When STD: Map inserts an existing key, the content of the key is not updated.

When STD: Map inserts an existing key, the content of the key is not updated. If you do not know this, the running result may be inconsistent with the Expected One.

 

"Because element keys in a map are unique, the insertion Operation checks whether each inserted element has a key equivalent to the one of an element already in the container, and if so, the element is not inserted, returning an iterator to this existing element (if the function returns a value )."

Http://www.cplusplus.com/reference/map/map/insert/