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<p>Library Version 18.1.40</p>
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<h2 class="title" style="clear: both"><a id="stl_primitive_rw"></a>Working with primitive types </h2>
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<dl>
<dt>
<span class="sect2">
<a href="stl_primitive_rw.html#idm140654539374368">Storing strings</a>
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<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">
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<div>
<h3 class="title"><a id="idm140654539374368"></a>Storing strings</h3>
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<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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