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<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<title>Store and Retrieve data or objects of complex types</title>
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<body>
<div xmlns="" class="navheader">
<div class="libver">
<p>Library Version 12.1.6.1</p>
</div>
<table width="100%" summary="Navigation header">
<tr>
<th colspan="3" align="center">Store and Retrieve data or
objects of complex types </th>
</tr>
<tr>
<td width="20%" align="left"><a accesskey="p" href="stl_primitive_rw.html">Prev</a> </td>
<th width="60%" align="center">Chapter 7. Standard Template Library API</th>
<td width="20%" align="right"> <a accesskey="n" href="stl_persistence.html">Next</a></td>
</tr>
</table>
<hr />
</div>
<div class="sect1" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h2 class="title" style="clear: both"><a id="stl_complex_rw"></a>Store and Retrieve data or
objects of complex types </h2>
</div>
</div>
</div>
<div class="toc">
<dl>
<dt>
<span class="sect2">
<a href="stl_complex_rw.html#idp808056">Storing varying length objects</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="stl_complex_rw.html#idp859120">Storing arbitrary sequences</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="stl_complex_rw.html#idp909008">Notes</a>
</span>
</dt>
</dl>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="idp808056"></a>Storing varying length objects</h3>
</div>
</div>
</div>
<p>
A structure like this:
</p>
<pre class="programlisting">class SMSMsg
{
public:
size_t mysize;
time_t when;
size_t szmsg;
int to;
char msg[1];
}; </pre>
<p>
with a varying length string in <code class="literal">msg</code>
cannot simply be stored in a
<code class="literal">db_vector<SMSMsg></code> without
some configuration on your part. This is because, by
default, dbstl uses the <span class="bold"><strong>sizeof()</strong></span> operator
to get the size of an object and then <code class="function">memcpy()</code> to copy the
object. This process is not suitable for this use-case as
it will fail to capture the variable length string
contained in <code class="literal">msg</code>.
</p>
<p>
There are currently two ways to store these kind of
objects:
</p>
<div class="orderedlist">
<ol type="1">
<li>
<p>
Register callback functions with dbstl that are
used to measure an object's size, and then
marshal/unmarshal the object.
</p>
</li>
<li>
<p>
Use a <code class="classname">DbstlDbt</code> wrapper
object.
</p>
</li>
</ol>
</div>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="idp838336"></a>Storing by marshaling objects</h4>
</div>
</div>
</div>
<p>
One way to store an object that contains
variable-sized fields is to marshall all of the
object's data into a single contiguous area in memory,
and then store the contents of that buffer. This means
that upon retrieval, the contents of the buffer must
be unmarshalled. To do these things, you must register
three callback functions:
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
<code class="function">typedef void
(*ElemRstoreFunct)(T& dest, const void
*srcdata);</code>
</p>
<p>
This callback is used to unmarshal an
object, updating <span class="bold"><strong>dest</strong></span> using
data found in <span class="bold"><strong>srcdata</strong></span>.
The data in <span class="bold"><strong>srcdata
</strong></span> contains the chunk of memory into
which the object was originally marshalled.
The default unmarshalling function simply
performs a cast (for example, <code class="literal">dest =
*((T*)srcdata)</code>), which assumes
the <span class="bold"><strong>srcdata</strong></span>
simply points to the memory layout of the
object.
</p>
</li>
<li>
<p>
<code class="function">typedef size_t
(*ElemSizeFunct)(const T&
elem);</code>
</p>
<p>
This callback returns the size in bytes
needed to store the <span class="bold"><strong>elem</strong></span> object.
By default this function simply uses <span class="bold"><strong>sizeof(elem)</strong></span>
to determine the size of <span class="bold"><strong>elem</strong></span>.
</p>
</li>
<li>
<p>
<code class="function">typedef void (*ElemCopyFunct)(void
*dest, const T&elem);</code>
</p>
<p>
This callback is used to arrange all data
contained by <span class="bold"><strong>elem</strong></span> into
the chunk of memory to which <span class="bold"><strong>dest</strong></span>
refers. The size of <span class="bold"><strong>dest</strong></span> is set
by the <code class="function">ElemSizeFunct</code>
function, discussed above. The default
marshalling function simply uses
<code class="function">memcpy()</code> to copy
<span class="bold"><strong>elem</strong></span> to
<span class="bold"><strong>dest</strong></span>.
</p>
</li>
</ul>
</div>
<p>
The
<code class="function">DbstlElemTraits<SMSMsg>::instance()->set_size_function()</code>,
<code class="function">set_copy_function()</code> and
<code class="function">set_restore_function()</code>
methods are used to register these callback functions.
If a callback is not registered, its default function
is used.
</p>
<p>
By providing non-default implementations of the
callbacks described here, you can store objects of
varying length and/or objects which do not reside in a
continuous memory chunk — for example, objects
containing a pointer which refers another object, or a
string, and so forth. As a result,
containers/iterators can manage variable length
objects in the same as they would manage objects that
reside in continuous chunks of memory and are of
identical size.
</p>
</div>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="idp880480"></a>Using a <code class="classname">DbstlDbt</code> wrapper
object</h4>
</div>
</div>
</div>
<p>
To use a <code class="classname">DbstlDbt</code> wrapper
object to store objects of variable length, a
<code class="literal">db_vector<DbstlDbt></code>
container is used to store complex objects in a
<code class="classname">db_vector</code>.
<code class="classname">DbstlDbt</code> derives from DB
C++ API's <code class="classname">Dbt</code>class, but can
manage its referenced memory properly and release it
upon destruction. The memory referenced by
<code class="classname">DbstlDbt</code> objects is
required to be allocated using the
<code class="function">malloc()</code>/<code class="function">realloc()</code>
functions from the standard C library.
</p>
<p>
Note that the use of
<code class="classname">DbstlDbt</code> wrapper class is
not ideal. It exists only to allow raw bytes of no
specific type to be stored in a container.
</p>
<p>
To store an <code class="classname">SMSMsg</code> object
into a <code class="literal">db_vector<DbstlDbt></code>
container using a <code class="classname">DbstlDbt</code>
object:
</p>
<div class="orderedlist">
<ol type="1">
<li>
Wrap the <code class="classname">SMSMSg</code>
object into a <code class="classname">DbstlDbt</code>
object, then marshal the SMSMsg object properly
into the memory chunk referenced by
<code class="methodname">DbstlDbt::data</code>.
</li>
<li>
Store the <code class="classname">DbstlDbt</code>
object into a
<code class="literal">db_vector<DbstlDbt></code>
container. The bytes in the memory chunk
referenced by the <code class="classname">DbstlDbt</code>
object's <span class="bold"><strong>data</strong></span>
member are stored in the
<code class="literal">db_vector<DbstlDbt></code>
container.
</li>
<li>
Reading from the container returns a
<code class="classname">DbstlDbt</code> object whose
<span class="bold"><strong>data</strong></span> field
points to the <code class="classname">SMSMsg</code> object
located in a continuous chunk of memory. The
application needs to perform its own
unmarshalling.
</li>
<li>
The memory referenced by
<code class="literal">DbstlDbt::data</code> is freed
automatically, and so the application should not
attempt to free the memory.
</li>
</ol>
</div>
<p>
<code class="classname">ElementHolder</code> should not be
used to store objects of a class because it doesn't
support access to object members using <span class="bold"><strong>(*iter).member</strong></span> or <span class="bold"><strong>iter->member</strong></span>
expressions. In this case, the default
<code class="literal">ElementRef<ddt></code> is used
automatically.
</p>
<p>
<code class="classname">ElementRef</code> inherits from
<code class="classname">ddt</code>, which allows <span class="bold"><strong>*iter</strong></span> to return the object
stored in the container. (Technically it is an
<code class="classname">ElementRef<ddt>
object</code>, whose "base class" part is the
object you stored). There are a few data members and
member functions in <code class="classname">ElementRef</code>,
which all start with <code class="literal">_DB_STL_</code>. To
avoid potential name clashes, applications should not
use names prefixing <code class="literal">_DB_STL_</code> in
classes whose instances may be stored into dbstl
containers.
</p>
<p>
Example code demonstrating this feature can be
found in the
<code class="methodname">StlAdvancedFeaturesExample::arbitrary_object_storage</code>
method.
</p>
</div>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="idp859120"></a>Storing arbitrary sequences</h3>
</div>
</div>
</div>
<p>
A sequence is a group of related objects, such as an
array, a string, and so forth. You can store sequences of
any structure using dbstl, so long as you implement and
register the proper callback functions. By using these
callbacks, each object in the sequence can be a complex
object with data members that are all not stored in a
continuous memory chunk.
</p>
<p>
Note that when using these callbacks, when you retrieve
a stored sequence from the database, the entire sequence
will reside in a single continuous block of memory with
the same layout as that constructed by your sequence copy
function.
</p>
<p>
For example, given a type RGB:
</p>
<pre class="programlisting">struct RGB{char r, g, b, bright;}; </pre>
<p>
and an array of RGB objects, the following steps
describe how to store an array into one key/data pair of a
<code class="classname">db_map</code> container.
</p>
<div class="orderedlist">
<ol type="1">
<li>
Use a <code class="classname">db_map<int, RGB *,
ElementHolder<RGB *> ></code>
container.
</li>
<li>
<p>
Define two functions. The first returns the
number of objects in a sequence, the second that
copies objects from a sequence to a defined
destination in memory:
</p>
<pre class="programlisting">typedef size_t (*SequenceLenFunct)(const RGB*); </pre>
<p>
and
</p>
<pre class="programlisting"> typedef void (*SequenceCopyFunct)(RGB*dest, const RGB*src); </pre>
</li>
<li>
Call
DbstlElemTraits<RGB>::set_sequence_len_function()/set_sequence_copy_function()
to register them as callbacks.
</li>
</ol>
</div>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="idp869312"></a>The <code class="function">SequenceLenFunct</code>
function</h4>
</div>
</div>
</div>
<pre class="programlisting">typedef size_t (*SequenceLenFunct)(const RGB*); </pre>
<p>
A <code class="function">SequenceLenFunct</code> function
returns the number of objects in a sequence. It is
called when inserting into or reading from the
database, so there must be enough information in the
sequence itself to enable the
<code class="function">SequenceLenFunct</code> function to
tell how many objects the sequence contains. The
<code class="literal">char*</code> and
<code class="literal">wchar_t*</code> strings use a
<code class="literal">'\0'</code> special character to do
this. For example, RGB(0, 0, 0, 0) could be used to
denote the end of the sequence. Note that for your
implementation of this callback, you are not required
to use a trailing object with a special value like
<code class="literal">'\0'</code> or <code class="literal">RGB(0, 0, 0,
0)</code> to denote the end of the sequence.
You are free to use what mechanism you want in your
<code class="function">SequenceLenFunct</code> function
implementation to figure out the length of the
sequence.
</p>
</div>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="idp903872"></a>The <code class="function">SequenceCopyFunct</code> function</h4>
</div>
</div>
</div>
<pre class="programlisting"> typedef void (*SequenceCopyFunct)(RGB*dest, const RGB*src); </pre>
<p>
<code class="function">SequenceCopyFunct</code> copies objects
from the sequence <span class="bold"><strong>src</strong></span>
into memory chunk <span class="bold"><strong>dest</strong></span>. If the
objects in the sequence do not reside in a continuous memory chunk, this
function must marshal each object in the sequence into
the <span class="bold"><strong>dest</strong></span> memory
chunk.
</p>
<p>
The sequence objects will reside in the continuous
memory chunk referred to by <span class="bold"><strong>dest</strong></span>, which
has been sized by
<code class="classname">SequenceLenFunct</code> and
<code class="classname">ElemSizeFunct</code> if available
(which is when objects in the sequence are of varying
lengths). <code class="classname">ElemSizeFunct</code>
function is not needed in this example because
<span class="bold"><strong>RGB</strong></span> is a simple
fixed length type, the <code class="literal">sizeof()</code>
operator is sufficient to return the size of the
sequence.
</p>
</div>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="idp909008"></a>Notes</h3>
</div>
</div>
</div>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
The get and set functions of this class are not
protected by any mutexes. When using multiple
threads to access the function pointers, the
callback functions must be registered to the
singleton of this class before any retrieval of
the callback function pointers. Isolation may also
be required among multiple threads. The best way
is to register all callback function pointers in a
single thread before making use of the any
containers.
</p>
</li>
<li>
<p>
If objects in a sequence are not of identical
sizes, or are not located in a consecutive chunk
of memory, you also need to implement and register
the
<code class="function">DbstlElemTraits<>::ElemSizeFunct</code>
callback function to measure the size of each
object. When this function is registered, it is
also used when allocating memory space. </p>
<p>
There is example code demonstrating the use
this feature in the
<code class="methodname">StlAdvancedFeaturesExample::arbitray_sequence_storage()</code>
method.
</p>
</li>
<li>
<p>
A consequence of this dbstl feature is that you
can not store a pointer value directly because
dbstl will think it is a sequence head pointer.
Instead, you need to convert the pointer into a
<code class="literal">long</code> and then store it into
a <code class="literal">long</code> container. And please
note that pointer values are probably meaningless
if the stored value is to be used across different
application run times.
</p>
</li>
</ul>
</div>
</div>
</div>
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