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8.2 KiB
HTML
185 lines
8.2 KiB
HTML
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<html xmlns="http://www.w3.org/1999/xhtml">
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
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<title>Working with primitive types</title>
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<link rel="stylesheet" href="gettingStarted.css" type="text/css" />
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<meta name="generator" content="DocBook XSL Stylesheets V1.73.2" />
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<link rel="start" href="index.html" title="Berkeley DB Programmer's Reference Guide" />
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<link rel="up" href="stl.html" title="Chapter 7. Standard Template Library API" />
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<link rel="prev" href="stl_mt_usage.html" title="Using dbstl in multithreaded applications" />
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<link rel="next" href="stl_complex_rw.html" title="Store and Retrieve data or objects of complex types" />
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</head>
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<body>
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<div xmlns="" class="navheader">
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<div class="libver">
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<p>Library Version 11.2.5.2</p>
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</div>
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<table width="100%" summary="Navigation header">
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<tr>
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<th colspan="3" align="center">Working with primitive types </th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="stl_mt_usage.html">Prev</a> </td>
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<th width="60%" align="center">Chapter 7. Standard Template Library API</th>
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<td width="20%" align="right"> <a accesskey="n" href="stl_complex_rw.html">Next</a></td>
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</tr>
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</table>
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<hr />
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</div>
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<div class="sect1" lang="en" xml:lang="en">
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<div class="titlepage">
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<div>
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<div>
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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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</div>
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</div>
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</div>
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<div class="toc">
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<dl>
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<dt>
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<span class="sect2">
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<a href="stl_primitive_rw.html#id3891729">Storing strings</a>
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</span>
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</dt>
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</dl>
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</div>
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<p>
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To store simple primitive types such as <code class="literal">int</code>,
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<code class="literal">long</code>, <code class="literal">double</code>, and so forth, an
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additional type parameter for the container class
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templates is needed. For example, to store an <code class="literal">int</code>
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in a <code class="classname">db_vector</code>, use this container class:
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</p>
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<pre class="programlisting">db_vector<int, ElementHolder<int> >;</pre>
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<p>
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To map integers to doubles, use this:
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</p>
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<pre class="programlisting">db_map<int, double, ElementHolder<double> >;</pre>
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<p>
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To store a <code class="literal">char*</code> string with <code class="literal">long</code> keys,
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use this:
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</p>
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<pre class="programlisting">db_map<long, char*, ElementHolder<char*> >;</pre>
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<p>
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Use this for <code class="literal">const char*</code> strings:
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</p>
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<pre class="programlisting">db_map<long, const char*, ElementHolder<const char*> >;</pre>
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<p>
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To map one const string to another, use this type:
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</p>
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<pre class="programlisting">db_map<const char*, const char*, ElementHolder<const char*> >;</pre>
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<p>
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The <code class="methodname">StlAdvancedFeaturesExample::primitive()</code> method demonstrates more of these examples.
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</p>
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<div class="sect2" lang="en" xml:lang="en">
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<div class="titlepage">
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<div>
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<div>
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<h3 class="title"><a id="id3891729"></a>Storing strings</h3>
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</div>
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</div>
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</div>
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<p>
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For <code class="literal">char*</code> and <code class="literal">wchar_t*</code> strings,
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<code class="methodname">_DB_STL_StoreElement()</code> must be called
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following partial or total modifications before iterator movement,
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<code class="literal">container::operator[]</code> or
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<code class="literal">iterator::operator*/-></code> calls. Without the
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<code class="methodname">_DB_STL_StoreElement()</code> call, the modified
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change will be lost. If storing an new value like this:
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</p>
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<pre class="programlisting">*iterator = new_char_star_string;</pre>
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<p>
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the call to <code class="methodname">_DB_STL_StoreElement()</code> is not needed.
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</p>
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<p>
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Note that passing a NULL pointer to a container of
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<code class="literal">char*</code> type or passing a
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<code class="classname">std::string</code> with no contents at all will insert
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an empty string of zero length into the database.
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</p>
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<p>
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The string returned from a container will not live beyond the next
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iterator movement call, <code class="literal">container::operator[]</code> or
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<code class="literal">iterator::operator*/-></code> call.
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</p>
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<p>
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A <span class="bold"><strong>db_map::value_type::second_type</strong></span> or
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<span class="bold"><strong>db_map::datatype_wrap</strong></span> should be used
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to hold a reference to a <code class="literal">container::operator[]</code>
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return value. Then the reference should be used for repeated
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references to that value. The *iterator is of type
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<code class="literal">ElementHolder<char *></code>, which can be automatically converted to a
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<code class="literal">char *</code> pointer using its type conversion operator.
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Wherever an auto conversion is done by the compiler, the conversion
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operator of <code class="literal">ElementHolder<T></code> is called. This
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avoids almost all explicit conversions, except for two use cases:
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</p>
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<div class="orderedlist">
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<ol type="1">
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<li>
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<p>
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The *iterator is used as a "..." parameter like this:
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</p>
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<pre class="programlisting">printf("this is the special case %s", *iterator);</pre>
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<p>
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This compiles but causes errors. Instead, an explicit cast
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should be used:
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</p>
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<pre class="programlisting">printf("this is the special case %s", (char *)*iterator);</pre>
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</li>
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<li>
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<p>
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For some old compilers, such as gcc3.4.6, the *iterator cannot be
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used with the ternary <code class="literal">?</code> operator, like
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this:
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</p>
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<pre class="programlisting">expr ? *iterator : var</pre>
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<p>
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Even when <span class="bold"><strong>var</strong></span> is the same
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type as the iterator's <code class="literal">value_type</code>, the
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compiler fails to perform an auto conversion.
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</p>
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</li>
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</ol>
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</div>
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<p>
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When using <code class="classname">std::string</code> or
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<code class="classname">std::wstring</code> as the data type for dbstl
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containers — that is, <code class="classname">db_vector<string></code>,
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and <code class="classname">db_map<string, wstring></code> — the
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string's content rather than the string object itself is stored in order
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to maintain persistence.
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</p>
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<p>
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You can find example code demonstrating string storage in the
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<code class="methodname">StlAdvancedFeaturesExample::char_star_string_storage()</code> and
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<code class="methodname">StlAdvancedFeaturesExample::storing_std_strings()</code> methods.
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</p>
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</div>
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</div>
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<div class="navfooter">
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<hr />
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||
<table width="100%" summary="Navigation footer">
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||
<tr>
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||
<td width="40%" align="left"><a accesskey="p" href="stl_mt_usage.html">Prev</a> </td>
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<td width="20%" align="center">
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<a accesskey="u" href="stl.html">Up</a>
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</td>
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<td width="40%" align="right"> <a accesskey="n" href="stl_complex_rw.html">Next</a></td>
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</tr>
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<tr>
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<td width="40%" align="left" valign="top">Using dbstl in multithreaded applications </td>
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<td width="20%" align="center">
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<a accesskey="h" href="index.html">Home</a>
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</td>
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<td width="40%" align="right" valign="top"> Store and Retrieve data or objects of complex types </td>
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</tr>
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</table>
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</div>
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</body>
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</html>
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