Inserting items efficiently and conditionally into std::map
Sometimes we want to fill a map with key-value pairs and while filling the map up, we might run into two different cases:
- The key does not exist yet. Create a fresh key-value pair.
- The key does exist already. Take the existing item and modify it.
We could just naively use the insert or emplace methods of map and see if they succeed. If it doesn't, we have case 2 and modify the existing item. In both cases, insert and emplace create the item which we try to insert, and in case 2 the freshly created item is dropped. We get a useless constructor call in both cases.
Since C++17, there is the try_emplace function, which enables us to create items only conditionally upon insertion. Let's implement a program that takes a list of billionaires and constructs a map that tells us the number of billionaires per country. In addition to that, it stores the wealthiest person in every country. Our example will not contain expensive to create items, but whenever we find ourselves in such a situation in real-life projects, we know how to master it with try_emplace.