Author: James CE Johnson

ACE Tutorial 016
Making ACE_Condition easier to use

We'll look first at the declaration of the wrapper class.

The way you use ACE_Condition is something like this:

- First, the setup...
  - Create a variable using your choice of data types
  - Create a mutex that will provide thread-safe access to that variable
  - Create an ACE_Condition that uses the mutex

- Waiting for the condition...

      the_mutex.acquire();
      while( the_variable != some_desired_state_or_value )
      	the_condition.wait();
      the_mutex.release();

  Note that when the_condition is created, it must be given a reference to the mutex. That's because the wait() method will release the mutex before waiting and reacquire it after being signaled.

- Setting the condition...

      the_mutex.acquire();
      the_variable = some_new_value_or_state;
      the_condition.signal() OR the_condition.broadcast()

The problem I have is remembering to setup everything and co-ordinate the locking, waiting and signaling. Even if I remember it all correctly it just makes my application code more complex than it should be.

To help out with that, I've created the class below to encapsulate the three elements necessary for the condition to work. I've then added methods for manipulation of the condition variable and waiting for the condition to occur.

    // page02.html,v 1.6 1999/04/03 21:56:31 jcej Exp

    #ifndef CONDITION_H
    #define CONDITION_H

    #include "ace/Synch.h"

    /** A wrapper for ACE_Condition<>.  When you're using an
        ACE_Condition<> you have to have three things: - Some variable
        that embodies the condition you're looking for - A mutex to
        prevent simultaneous access to that variable from different
        threads - An ACE_Condition<> that enables blocking on state
        changes in the variable The class I create here will contain those
        three things.  For the actual condition variable I've chosen an
        integer.  You could easily turn this class into a template
        parameterized on the condition variable's data type if 'int' isn't
        what you want.  */
    class Condition
    {
    public:
      // From here on I'll use value_t instead of 'int' to make any
      // future upgrades easier.
      typedef int value_t;

      // Initialize the condition variable
      Condition (value_t value = 0);
      ~Condition (void);

      /* I've created a number of arithmetic operators on the class that
         pass their operation on to the variable.  If you turn this into a
         template then some of these may not be appropriate...  For the
         ones that take a parameter, I've stuck with 'int' instead of
         'value_t' to reinforce the fact that you'll need a close look at
         these if you choose to change the 'value_t' typedef.  */

      // Increment & decrement
      Condition &operator++ (void);
      Condition &operator-- (void);

      // Increase & decrease
      Condition &operator+= (int inc);
      Condition &operator-= (int inc);

      // Just to be complete
      Condition &operator*= (int inc);
      Condition &operator/= (int inc);
      Condition &operator%= (int inc);

      // Set/Reset the condition variable's value
      Condition &operator= (value_t value);

      /* These four operators perform the actual waiting.  For instance:

         operator!=(int _value)

         is implemented as:

         Guard guard(mutex_)
         while( value_ != _value )
         condition_.wait();

         This is the "typical" use for condition mutexes.  Each of the
         operators below behaves this way for their respective
         comparisions.

         To use one of these in code, you would simply do:

         Condition mycondition;
         ...
         // Wait until the condition variable has the value 42
         mycondition != 42
         ...  */

      // As long as the condition variable is NOT EQUAL TO <value>, we wait
      int operator!= (value_t value);

      // As long as the condition variable is EXACTLY EQUAL TO <value>, we
      // wait
      int operator== (value_t value);

      // As long as the condition variable is LESS THAN OR EQUAL TO
      // <value>, we wait
      int operator<= (value_t value);

      // As long as the condition variable is GREATER THAN OR EQUAL TO
      // <value>, we wait
      int operator>= (value_t value);

      // Return the value of the condition variable
      operator value_t (void);

      /* In addition to the four ways of waiting above, I've also create a
         method that will invoke a function object for each iteration of
         the while() loop.  Derive yourself an object from
         Condition::Compare and overload operator()(value_t) to take
         advantage of this.  Have the function return non-zero when you
         consider the condition to be met.  */
      class Compare
      {
      public:
        virtual int operator() (value_t value) = 0;
      };

      /* Wait on the condition until _compare(value) returns non-zero.
         This is a little odd since we're not really testing equality.
         Just be sure that _compare(value_) will return non-zero when you
         consider the condition to be met.  */
      int operator== (Compare & compare);

    private:
      // Prevent copy construction and assignment.
      Condition (const Condition &condition);
      Condition &operator= (const Condition &condition);

      /* Typedefs make things easier to change later.
         ACE_Condition_Thread_Mutex is used as a shorthand for
         ACE_Condition<ACE_Thread_Mutex> and also because it may provide
         optimizations we can use.  */
      typedef ACE_Thread_Mutex mutex_t;
      typedef ACE_Condition_Thread_Mutex condition_t;
      typedef ACE_Guard<mutex_t> guard_t;

      // The mutex that keeps the data save
      mutex_t mutex_;

      // The condition mutex that makes waiting on the condition easier.
      condition_t *condition_;

      // The acutal variable that embodies the condition we're waiting
      // for.
      value_t value_;

      // Accessors for the two mutexes.
      mutex_t &mutex (void)
      {
        return this->mutex_;
      }

      condition_t &condition (void)
      {
        return *this->condition_;
      }

      // This particular accessor will make things much easier if we
      // decide that 'int' isn't the correct datatype for value_.  Note
      // that we keep this private and force clients of the class to use
      // the cast operator to get a copy of the value.
      value_t &value (void)
      {
        return this->value_;
      }
    };

    #endif /* CONDITION_H */

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