When Should You Use `map::insert` vs. Bracket Notation in STL Maps?

Insert vs. Bracket Notation in STL Maps

In STL maps, the choice between using map::insert and the bracket notation (map[key] = value) for value insertion has often been a topic of debate. While the reference documentation claims insert serves only as a convenience, the question remains: is there any real advantage to using one over the other?

Map::insert vs. Bracket Notation

The map::insert function explicitly adds a key-value pair to the map and returns a pair indicating whether the insertion was successful and, if not, the iterator pointing to the existing key. On the other hand, the bracket notation assigns the value to the key, and if the key does not exist, it is created and the value is assigned to it.

Technical Advantage of Map::insert

The technical advantage of map::insert lies in its explicit nature. Unlike the bracket notation, which cannot differentiate between value updates and key creations, map::insert ensures that the operation is strictly an insertion. This can be useful in scenarios where you need to handle both scenarios separately, as the following example demonstrates:

<code class="cpp">std::map<int, std::string> map;
std::pair<std::map<int, std::string>::iterator, bool> res = map.insert(std::make_pair(key, value));
if (!res.second) {
  std::cout << "Key " << key << " already exists with value " << (res.first)->second << "\n";
} else {
  std::cout << "Created key " << key << " with value " << value << "\n";
}</code>

This code snippet ensures that when you use map::insert, you are clear on whether you are creating a new key or updating an existing one. It is particularly useful when dealing with scenarios where handling the two cases differently is important. However, for common use cases where the distinction is not necessary, the simpler bracket notation is often more preferred.

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