cpp/io/cin

The global objects and  control input from a stream buffer of implementation-defined type (derived from std), associated with the standard C input stream stdin.

These objects are guaranteed to be initialized during or before the first time an object of type std is constructed and are available for use in the constructors and destructors of static objects with ordered initialization (as long as is included before the object is defined).

Unless has been issued, it is safe to concurrently access these objects from multiple threads for both formatted and unformatted input.

Once is constructed,  returns, and likewise,  returns. This means that any formatted input operation on forces a call to  if any characters are pending for output.