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<title>Serialization - extended_type_info</title>
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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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    <td valign="top"> 
      <h1 align="center">Serialization</h1>
      <h2 align="center"><code style="white-space: normal">extended_type_info</code></h2>
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<hr>
<dl class="page-index">
  <dt><a href="#motivation">Motivation</a>
  <dt><a href="#runtime">Runtime Interface</a>
  <dt><a href="#requirements">Requirements</a>
  <dt><a href="#models">Models</a>
  <dt><a href="#example">Example</a>
</dl>

<h3><a name="motivation">Motivation</a></h3>
The serialization library needs a system like 
<code style="white-space: normal">type_info/typeid()</code> to perform
the following functions
<ol>
  <li>
     given a pointer to a type T discover the true type pointed to.
  <li>
     given an "external" key - determine what type of object to create.
</ol>
<h3>The problem with <code style="white-space: normal">std::type_info</code></h3>
<ul>
  <li>
    The main function we require - <code style="white-space: normal">std::typeid()</code>
    is not available in all environments. Support for this function depends upon
    runtime typing(RTTI) support from the compiler.  This may be non-existent
    or not enabled for reasons such as a percieved inefficiency.
  <li>
    <code style="white-space: normal">std::type_info</code> includes a string
    containing type name.  This would seem to satisfy 2) above.
    But the format of this string is not consistent accross compilers, libraries, 
    and operating systems. This makes it unusable for support of portable archives.
  <li>
    Even if the type name string could somehow be made portable, there is no 
    guarantee that class headers would be included in the same namespace accross 
    different applications. In fact, including different headers in different
    namespaces is an accepted method used to avoid namespace conflicts.
    Thus the namespace::class_name can't be used as a key.
  <li>
    There exists the possibility that different classes use different type id 
    mechanisms.  The class header might include this information. If we want to 
    import class headers accross applications, it's convenient that the type id
    mechanism support inter-operability accross different type id systems.
</ul>
<h3>Features</h3>
<code style="white-space: normal"><a target="extended_type_info.hpp" href = "../../../boost/serialization/extended_type_info.hpp">
extended_type_info</a></code> is an implementation
of <code style="white-space: normal">std::type_info</code> functionality with the 
following features:
<ul>
  <li>
    Builds a set of <a target="extended_type_info.hpp" href = "../../../boost/serialization/extended_type_info.hpp">
    <code style="white-space: normal">extended_type_info</a></code> records - one for each type
    serialized.
  <li>
    permits association of an arbitrary string key with a type.  Often this key would
    be the class name - but it doesn't have to be.  This key is referred to as
    a GUID - Globally Unique IDentifier.  Presumably it should be unique in the universe.
    Typically this GUID would be in header files and be used to match type accross
    applications.  The macro BOOST_CLASS_EXPORT can be invoked to associate a string
    key with any known type.  We'll refer to these types as "exported types"
  <li>
    permits the "mixing" of type info systems.  For example, one class might use 
    <code style="white-space: normal">typeid()</code> to find the external identifier 
    of a class while another might not.
</ul>

Exported types are maintained in a global table so that given a string key, the
corresponding type can be found.  This facility is used by the serialization library
in order to construct types serialized through a base class pointer.

<h3><a name="runtime">Runtime Interface</a></h3>
<pre><code">
namespace boost { 
namespace serialization {

class extended_type_info
{
protected:
    // this class can't be used as is. It's just the 
    // common functionality for all type_info replacement
    // systems.  Hence, make these protected
    extended_type_info(
        const unsigned int type_info_key,
        const char * key
    );
    ~extended_type_info();
    void key_register();
    void key_unregister();
public:
    const char * get_key() const;
    bool operator<(const extended_type_info &rhs) const;
    bool operator==(const extended_type_info &rhs) const;
    bool operator!=(const extended_type_info &rhs) const {
        return !(operator==(rhs));
    }
    // for plugins
    virtual void * construct(unsigned int count = 0, ...) const;
    virtual void destroy(void const * const p) const;
    static const extended_type_info * find(const char *key);
};

} // namespace serialization 
} // namespace boost
</code></pre>
<p>
Generally, there will be one and only one
<code style="white-space: normal"><a target="extended_type_info.hpp" href = "../../../boost/serialization/extended_type_info.hpp">extended_type_info</a></code>
instance created for each type.  However, this is enforced only at the executable
module level.  That is, if a program includes some shared libraries or DLLS,
there may be more than one instance of this class correponding to a particular type.
For this reason the comparison functions below can't just compare the addresses of 
this instance but rather must be programmed to compare the actual information 
the instances contain.
<dl>

<dt><h4><pre><code>
extended_type_info(unsigned int type_info_key, const char *key);
</code></pre></h4></dt>
<dd>
This constructor should be called by all derived classes.
The first argument should be the particular implementation. 
For this default implementation base on typeid(), this is the 
value 1.  Each system must have its own integer.  This value
is used to permit the inter-operability of different typeinfo
systems.
<p>
The second argument is a const string which is the external
name of the type to which this record corresponds.  
It may sometimes be referred to as a GUID - a <b>G</b>lobal <b>U</b>nique <b>ID</b>entifier.
It is passed through archives from one program invocation to
another to uniquely identify the types that the archive contains.
If the "export" facility is not going to be used, 
this value may be NULL.
</dd>

<dt><h4><pre><code>
void key_register();
void key_unregister();
</code></pre></h4></dt>
<dd>
This system maintains a global table which relates
external strings to 
<code style="white-space: normal">extended_type_info</code> records.
This table is used when loading pointers to objects serialized
through a base class pointer.  In this case, the archive 
contains a string which is looked up in this table to
determine which <code style="white-space: normal">extended_type_info</code>
to use for creating a new object.
<p>
These functions are called by constructors and
destructors of classes which implement 
<code style="white-space: normal">extended_type_info</code>
to add and remove entries from this table.
</dd>

<dt><h4><pre><code>
const char *get_key() const;
</code></pre></h4></dt>
<dd>
Retrieves the key for <code style="white-space: normal"><a target="extended_type_info.hpp" href = "../../../boost/serialization/extended_type_info.hpp">extended_type_info</a></code>
instance.  If no key has been associated with the instance, then a NULL is returned.
</dd>

<dt><h4><pre><code>
bool operator<(const extended_type_info & rhs) const;
bool operator==(const extended_type_info & rhs) const;
bool operator!=(const extended_type_info & rhs) const;
</code></pre></h4></dt>
<dd>
These functions are used to compare
<a target="extended_type_info.hpp" href = "../../../boost/serialization/extended_type_info.hpp">
<code style="white-space: normal">
extended_type_info
</code> 
</a>
objects. They impose a strict total ordering on all
<code style="white-space: normal">extended_type_info</code> records.
</dd>

<dt><h4><pre><code>
virtual void * construct(unsigned int count = 0, ...) const;
</code></pre></h4></dt>
<dd>
Construct a new instance of the type to which this
<a target="extended_type_info.hpp" href = "../../../boost/serialization/extended_type_info.hpp">
<code style="white-space: normal">
extended_type_info
</code>
</a>
record corresponds. This function takes a variable list of up to 4 arguments
of any type.  These arguments are passed to the type's constructor
at runtime.  In order to use the facility, 
one must declare a type sequence for the constructor arguments.  
Arguments for this function must match in number and type
with those specified when the type was exported.  
This function permits one to create instances of
any exported type given only the exported <strong>GUID</strong> assigned
with BOOST_CLASS_EXPORT.  
If these types are defined in DLLS or shared libraries loaded at runtime, 
these constructors can be called until the module is unloaded.  
Such modules are referred to as <b>plugins</b>. 
</code>
</dd>

<dt><h4><pre><code>
virtual void destroy(void const * const p) const;
</code></pre></h4></dt>
<dd>
Destroy an instance created by the above constructor.
</dd>

<dt><h4><pre><code>
static const extended_type_info * find(const char *key);
</code></pre></h4></dt>
<dd>
Given a character string key or <strong>GUID</strong>, return the address of a 
corresponding <code style="white-space: normal"><a target="extended_type_info.hpp" href = "../../../boost/serialization/extended_type_info.hpp">extended_type_info</a></code>
object. 

</dl>

<h3><a name="requirements">Requirements for an Implementation</a></h3>
In order to be used by the serialization library, an implementation of
<code style="white-space: normal">extended_type_info</code>, 
(referred to as ETI here), must be derived from 
<a target="extended_type_info.hpp" href = "../../../boost/serialization/extended_type_info.hpp">
<code style="white-space: normal">
extended_type_info
</code> 
</a>
and also implement the following:

<dl>

<dt><h4><code style="white-space: normal"><pre>
template&lt;class ETI&gt;
const extended_type_info *
ETI::get_derived_extended_type_info(const T & t) const;
</pre></code></h4></dt>
<dd>
Return a pointer to the 
<code style="white-space: normal">extended_type_info</code>
instance that corresponds to
the "true type" of the type T.  The "true type" is the lowest type in the
hierarchy of classes.  The type T can always be cast to the "true type" with
a static cast.  Implementation of this function will vary among type id systems
and sometimes will make presumptions about the type T than can be identified
with a particular <code style="white-space: normal">extended_type_info</code> implementation.
</dd>

<dt><h4><code style="white-space: normal"><pre>
virtual bool ETI::is_less_than(const extended_type_info &rhs) const;
</pre></code></h4></dt>
<dd>
Compare this instance to another one using the same
<code style="white-space: normal">extended_type_info</code> implementation.
</dd>

<dt><h4><code style="white-space: normal"><pre>
virtual bool ETI::is_equal(const extended_type_info &rhs) const;
</pre></code></h4></dt>
<dd>
Compare this instance to another one using the same
<code style="white-space: normal">extended_type_info</code> implementation.
Return <code style="white-space: normal">true</code> if the types referred
to are the same.  Otherwise return 
<code style="white-space: normal">false</code>
</dd>

<dt><h4><code style="white-space: normal"><pre>
const char ETI::get_key() const;
</pre></code></h4></dt>
<dd>
Retrieve the external key (aka GUID) for this class.
</dd>

<dt><h4><code style="white-space: normal"><pre>
virtual void * construct(unsigned int count, ...) const;
</pre></code></h4></dt>
<dd>
Construct an instance of the corresponding type
with the include argument list.
</dd>

<dt><h4><code style="white-space: normal"><pre>
virtual void * destroy(void const * const ptr ) const;
</pre></code></h4></dt>
<dd>
Destroy an instance of this type. This calls the
proper destructor and recovers allocated memory.
</dd>

</dl>

<h3><a name="models">Models</a></h3>
The serialization library includes two distinct
<code style="white-space: normal"><a target="extended_type_info.hpp" href="../../../boost/serialization/extended_type_info.hpp">extended_type_info</a></code>
implementations.
<p>
<code style="white-space: normal"><h4><a target="extended_type_info_typeid.hpp" href = "../../../boost/serialization/extended_type_info_typeid.hpp">
extended_type_info_typeid</a></h4></code>is implemented in terms of the standard typeid(). It presumes that RTTI support is enabled
by the compiler.
<p>
<code style="white-space: normal"><h4><a target="extended_type_info_no_rtti.hpp" href="../../../boost/serialization/extended_type_info_no_rtti.hpp">
extended_type_info_no_rtti</a></h4></code>
is implemented in a way that doesn't rely on the existence RTTI.
Instead, it requires that all polymorphic types be explictly exported.
In addition, if the export facility is to be used to serialize types
through base class pointers, those types are required to implement
a virtual function with the signature:

<code><pre>
virtual const char * get_key();
</pre></code>
which returns a unique string the most derived object this class.  
This function must be virtual in order to implement the functionality required by
<code style="white-space: normal">ETI::get_derived_extended_type_info</code>
as described above.

<h3><a name="example">Example</a></h3>
The test program <code style="white-space: normal"><a target="test_no_rtti" href="../test/test_no_rtti.cpp">test_no_rtti</a></code>
implements this function in terms of the <code style="white-space: normal"><a target="extended_type_info_no_rtti.hpp" href="../../../boost/serialization/extended_type_info_no_rtti.hpp">
extended_type_info</a></code> API above to return the export key associated with the class.  
This requires that non-abstract types be exported. It also demonstrates the
inter-operability between two different implementations of
<code style="white-space: normal">extended_type_info</code>.

<h3><a name="type_requirements">Requirements for Each Type</a></h3>
Each type to be managed by the system must be
"registered" individually. This is accomplished by instantiating
templates.  For example, if the type T is to use the type_info system
one would include the following code:

<code style="white-space: normal"><pre>
namespace boost {
namespace serialization {
template
struct extended_type_info_typeid&gt;T&gt;;
template
struct extended_type_info_typeid&gt;const T&gt;;
} // serialization
} // boost
</pre></code>

For those using the serialization library, this step can be skipped
as it is done automatically.  The serialization library includes
the macro:

<code style="white-space: normal"><pre>
BOOST_CLASS_TYPE_INFO(
    my_type, 
    extended_type_info_typeid&gt;my_class&gt;
)
</pre></code>

which is used to specify which <code>extended_type_info</code> system is to 
be used for a given type.
<p>
<code>extended_type_info</code> includes a facility for constructing
instances of types without knowing what the exact types are. This is done
with the function 
<code>
virtual void * extended_type_info::construct(unsigned int count = 0, ...) const;
</code>
. For example:
<br>
<code><pre>
struct base {
...
};
struct derived : public base {
...
};
...
extended_type_info *eti = extended_type_info::find("my_class")
base * b = eti->construct(...);
</pre></code>
<br>
The <code>construct</code> takes an argument count and up to
four parameters of any type.  The arguments are passed to the
constructor of "my_class".


A complete example of this can be found
<a target="test_dll_plugin.cpp" href="../test/test_dll_plugin.cpp">here</a>
<hr>
<p><i>&copy; Copyright <a href="http://www.rrsd.com">Robert Ramey</a> 2005-2009. 
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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