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<th colspan="3" align="center">Working with primitive types </th>
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<td width="20%" align="left"><a accesskey="p" href="stl_mt_usage.html">Prev</a> </td>
<th width="60%" align="center">Chapter 7. Standard Template Library API</th>
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<div class="sect1" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h2 class="title" style="clear: both"><a id="stl_primitive_rw"></a>Working with primitive types </h2>
</div>
</div>
</div>
<div class="toc">
<dl>
<dt>
<span class="sect2">
<a href="stl_primitive_rw.html#idp51467888">Storing strings</a>
</span>
</dt>
</dl>
</div>
<p>
To store simple primitive types such as <code class="literal">int</code>,
<code class="literal">long</code>, <code class="literal">double</code>, and so forth, an
additional type parameter for the container class
templates is needed. For example, to store an <code class="literal">int</code>
in a <code class="classname">db_vector</code>, use this container class:
</p>
<pre class="programlisting">db_vector<int, ElementHolder<int> >;</pre>
<p>
To map integers to doubles, use this:
</p>
<pre class="programlisting">db_map<int, double, ElementHolder<double> >;</pre>
<p>
To store a <code class="literal">char*</code> string with <code class="literal">long</code> keys,
use this:
</p>
<pre class="programlisting">db_map<long, char*, ElementHolder<char*> >;</pre>
<p>
Use this for <code class="literal">const char*</code> strings:
</p>
<pre class="programlisting">db_map<long, const char*, ElementHolder<const char*> >;</pre>
<p>
To map one const string to another, use this type:
</p>
<pre class="programlisting">db_map<const char*, const char*, ElementHolder<const char*> >;</pre>
<p>
The <code class="methodname">StlAdvancedFeaturesExample::primitive()</code> method demonstrates more of these examples.
</p>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="idp51467888"></a>Storing strings</h3>
</div>
</div>
</div>
<p>
For <code class="literal">char*</code> and <code class="literal">wchar_t*</code> strings,
<code class="methodname">_DB_STL_StoreElement()</code> must be called
following partial or total modifications before iterator movement,
<code class="literal">container::operator[]</code> or
<code class="literal">iterator::operator*/-></code> calls. Without the
<code class="methodname">_DB_STL_StoreElement()</code> call, the modified
change will be lost. If storing an new value like this:
</p>
<pre class="programlisting">*iterator = new_char_star_string;</pre>
<p>
the call to <code class="methodname">_DB_STL_StoreElement()</code> is not needed.
</p>
<p>
Note that passing a NULL pointer to a container of
<code class="literal">char*</code> type or passing a
<code class="classname">std::string</code> with no contents at all will insert
an empty string of zero length into the database.
</p>
<p>
The string returned from a container will not live beyond the next
iterator movement call, <code class="literal">container::operator[]</code> or
<code class="literal">iterator::operator*/-></code> call.
</p>
<p>
A <span class="bold"><strong>db_map::value_type::second_type</strong></span> or
<span class="bold"><strong>db_map::datatype_wrap</strong></span> should be used
to hold a reference to a <code class="literal">container::operator[]</code>
return value. Then the reference should be used for repeated
references to that value. The *iterator is of type
<code class="literal">ElementHolder<char *></code>, which can be automatically converted to a
<code class="literal">char *</code> pointer using its type conversion operator.
Wherever an auto conversion is done by the compiler, the conversion
operator of <code class="literal">ElementHolder<T></code> is called. This
avoids almost all explicit conversions, except for two use cases:
</p>
<div class="orderedlist">
<ol type="1">
<li>
<p>
The *iterator is used as a "..." parameter like this:
</p>
<pre class="programlisting">printf("this is the special case %s", *iterator);</pre>
<p>
This compiles but causes errors. Instead, an explicit cast
should be used:
</p>
<pre class="programlisting">printf("this is the special case %s", (char *)*iterator);</pre>
</li>
<li>
<p>
For some old compilers, such as gcc3.4.6, the *iterator cannot be
used with the ternary <code class="literal">?</code> operator, like
this:
</p>
<pre class="programlisting">expr ? *iterator : var</pre>
<p>
Even when <span class="bold"><strong>var</strong></span> is the same
type as the iterator's <code class="literal">value_type</code>, the
compiler fails to perform an auto conversion.
</p>
</li>
</ol>
</div>
<p>
When using <code class="classname">std::string</code> or
<code class="classname">std::wstring</code> as the data type for dbstl
containers — that is, <code class="classname">db_vector<string></code>,
and <code class="classname">db_map<string, wstring></code> — the
string's content rather than the string object itself is stored in order
to maintain persistence.
</p>
<p>
You can find example code demonstrating string storage in the
<code class="methodname">StlAdvancedFeaturesExample::char_star_string_storage()</code> and
<code class="methodname">StlAdvancedFeaturesExample::storing_std_strings()</code> methods.
</p>
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