tryemplace

Try_emplace is a member function of associative containers such as std::map and std::unordered_map introduced in C++17. It provides a way to insert a new element with a given key and a value constructed in place from forwarded arguments, but only if the key does not already exist in the container. This allows avoiding unnecessary construction costs when the key is already present.

The function returns a pair consisting of an iterator to the element and a boolean flag. If insertion succeeds, the boolean is true and the iterator points to the newly inserted element. If the key already exists, no new element is created, the boolean is false, and the iterator points to the existing element with that key. The arguments following the key are forwarded to construct the mapped value in place only when insertion occurs.

Usage is straightforward. For example, using std::map<int, std::string>:

- auto res = m.try_emplace(42, "Answer");

- if (res.second) { // inserted

- // res.first points to the new element

- } else {

- // key 42 already existed; res.first points to the existing element

- }

The primary advantage of try_emplace over other insertion methods is that it avoids constructing the mapped value if the key is already present, reducing unnecessary work and copies. It complements emplace by providing a safe, cost-efficient path for insertion when the existence of the key is uncertain. The exact signature is template<class... Args> pair<iterator, bool> try_emplace(const key_type& k, Args&&... args); with equivalent overloads for other key types and for unordered_map.