boost_1_45_0/libs/serialization/doc/singleton.html

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      <h3><a href="../../../index.htm"><img height="86" width="277" alt="C++ Boost" src="../../../boost.png" border="0"></a></h3>
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      <h1 align="center">Serialization</h1>
      <h2 align="center"><code style="white-space: normal">singleton</code></h2>
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<hr>
<dl class="page-index">
  <dt><a href="#motivation">Motivation</a>
  <dt><a href="#features">Features</a>
  <dt><a href="#classinterface">Class Interface</a>
  <dt><a href="#requirements">Requirements</a>
  <dt><a href="#example">Examples</a>
  <dt><a href="#multithreading">Multi-Threading</a>
</dl>

<h3><a name="motivation">Motivation</a></h3>
The serialization library relies on the existence of a number
of static variables and tables to store information related
to runtime types.  Examples are tables which relate exported
names to types and tables which relate base classes to derived
classes.  Construction, destruction and usage of these variables
requires consideration of the following issues:
<ul>
  <li>Some static data variables and constants  entries refer to others.  
  The sequence of initialization cannot be arbitrary but must be in proper
  sequence.</li>
  <li>A number of static variables aren't referred explicitly and, without
  special precautions, will be stripped by most code optimizers</li>
  <li>Many of these variables are created by templates and special care must
  be taken to be sure that they are instantiated</li>
  <li>In a multi-threading system, its possible that these static variables
  will be accessed concurrently by separate threads.  This would create a
  race condition with unpredictabe behavior</li>
</ul>
This singleton class addresses all of the above issues.

<h3><a name="features">Features</a></h3>
This singleton implementation has the following features:
<ul>
  <li>
    Any instance will be constructed before any attempt is made to access it.</li>
  <li>
    Any instance created with a template is guarenteed to be instantiated.
  <li>
    Regardless of whether or not an instance has been explicitly
    referred to, it will not be stripped by the optimizer when the
    executable is built in release mode.
  <li>
    All instances are constructed before 
    <code style="white-space: normal">main</code> is called
    regardless of where they might be referenced within the program.
    In a multi-tasking system, this guarentees that there will be no 
    race conditions during the construction of any instance.  No
    thread locking is required to guarentee this.
  <li>
    The above implies that any <code style="white-space: normal">const</code>
    instances are thread-safe during the whole program.  Again, no
    thread locking is required.
  <li>
    If a mutable instance is created, and such an instance is modified
    after main is called in a mult-threading system, there exists
    the possibility that a race condition will occur.  The serialization
    library takes care that in the few places where a mutable
    singleton is required, it is not altered after 
    <code style="white-space: normal">main</code> is called.
    For a more general purpose usage, thread locking on this
    singleton could easily be implemented.  But as the serialization
    library didn't require it, it wasn't implemented.
</ul>

<h3><a name="classinterface">Class Interface</a></h3>
<pre><code>
namespace boost { 
namespace serialization {

template <class T>
class singleton : public boost::noncopyable
{
public:
    static const T & get_const_instance();
    static T & get_mutable_instance();
    static bool is_destroyed();
};

} // namespace serialization 
} // namespace boost
</code></pre>

<dl>

<dt><h4><pre><code>
static const T & get_const_instance();
</code></pre></h4></dt>
<dd>
Retrieve a constant reference to the singleton for this type.
</dd>

<dt><h4><pre><code>
static T & get_mutable_instance();
</code></pre></h4></dt>
<dd>
Retrieve a mutable reference to the singleton for this type.
</dd>

<dt><h4><pre><code>
static bool is_destroyed();
</code></pre></h4></dt>
<dd>
Return <code>true</code> if the destructor on this singleton has been 
called.  Otherwise, return <code>false</code>.
</dd>

</dl>

<h3><a name="requirements">Requirements</a></h3>
In order to be used as 
<a target="singleton.hpp" href = "../../../boost/serialization/singleton.hpp">
<code style="white-space: normal">
singleton&lt;T&gt;
</code> 
</a>
, the type T must be default constructable.
It doesn't require static variables - though it may have them.
Since the library guarentees that only one instance of 
<a target="singleton.hpp" href = "../../../boost/serialization/singleton.hpp">
<code style="white-space: normal">
singleton&lt;T&gt;
</code> 
</a>
and all accesss is through the above static interface
functions, common member functions of T become
the functional equivalent of 
<code style="white-space: normal">static</code> functions.

<h3><a name="example">Examples</a></h3>
There are at least two different ways to use this class template.
Both are used in the serialization library.
<p>
The first way is illustrated by and excerpt from the file
<code style="white-space: normal"><a target="extended_type_info" href="../src/extended_type_info.cpp">extended_type_info.cpp</a></code>.
which contains the following code:

<pre><code>
typedef std::set<const extended_type_info *, key_compare> ktmap;
...
void
extended_type_info::key_register(const char *key) {
    ...
    result = singleton<ktmap>::get_mutable_instance().insert(this);
    ...
}
</code></pre>
Just by referring to the singleton instance anywhere in the program
will guarentee that one and only one instance for the specified
type (<code style="white-space: normal">ktmap</code> in this example) 
will exist throughout the program.  There is no need for anyother
declaration or definition.
<p>
A second way is to use 
<a target="singleton.hpp" href = "../../../boost/serialization/singleton.hpp">
<code style="white-space: normal">
singleton&lt;T&gt;
</code> 
</a>
as one of the base classes of the type.  This is illustrated by a simplified
excerpt from
<a target="extended_type_info_typeid.hpp" href = "../../../boost/serialization/extended_type_info_typeid.hpp">
<code style="white-space: normal">
extended_type_info_typeid.hpp
</code> 
</a>

<pre><code>
template&lt;class T&gt;
class extended_type_info_typeid : 
    public detail::extended_type_info_typeid_0,
    public singleton&lt;extended_type_info_typeid&lt;const T&gt; &gt;
{
    friend class singleton&lt;extended_type_info_typeid&lt;const T&gt; &gt;;
private:
    // private constructor to inhibit any existence other than the 
    // static one.  Note: not all compilers support this !!!
    extended_type_info_typeid() :
        detail::extended_type_info_typeid_0()
    {
        type_register(typeid(T));
    }
    ~extended_type_info_typeid(){}
    ...
};
</code></pre>

This usage will permit a more natural syntax to be used:
<pre><code>
extended_type_info_typeid&lt;T&gt;::get_const_instance()
</code></pre>

Again, including one or more of the above statements anywhere
in the program will guarentee that one and only one instance
is created and referred to.

<h3><a name="multithreading">Multi-Threading</a></h3>
This singleton CAN be safely used in multi-threading applications if one
is careful follow a simple rule:
<p>
<b>Do not call get_mutable_instance when more than one thread is running!</b>
<'>
All singletons used in the serialization library follow this rule.  
In order to help detect accidental violations of this rule there
exists an singleton lock/unlock functions.
<pre><code>
void boost::serialization::singleton_module::lock();
void boost::serialization::singleton_module::unlock();
bool boost::serialization::singleton_module::is_locked();
</code></pre>
In a program compiled for debug, any invocation of 
<code style="white-space: normal">get_mutable_instance()</code>
while the library is in a "locked" state will trap in an assertion.
The singleton module lock state is initialized as "unlocked" to permit
alteration of static variables before 
<code style="white-space: normal">main</code> is called. 
The <code style="white-space: normal">lock()</code> and
<code style="white-space: normal">unlock()</code> are "global"
in they affect ALL the singletons defined by this template.
All serialization tests invoke <code style="white-space: normal">lock()</code>
at the start of the progam.  For programs compiled in release
mode these functions have no effect.

<hr>
<p><i>&copy; Copyright <a href="http://www.rrsd.com">Robert Ramey</a> 2007. 
Distributed under the Boost Software License, Version 1.0. (See
accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
</i></p>
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