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<!-- $Id$ -->
<HTML>
<HEAD>
<META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=iso-8859-1">
<META NAME="Author" CONTENT="James CE Johnson">
<TITLE>ACE Tutorial 016</TITLE>
</HEAD>
<BODY TEXT="#000000" BGCOLOR="#FFFFFF" LINK="#000FFF" VLINK="#FF0F0F">
<CENTER><B><FONT SIZE=+2>ACE Tutorial 016</FONT></B></CENTER>
<CENTER><B><FONT SIZE=+2>Making ACE_Condition easier to use</FONT></B></CENTER>
<P>
<HR WIDTH="100%">
We'll look first at the declaration of the wrapper class.
<P>
The way you use ACE_Condition is something like this:
<UL>
<LI>First, the setup...
<UL>
<LI>Create a variable using your choice of data types
<LI>Create a mutex that will provide thread-safe access to that
variable
<LI>Create an ACE_Condition that uses the mutex
</UL>
<P>
<LI>Waiting for the condition...
<UL>
<PRE>
the_mutex.acquire();
while( the_variable != some_desired_state_or_value )
the_condition.wait();
the_mutex.release();
</PRE>
Note that when <i>the_condition</i> 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.
</UL>
<P>
<LI>Setting the condition...
<UL>
<PRE>
the_mutex.acquire();
the_variable = some_new_value_or_state;
the_condition.signal() <i>OR</i> the_condition.broadcast()
</pre>
</UL>
</UL>
<P>
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.
<P>
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.
<HR>
<PRE>
<font color=red>// $Id$</font>
<font color=blue>#ifndef</font> <font color=purple>CONDITION_H</font>
<font color=blue>#define</font> <font color=purple>CONDITION_H</font>
<font color=blue>#include</font> "<A HREF="../../../ace/Synch.h">ace/Synch.h</A>"
<font color=red>/** 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. */</font>
class Condition
{
public:
<font color=red>// From here on I'll use value_t instead of 'int' to make any</font>
<font color=red>// future upgrades easier.</font>
typedef int value_t;
<font color=red>// Initialize the condition variable</font>
Condition (value_t value = 0);
~Condition (void);
<font color=red>/* 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. */</font>
<font color=red>// Increment & decrement</font>
Condition &operator++ (void);
Condition &operator-- (void);
<font color=red>// Increase & decrease</font>
Condition &operator+= (int inc);
Condition &operator-= (int inc);
<font color=red>// Just to be complete</font>
Condition &operator*= (int inc);
Condition &operator/= (int inc);
Condition &operator%= (int inc);
<font color=red>// Set/Reset the condition variable's value</font>
Condition &operator= (value_t value);
<font color=red>/* 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 "<font color=green>typical</font>" 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;
...
<font color=red>// Wait until the condition variable has the value 42</font>
mycondition != 42
... */</font>
<font color=red>// As long as the condition variable is NOT EQUAL TO <value>, we wait</font>
int operator!= (value_t value);
<font color=red>// As long as the condition variable is EXACTLY EQUAL TO <value>, we</font>
<font color=red>// wait</font>
int operator== (value_t value);
<font color=red>// As long as the condition variable is LESS THAN OR EQUAL TO</font>
<font color=red>// <value>, we wait</font>
int operator<= (value_t value);
<font color=red>// As long as the condition variable is GREATER THAN OR EQUAL TO</font>
<font color=red>// <value>, we wait</font>
int operator>= (value_t value);
<font color=red>// Return the value of the condition variable</font>
operator value_t (void);
<font color=red>/* 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
<font color=#008888>Condition::Compare</font> and overload operator()(value_t) to take
advantage of this. Have the function return non-zero when you
consider the condition to be met. */</font>
class Compare
{
public:
virtual int operator() (value_t value) = 0;
};
<font color=red>/* 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. */</font>
int operator== (Compare & compare);
private:
<font color=red>// Prevent copy construction and assignment.</font>
Condition (const Condition &condition);
Condition &operator= (const Condition &condition);
<font color=red>/* 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. */</font>
typedef ACE_Thread_Mutex mutex_t;
typedef ACE_Condition_Thread_Mutex condition_t;
typedef ACE_Guard<mutex_t> guard_t;
<font color=red>// The mutex that keeps the data save</font>
mutex_t mutex_;
<font color=red>// The condition mutex that makes waiting on the condition easier.</font>
condition_t *condition_;
<font color=red>// The acutal variable that embodies the condition we're waiting</font>
<font color=red>// for.</font>
value_t value_;
<font color=red>// Accessors for the two mutexes.</font>
mutex_t &mutex (void)
{
return this->mutex_;
}
condition_t &condition (void)
{
return *this->condition_;
}
<font color=red>// This particular accessor will make things much easier if we</font>
<font color=red>// decide that 'int' isn't the correct datatype for value_. Note</font>
<font color=red>// that we keep this private and force clients of the class to use</font>
<font color=red>// the cast operator to get a copy of the value.</font>
value_t &value (void)
{
return this->value_;
}
};
<font color=blue>#endif</font> <font color=red>/* CONDITION_H */</font>
</PRE>
<P><HR WIDTH="100%">
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