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<h2 class="title" style="clear: both"><a id="stl_misc"></a>Dbstl miscellaneous notes</h2>
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<dt>
<span class="sect2">
<a href="stl_misc.html#idp51587208">Special notes about trivial methods</a>
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<span class="sect2">
<a href="stl_misc.html#idp51603304">Using correct container and iterator public types</a>
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<h3 class="title"><a id="idp51587208"></a>Special notes about trivial methods</h3>
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<p>
There are some standard STL methods which are meaningless in dbstl, but they are kept in
dbstl as no-ops so as to stay consistent with the standard. These are:
</p>
<table class="simplelist" border="0" summary="Simple list">
<tr>
<td>
<code class="methodname">db_vecter::reserve();</code>
</td>
</tr>
<tr>
<td>
<code class="methodname">db_vector::max_size();</code>
</td>
</tr>
<tr>
<td>
<code class="methodname">db_vector::capacity();</code>
</td>
</tr>
<tr>
<td>
<code class="methodname">db_map::reserve();</code>
</td>
</tr>
<tr>
<td>
<code class="methodname">db_map::max_size();</code>
</td>
</tr>
</table>
<p>
<code class="methodname">db_vector<>::max_size()</code> and
<code class="methodname">db_map<>::max_size()</code> both return 2^30. This does not mean
that Berkeley DB can only hold that much data. This value is returned to conform to some
compilers' overflow rules — if we set bigger numbers like 2^32 or 2^31, some compilers
complain that the number has overflowed.
</p>
<p>
See the Berkeley DB documentation for information about limitations on how much data a
database can store.
</p>
<p>
There are also some read-only functions. You set the configuration
for these using the Berkeley DB API. You access them using the container's methods. Again,
this is to keep consistent with C++ standard STL containers, such as:
</p>
<table class="simplelist" border="0" summary="Simple list">
<tr>
<td>
<code class="methodname">db_map::key_comp();</code>
</td>
</tr>
<tr>
<td>
<code class="methodname">db_map::value_comp();</code>
</td>
</tr>
<tr>
<td>
<code class="methodname">db_map::hash_funct();</code>
</td>
</tr>
<tr>
<td>
<code class="methodname">db_map::key_eq();</code>
</td>
</tr>
</table>
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<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="idp51603304"></a>Using correct container and iterator public types</h3>
</div>
</div>
</div>
<p>
All public types defined by the C++ STL specification are present in dbstl. One thing to
note is the <span class="bold"><strong>value_type</strong></span>. dbstl defines the <span class="bold"><strong>value_type</strong></span> for each iterator and container class to be the raw
type without the <code class="classname">ElementRef</code>/<code class="classname">ElementHolder</code>
wrapper, so this type of variable can not be used to store data in a database. There is a
<span class="bold"><strong>value_type_wrap</strong></span> type for each container and iterator type,
with the raw type wrapped by the
<code class="classname">ElementRef</code>/<code class="classname">ElementHolder</code>.
</p>
<p>
For example, when type <code class="literal">int_vector_t</code> is defined as
</p>
<pre class="programlisting">db_vector<int, ElementHolder<int> ></pre>
<p>
its <span class="bold"><strong>value_type</strong></span> is <code class="literal">int</code>, its
<span class="bold"><strong>value_type_wrap</strong></span> is <code class="literal">ElementHolder<int></code>,
and its reference and pointer types are <code class="literal">ElementHolder<int>&</code> and
<code class="literal">ElementHolder<int>*</code> respectively. If you need to store data, use
<span class="bold"><strong>value_type_wrap</strong></span> to make use of the wrapper to store data
into database.
</p>
<p>
The reason we leave <span class="bold"><strong>value_type</strong></span> as the raw type is that we
want the existing algorithms in the STL library to work with dbstl because we have seen that
without doing so, a few tests will fail.
</p>
<p>
You need to use the same type as the return type of the data element retrieval functions to
hold a value in order to properly manipulate the data element. For example, when calling
</p>
<pre class="programlisting">db_vector<T>::operator[]</pre>
<p>
check that the return type for this function is
</p>
<pre class="programlisting">db_vector<T>::datatype_wrap</pre>
<p>
Then, hold the return value using an object of the same type:
</p>
<pre class="programlisting">db_vector<T>::datatype_wrap refelem = vctr[3];</pre>
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