[container.insert.return]
23 Containers library [containers]
23.2 Requirements [container.requirements]
23.2.6 Insert return type [container.insert.return]
The associative containers with unique keys and the unordered containers with unique keys have a member function insert that returns a nested type insert_return_type.
That return type is a specialization of the template specified in this subclause.
template<class Iterator, class NodeType> struct insert-return-type { Iterator position; bool inserted; NodeType node; };