Question

I am working with a memory manager that, on occasion, wants to defragment memory. Basically, I will go through a list of objects allocated by the memory manager and relocate them:

class A {
  SomeClass* data; // This member is allocated by the special manager
};

for(... each instance of A ...) 
    a.data = memory_manager.relocate(a.data);

memory_manager.relocate() will memcpy() the contents of data to a new location, and return the pointer.

Although it’s generally not idiomatic to memcpy() C++ classes, it seems to be a useful solution in this case, considering that I control the implementation of the (few) classes that will be used with the memory manager.

The problem is that one of those classes uses std::map, which is an opaque class as far as I am concerned. I certainly don’t imagine I can memcpy() it. I may not be able to use std::map in any case. For all I know it could allocate several pieces of memory.

The best workaround I can think of is simple enough. Due to the fact that the fragmented memory manager will put new allocations at more beneficial locations, all I need to do is allocate it anew and then delete the old:

for(... each instance of A ...) {
    stl::map<something>* tmp = a.the_map;
    a.the_map = new stl::map<something>(tmp);
    delete tmp;
}

In any case, this lead me to wonder:

Does C++ have semantics or idioms to move/copy an object into a specific memory location?

Is it possible to move the contents of an stl container to a specific memory location?

Edit: Although I didn’t point it out, I would obviously pass an allocator parameter to std::map. Based on the informative answers I got, I realize the workaround I posted in my initial question would probably be the only way to reduce fragmentation. By using the map’s copy constructor (and the allocator template parameter), all memory used by the map would be properly re-allocated.

As a comment pointed out, this is mostly a theoretical problem. Memory fragmentation is rarely something to worry about.

Answer 1

Everytime you insert a new key,value pair the map will allocate a node to store it. The details of how this allocation takes place are determined by the allocator that the map uses.

By default when you create a map as in std::map<K,V> the default allocator is used, which creates nodes on the heap (i.e., with new/delete).

You don’t want that, so you’ll have to create a custom allocator class that creates nodes as dictated by your memory manager.

Creating an allocator class is not trivial. This code shows how it can be done, you’ll have to adapt it to your own needs.

Once you have your allocator class (let’s say you call it MemManagerAllocator) you’ll have to define your map as std::map<K, V, MemManagerAllocator> and then use it like you would use a regular map.

Personally, I would need to have a really bad problem of memory fragmentation to go into all that trouble.