your own allocator for the map, it might have its own cache. However, even in that case, it should be cached in order to be reused immediately anyway.

Maps don't have a concept of capacity vs size the way vectors do.