It is not a cast. std::pair<const std::string, ...> and std::pair<std::string,...> are different types, although there is an implicit conversion. So a temporary is implicitly created and bound to the const reference. So not only there is a copy, you have a reference to an object that is destroyed at end of scope when you might expect it to live further.

Each entry in the map will be copied. In C++, const T& is allowed to bind to a temporary object (whose lifetime will be extended). So a new pair is implicitly constructed, and the reference binds to this object.