Haven't got an example online to demonstrate this vividly. Saw an example at http://en.cppreference.com/w/cpp/header/shared_mutex but it is still unclear. Can somebody help?
By use of normal mutexes, you can guarantee exclusive access to some kind of critical resource – and nothing else. Shared mutexes extend this feature by allowing two levels of access: shared and exclusive as follows:
A typical scenario is a database: It does not matter if several threads read one and the same data simultaneously. But modification of the database is critical - if some thread reads data while another one is writing it might receive inconsistent data. So all reads must have finished before writing is allowed and new reading must wait until writing has finished. After writing, further reads can occur simultaneously again.
Edit: Sidenote:
Why readers need a lock?
This is to prevent the writer from acquiring the lock while reading yet occurs. Additionally, it prevents new readers from acquiring the lock if it is yet held exclusively.