CODEWITHSUNDEEP
c++stl
Yuri
Yuri

Reputation: 1219

C++ map, find element and insert it into another map (without copying)

How to correctly find element from source map and insert it into another map?

std::map<int, std::shared_prt<Obj>> src_map
std::map<int, std::shared_prt<Obj>> target_map

int key = 6;
auto found_elem = src_map.find(key);

if (found_elem != src_map.end()) {
  if (target_map.find(key) == target_map.end()) {
     target_map.insert(found_elem ); <---- How to correctly insert found element from src_map to target_map
  }
}

Upvotes: 0

Views: 661

Answers (2)

Jonathan Wakely
Jonathan Wakely

Reputation: 171263

  target_map.insert(found_elem);

found_elem is an iterator, you need to insert the value it refers to:

  target_map.insert(*found_elem);

Also this could be done more efficiently:

  if (target_map.find(key) == target_map.end()) {
     target_map.insert(found_elem);
  }

You do the lookup twice. Once in find and again in insert.

It's better to just try to insert it, and if you need to know whether it was inserted check the return value:

  auto inserted = target_map.insert(*found_elem);
  // inserted.first is the iterator to the element with the desired key
  // inserted.second is true if a new element was inserted, false if the key already existed

Other options for putting it in the map are to find the position where it belongs, then insert at that position if it's not there already:

auto lower = target_map.lower_bound(key);
if (lower == target_map.end() || lower->first != key) {
  target_map.insert(lower, *found_elem);
}

Another option is:

auto& val = target_map[found_elem->first];
if (!val)
  val = found_elem->second;

but this is not exactly the same, because if the key already exists in the map with an empty shared_ptr as the value then the value will get replaced. Depending whether you can have empty shared_ptr objects in the map that might not be correct for your program.

Yet another, with slightly different meaning again, is:

target_map[found_elem->first] = found_elem->second;

Upvotes: 5

Kuba
Kuba

Reputation: 839

In current declaration

std::map<int, Obj> src_map
std::map<int, Obj> target_map

You can't have one Obj instance in memory connected to both maps. Either you remove Obj from src_map and put in target_map or change declaration to;

std::map<int, Obj*> src_map
std::map<int, Obj*> target_map

or any other pointer type (shared_ptr as suggested in comment), without this you will always have two independent objects in memory.

Upvotes: 0

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