49.3. Using CGaurdedObject to implement synchronized methods

The CGaurdedObject class provides a base class that can be used for classes that must require some or all of their methods only be run by one thread at a time. The class incorporates a mutex as private class data. It also provides Enter and Leave methods that take and release the mutex respectively.

Below is a typical use pattern for this class:

Example 49-5. Using CGaurdedObject

#include <GaurdedObject.h>              (1)

class MyClass : public CGaurdedObject    (2)
   void someProtecedMethod();            (3)

   Enter();                             (4)
   Leave();                             (5)
The header CGaurdedObject.h defines the CGaurdedObject class and must be included by classes that derive from that class.
The CGaurdedObject must be derived from to be useful. Here we define the MyClass as a derivation of CGaurdedObject. What this normally means is that MyClass has one or more methods that must be synchronized between threads. The concept of a method that can only run in one thread of execution was developed/discovered by Per Brinch-Hansen, and C.A. Hoare in 1972.
Invoking the Enter method of the base class enters the monitor. You can think of this call as your execution flow entering a gate that only admits one execution thread at a time. If a thread is already executing code following an Enter call any other thread will block until the corresponding Leave call is executed. Note that the underying mutex is a recursive mutex so that if a thread already in a monitor needs to call another protected method in the same class it will not block.
The Leave indicates the end of execution of a monitor. If there are threads waiting at an Enter for the same object one of them is scheduled for execution, and therefore enteres the monitor.

Note that the monitors implemented by CGaurdedObject are object global, that is only one thread can execute in an Enter, Leave over the entire object.