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<div class="title">Wt::Dbo::weak_ptr< C > Class Template Reference<div class="ingroups"><a class="el" href="group__dbo.html">Database Objects (Wt::Dbo)</a></div></div> </div>
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<!-- doxytag: class="Wt::Dbo::weak_ptr" -->
<p>A weak pointer for a database object.
<a href="classWt_1_1Dbo_1_1weak__ptr.html#details">More...</a></p>
<p><code>#include <Wt/Dbo/weak_ptr></code></p>
<p><a href="classWt_1_1Dbo_1_1weak__ptr-members.html">List of all members.</a></p>
<table class="memberdecls">
<tr><td colspan="2"><h2><a name="pub-methods"></a>
Public Member Functions</h2></td></tr>
<tr><td class="memItemLeft" align="right" valign="top"> </td><td class="memItemRight" valign="bottom"><a class="el" href="classWt_1_1Dbo_1_1weak__ptr.html#ad66ef2f74e0242eec03488f4a534d161">weak_ptr</a> ()</td></tr>
<tr><td class="mdescLeft"> </td><td class="mdescRight">Creates a new weak pointer. <a href="#ad66ef2f74e0242eec03488f4a534d161"></a><br/></td></tr>
<tr><td class="memItemLeft" align="right" valign="top"><a class="anchor" id="a6fcd8683e81f1a8e30b3b72f9bd98eaa"></a><!-- doxytag: member="Wt::Dbo::weak_ptr::weak_ptr" ref="a6fcd8683e81f1a8e30b3b72f9bd98eaa" args="(const weak_ptr< C > &other)" -->
 </td><td class="memItemRight" valign="bottom"><a class="el" href="classWt_1_1Dbo_1_1weak__ptr.html#a6fcd8683e81f1a8e30b3b72f9bd98eaa">weak_ptr</a> (const <a class="el" href="classWt_1_1Dbo_1_1weak__ptr.html">weak_ptr</a>< C > &other)</td></tr>
<tr><td class="mdescLeft"> </td><td class="mdescRight">Copy constructor. <br/></td></tr>
<tr><td class="memItemLeft" align="right" valign="top">void </td><td class="memItemRight" valign="bottom"><a class="el" href="classWt_1_1Dbo_1_1weak__ptr.html#a3ab3b78fba6dcdb657688cce05abb622">reset</a> (C *obj=0)</td></tr>
<tr><td class="mdescLeft"> </td><td class="mdescRight">Sets the value. <a href="#a3ab3b78fba6dcdb657688cce05abb622"></a><br/></td></tr>
<tr><td class="memItemLeft" align="right" valign="top"><a class="el" href="classWt_1_1Dbo_1_1weak__ptr.html">weak_ptr</a>< C > & </td><td class="memItemRight" valign="bottom"><a class="el" href="classWt_1_1Dbo_1_1weak__ptr.html#a0e9ad1c975f81deb6c84f2e0729441e2">operator=</a> (const <a class="el" href="classWt_1_1Dbo_1_1weak__ptr.html">weak_ptr</a>< C > &other)</td></tr>
<tr><td class="mdescLeft"> </td><td class="mdescRight">Assignment operator. <a href="#a0e9ad1c975f81deb6c84f2e0729441e2"></a><br/></td></tr>
<tr><td class="memItemLeft" align="right" valign="top"><a class="el" href="classWt_1_1Dbo_1_1weak__ptr.html">weak_ptr</a>< C > & </td><td class="memItemRight" valign="bottom"><a class="el" href="classWt_1_1Dbo_1_1weak__ptr.html#a0ca7f9b95f69c755cc8c7df1d0f211fd">operator=</a> (const <a class="el" href="classWt_1_1Dbo_1_1ptr.html">ptr</a>< C > &other)</td></tr>
<tr><td class="mdescLeft"> </td><td class="mdescRight">Assignment operator. <a href="#a0ca7f9b95f69c755cc8c7df1d0f211fd"></a><br/></td></tr>
<tr><td class="memItemLeft" align="right" valign="top">const C * </td><td class="memItemRight" valign="bottom"><a class="el" href="classWt_1_1Dbo_1_1weak__ptr.html#a538df66d4e143c372df1e4b6d6cd42fa">operator-></a> () const </td></tr>
<tr><td class="mdescLeft"> </td><td class="mdescRight">Dereference operator. <a href="#a538df66d4e143c372df1e4b6d6cd42fa"></a><br/></td></tr>
<tr><td class="memItemLeft" align="right" valign="top">C * </td><td class="memItemRight" valign="bottom"><a class="el" href="classWt_1_1Dbo_1_1weak__ptr.html#a8ed7e6eb2807e52dfbe2e579f4fd1802">modify</a> () const </td></tr>
<tr><td class="mdescLeft"> </td><td class="mdescRight">Dereference operator, for writing. <a href="#a8ed7e6eb2807e52dfbe2e579f4fd1802"></a><br/></td></tr>
<tr><td class="memItemLeft" align="right" valign="top">bool </td><td class="memItemRight" valign="bottom"><a class="el" href="classWt_1_1Dbo_1_1weak__ptr.html#a57b32ae3897b65ef98496c4b52fc31f3">operator==</a> (const <a class="el" href="classWt_1_1Dbo_1_1weak__ptr.html">weak_ptr</a>< C > &other) const </td></tr>
<tr><td class="mdescLeft"> </td><td class="mdescRight">Comparison operator. <a href="#a57b32ae3897b65ef98496c4b52fc31f3"></a><br/></td></tr>
<tr><td class="memItemLeft" align="right" valign="top">bool </td><td class="memItemRight" valign="bottom"><a class="el" href="classWt_1_1Dbo_1_1weak__ptr.html#a47d41108ffbb8bf34b697404d4b32883">operator==</a> (const <a class="el" href="classWt_1_1Dbo_1_1ptr.html">ptr</a>< C > &other) const </td></tr>
<tr><td class="mdescLeft"> </td><td class="mdescRight">Comparison operator. <a href="#a47d41108ffbb8bf34b697404d4b32883"></a><br/></td></tr>
<tr><td class="memItemLeft" align="right" valign="top">bool </td><td class="memItemRight" valign="bottom"><a class="el" href="classWt_1_1Dbo_1_1weak__ptr.html#a54d6de12820954bfee289fdcc2d051cf">operator!=</a> (const <a class="el" href="classWt_1_1Dbo_1_1weak__ptr.html">weak_ptr</a>< C > &other) const </td></tr>
<tr><td class="mdescLeft"> </td><td class="mdescRight">Comparison operator. <a href="#a54d6de12820954bfee289fdcc2d051cf"></a><br/></td></tr>
<tr><td class="memItemLeft" align="right" valign="top">bool </td><td class="memItemRight" valign="bottom"><a class="el" href="classWt_1_1Dbo_1_1weak__ptr.html#a77999ff9a0499bbcd77131b284aedd6b">operator!=</a> (const <a class="el" href="classWt_1_1Dbo_1_1ptr.html">ptr</a>< C > &other) const </td></tr>
<tr><td class="mdescLeft"> </td><td class="mdescRight">Comparison operator. <a href="#a77999ff9a0499bbcd77131b284aedd6b"></a><br/></td></tr>
<tr><td class="memItemLeft" align="right" valign="top"> </td><td class="memItemRight" valign="bottom"><a class="el" href="classWt_1_1Dbo_1_1weak__ptr.html#a9083ae4fd9eda8d0ee695bd0785b81d7">operator bool</a> () const </td></tr>
<tr><td class="mdescLeft"> </td><td class="mdescRight">Checks for null. <a href="#a9083ae4fd9eda8d0ee695bd0785b81d7"></a><br/></td></tr>
<tr><td class="memItemLeft" align="right" valign="top"> </td><td class="memItemRight" valign="bottom"><a class="el" href="classWt_1_1Dbo_1_1weak__ptr.html#a7ad82f37f30b4dc467a3017183d83ba1">operator ptr< C ></a> () const </td></tr>
<tr><td class="mdescLeft"> </td><td class="mdescRight">Casting operator to a ptr. <a href="#a7ad82f37f30b4dc467a3017183d83ba1"></a><br/></td></tr>
<tr><td class="memItemLeft" align="right" valign="top"><a class="el" href="classWt_1_1Dbo_1_1ptr.html">ptr</a>< C > </td><td class="memItemRight" valign="bottom"><a class="el" href="classWt_1_1Dbo_1_1weak__ptr.html#a0644010958fecb882ddb64510a2c377d">query</a> () const </td></tr>
<tr><td class="mdescLeft"> </td><td class="mdescRight">Promotes to a ptr. <a href="#a0644010958fecb882ddb64510a2c377d"></a><br/></td></tr>
<tr><td class="memItemLeft" align="right" valign="top"><a class="el" href="classWt_1_1Dbo_1_1ptr.html">ptr</a>< C > </td><td class="memItemRight" valign="bottom"><a class="el" href="classWt_1_1Dbo_1_1weak__ptr.html#abd9ba3eca68c4de65ecfdd493e35aed4">lock</a> () const </td></tr>
<tr><td class="mdescLeft"> </td><td class="mdescRight">Promotes to a ptr. <a href="#abd9ba3eca68c4de65ecfdd493e35aed4"></a><br/></td></tr>
<tr><td class="memItemLeft" align="right" valign="top"><a class="el" href="structWt_1_1Dbo_1_1dbo__traits.html">dbo_traits</a>< C >::IdType </td><td class="memItemRight" valign="bottom"><a class="el" href="classWt_1_1Dbo_1_1weak__ptr.html#ac8d8a7184c97bbc991b30ab2e57a0abe">id</a> () const </td></tr>
<tr><td class="mdescLeft"> </td><td class="mdescRight">Returns the object id. <a href="#ac8d8a7184c97bbc991b30ab2e57a0abe"></a><br/></td></tr>
</table>
<hr/><a name="details" id="details"></a><h2>Detailed Description</h2>
<div class="textblock"><h3>template<class C><br/>
class Wt::Dbo::weak_ptr< C ></h3>
<p>A weak pointer for a database object. </p>
<p>A <a class="el" href="classWt_1_1Dbo_1_1weak__ptr.html" title="A weak pointer for a database object.">weak_ptr</a> has similar API and capabilities as a ptr, but does not actually store a reference to the object itself. Instead, it uses a query to retrieve the object.</p>
<p>It is used only to implement a OneToOne relation:</p>
<ul>
<li>class A hasOne class B object, in a <a class="el" href="classWt_1_1Dbo_1_1weak__ptr.html" title="A weak pointer for a database object.">weak_ptr</a></li>
<li>class B belongsTo a class A object, in a ptr</li>
</ul>
<p>This cannot be implemented using two plain ptr's because otherwise a cycle would be created which interferes with the count-based shared pointer implementation of ptr.</p>
<p>Only in a OneToOne relation, the required querying logic for the <a class="el" href="classWt_1_1Dbo_1_1weak__ptr.html" title="A weak pointer for a database object.">weak_ptr</a> to function properly is provided by the <a class="el" href="group__dbo.html#ga00a060d5f4764f673948989f21837548" title="Maps the "One"-side of a OneToOne relation.">hasOne()</a> call, and thus a <a class="el" href="classWt_1_1Dbo_1_1weak__ptr.html" title="A weak pointer for a database object.">weak_ptr</a> cannot be used outside of a OneToOne relation.</p>
<p>The <a class="el" href="classWt_1_1Dbo_1_1weak__ptr.html" title="A weak pointer for a database object.">weak_ptr</a> has almost the same capabilities as a ptr, except that it does not provide a dereference operator (*).</p>
<p>It's important to realize that reading a <a class="el" href="classWt_1_1Dbo_1_1weak__ptr.html" title="A weak pointer for a database object.">weak_ptr</a> results in a query, and thus you may want to first convert the <a class="el" href="classWt_1_1Dbo_1_1weak__ptr.html" title="A weak pointer for a database object.">weak_ptr</a> into a plain ptr when needing multiple operations. </p>
</div><hr/><h2>Constructor & Destructor Documentation</h2>
<a class="anchor" id="ad66ef2f74e0242eec03488f4a534d161"></a><!-- doxytag: member="Wt::Dbo::weak_ptr::weak_ptr" ref="ad66ef2f74e0242eec03488f4a534d161" args="()" -->
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template<class C > </div>
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<td class="memname"><a class="el" href="classWt_1_1Dbo_1_1weak__ptr.html">Wt::Dbo::weak_ptr</a>< C >::<a class="el" href="classWt_1_1Dbo_1_1weak__ptr.html">weak_ptr</a> </td>
<td>(</td>
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<p>Creates a new weak pointer. </p>
<p>The ptr does not point to anything. </p>
</div>
</div>
<hr/><h2>Member Function Documentation</h2>
<a class="anchor" id="ac8d8a7184c97bbc991b30ab2e57a0abe"></a><!-- doxytag: member="Wt::Dbo::weak_ptr::id" ref="ac8d8a7184c97bbc991b30ab2e57a0abe" args="() const " -->
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<div class="memtemplate">
template<class C > </div>
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<tr>
<td class="memname"><a class="el" href="structWt_1_1Dbo_1_1dbo__traits.html">dbo_traits</a>< C >::IdType <a class="el" href="classWt_1_1Dbo_1_1weak__ptr.html">Wt::Dbo::weak_ptr</a>< C >::id </td>
<td>(</td>
<td class="paramname"></td><td>)</td>
<td> const</td>
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<p>Returns the object id. </p>
<p>Since this needs to query the value, you should have an active transaction. </p>
</div>
</div>
<a class="anchor" id="abd9ba3eca68c4de65ecfdd493e35aed4"></a><!-- doxytag: member="Wt::Dbo::weak_ptr::lock" ref="abd9ba3eca68c4de65ecfdd493e35aed4" args="() const " -->
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template<class C > </div>
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<td class="memname"><a class="el" href="classWt_1_1Dbo_1_1ptr.html">ptr</a><C> <a class="el" href="classWt_1_1Dbo_1_1weak__ptr.html">Wt::Dbo::weak_ptr</a>< C >::lock </td>
<td>(</td>
<td class="paramname"></td><td>)</td>
<td> const</td>
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<p>Promotes to a ptr. </p>
<p>Returns the value as a plain ptr.</p>
<p>Since this needs to query the value, you should have an active transaction.</p>
<p>This is equivalent to <a class="el" href="classWt_1_1Dbo_1_1weak__ptr.html#a0644010958fecb882ddb64510a2c377d" title="Promotes to a ptr.">query()</a> or the ptr<C> cast operator. </p>
</div>
</div>
<a class="anchor" id="a8ed7e6eb2807e52dfbe2e579f4fd1802"></a><!-- doxytag: member="Wt::Dbo::weak_ptr::modify" ref="a8ed7e6eb2807e52dfbe2e579f4fd1802" args="() const " -->
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template<class C > </div>
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<td class="memname"><a class="el" href="classWt_1_1Dbo_1_1weak__ptr.html">weak_ptr</a>< C >::mutator <a class="el" href="classWt_1_1Dbo_1_1weak__ptr.html">Wt::Dbo::weak_ptr</a>< C >::modify </td>
<td>(</td>
<td class="paramname"></td><td>)</td>
<td> const</td>
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<p>Dereference operator, for writing. </p>
<p>Returns the underlying object (or, rather, a proxy for it) with the intention to modify it. The proxy object will mark the object as dirty from its destructor. An involved modification should therefore preferably be implemented as a separate method or function to make sure that the object is marked as dirty after the whole modification: </p>
<div class="fragment"><pre class="fragment"> weak_ptr<A> a = ...;
a.modify()->doSomething();
</pre></div><p>Since this needs to query the value, you should have an active transaction. </p>
</div>
</div>
<a class="anchor" id="a9083ae4fd9eda8d0ee695bd0785b81d7"></a><!-- doxytag: member="Wt::Dbo::weak_ptr::operator bool" ref="a9083ae4fd9eda8d0ee695bd0785b81d7" args="() const " -->
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<td> const</td>
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<p>Checks for null. </p>
<p>Returns true if the pointer is pointing to a non-null object.</p>
<p>Since this needs to query the value, you should have an active transaction. </p>
</div>
</div>
<a class="anchor" id="a7ad82f37f30b4dc467a3017183d83ba1"></a><!-- doxytag: member="Wt::Dbo::weak_ptr::operator ptr< C >" ref="a7ad82f37f30b4dc467a3017183d83ba1" args="() const " -->
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<td class="memname"><a class="el" href="classWt_1_1Dbo_1_1weak__ptr.html">Wt::Dbo::weak_ptr</a>< C >::operator <a class="el" href="classWt_1_1Dbo_1_1ptr.html">ptr</a>< C > </td>
<td>(</td>
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<p>Casting operator to a ptr. </p>
<p>Returns the value as a plain ptr.</p>
<p>Since this needs to query the value, you should have an active transaction. </p>
</div>
</div>
<a class="anchor" id="a54d6de12820954bfee289fdcc2d051cf"></a><!-- doxytag: member="Wt::Dbo::weak_ptr::operator!=" ref="a54d6de12820954bfee289fdcc2d051cf" args="(const weak_ptr< C > &other) const " -->
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<td class="memname">bool <a class="el" href="classWt_1_1Dbo_1_1weak__ptr.html">Wt::Dbo::weak_ptr</a>< C >::operator!= </td>
<td>(</td>
<td class="paramtype">const <a class="el" href="classWt_1_1Dbo_1_1weak__ptr.html">weak_ptr</a>< C > & </td>
<td class="paramname"><em>other</em></td><td>)</td>
<td> const</td>
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<p>Comparison operator. </p>
<p>Two pointers are equal if and only if they reference the same database object.</p>
<p>Since this needs to query the value, you should have an active transaction. </p>
</div>
</div>
<a class="anchor" id="a77999ff9a0499bbcd77131b284aedd6b"></a><!-- doxytag: member="Wt::Dbo::weak_ptr::operator!=" ref="a77999ff9a0499bbcd77131b284aedd6b" args="(const ptr< C > &other) const " -->
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<td class="memname">bool <a class="el" href="classWt_1_1Dbo_1_1weak__ptr.html">Wt::Dbo::weak_ptr</a>< C >::operator!= </td>
<td>(</td>
<td class="paramtype">const <a class="el" href="classWt_1_1Dbo_1_1ptr.html">ptr</a>< C > & </td>
<td class="paramname"><em>other</em></td><td>)</td>
<td> const</td>
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<div class="memdoc">
<p>Comparison operator. </p>
<p>Two pointers are equal if and only if they reference the same database object.</p>
<p>Since this needs to query the value, you should have an active transaction. </p>
</div>
</div>
<a class="anchor" id="a538df66d4e143c372df1e4b6d6cd42fa"></a><!-- doxytag: member="Wt::Dbo::weak_ptr::operator->" ref="a538df66d4e143c372df1e4b6d6cd42fa" args="() const " -->
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<td class="memname"><a class="el" href="classWt_1_1Dbo_1_1weak__ptr.html">weak_ptr</a>< C >::accessor <a class="el" href="classWt_1_1Dbo_1_1weak__ptr.html">Wt::Dbo::weak_ptr</a>< C >::operator-> </td>
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<td class="paramname"></td><td>)</td>
<td> const</td>
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<p>Dereference operator. </p>
<p>Note that this operator returns a const copy of the referenced object. Use <a class="el" href="classWt_1_1Dbo_1_1weak__ptr.html#a8ed7e6eb2807e52dfbe2e579f4fd1802" title="Dereference operator, for writing.">modify()</a> to get a non-const reference.</p>
<p>Since this needs to query the value, you should have an active transaction. </p>
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<a class="anchor" id="a0e9ad1c975f81deb6c84f2e0729441e2"></a><!-- doxytag: member="Wt::Dbo::weak_ptr::operator=" ref="a0e9ad1c975f81deb6c84f2e0729441e2" args="(const weak_ptr< C > &other)" -->
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template<class C > </div>
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<td class="memname"><a class="el" href="classWt_1_1Dbo_1_1weak__ptr.html">weak_ptr</a>< C > & <a class="el" href="classWt_1_1Dbo_1_1weak__ptr.html">Wt::Dbo::weak_ptr</a>< C >::operator= </td>
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<td class="paramtype">const <a class="el" href="classWt_1_1Dbo_1_1weak__ptr.html">weak_ptr</a>< C > & </td>
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<p>Assignment operator. </p>
<p>Since this needs to query the previous value, you should have an active transaction. </p>
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<a class="anchor" id="a0ca7f9b95f69c755cc8c7df1d0f211fd"></a><!-- doxytag: member="Wt::Dbo::weak_ptr::operator=" ref="a0ca7f9b95f69c755cc8c7df1d0f211fd" args="(const ptr< C > &other)" -->
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template<class C > </div>
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<td class="memname"><a class="el" href="classWt_1_1Dbo_1_1weak__ptr.html">weak_ptr</a>< C > & <a class="el" href="classWt_1_1Dbo_1_1weak__ptr.html">Wt::Dbo::weak_ptr</a>< C >::operator= </td>
<td>(</td>
<td class="paramtype">const <a class="el" href="classWt_1_1Dbo_1_1ptr.html">ptr</a>< C > & </td>
<td class="paramname"><em>other</em></td><td>)</td>
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<p>Assignment operator. </p>
<p>Since this needs to query the previous value, you should have an active transaction. </p>
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</div>
<a class="anchor" id="a57b32ae3897b65ef98496c4b52fc31f3"></a><!-- doxytag: member="Wt::Dbo::weak_ptr::operator==" ref="a57b32ae3897b65ef98496c4b52fc31f3" args="(const weak_ptr< C > &other) const " -->
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template<class C > </div>
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<td class="memname">bool <a class="el" href="classWt_1_1Dbo_1_1weak__ptr.html">Wt::Dbo::weak_ptr</a>< C >::operator== </td>
<td>(</td>
<td class="paramtype">const <a class="el" href="classWt_1_1Dbo_1_1weak__ptr.html">weak_ptr</a>< C > & </td>
<td class="paramname"><em>other</em></td><td>)</td>
<td> const</td>
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<p>Comparison operator. </p>
<p>Two pointers are equal if and only if they reference the same database object.</p>
<p>Since this needs to query the value, you should have an active transaction. </p>
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<a class="anchor" id="a47d41108ffbb8bf34b697404d4b32883"></a><!-- doxytag: member="Wt::Dbo::weak_ptr::operator==" ref="a47d41108ffbb8bf34b697404d4b32883" args="(const ptr< C > &other) const " -->
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template<class C > </div>
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<td class="memname">bool <a class="el" href="classWt_1_1Dbo_1_1weak__ptr.html">Wt::Dbo::weak_ptr</a>< C >::operator== </td>
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<td class="paramtype">const <a class="el" href="classWt_1_1Dbo_1_1ptr.html">ptr</a>< C > & </td>
<td class="paramname"><em>other</em></td><td>)</td>
<td> const</td>
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<p>Comparison operator. </p>
<p>Two pointers are equal if and only if they reference the same database object.</p>
<p>Since this needs to query the value, you should have an active transaction. </p>
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<a class="anchor" id="a0644010958fecb882ddb64510a2c377d"></a><!-- doxytag: member="Wt::Dbo::weak_ptr::query" ref="a0644010958fecb882ddb64510a2c377d" args="() const " -->
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template<class C > </div>
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<td class="memname"><a class="el" href="classWt_1_1Dbo_1_1ptr.html">ptr</a>< C > <a class="el" href="classWt_1_1Dbo_1_1weak__ptr.html">Wt::Dbo::weak_ptr</a>< C >::query </td>
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<p>Promotes to a ptr. </p>
<p>Returns the value as a plain ptr.</p>
<p>Since this needs to query the value, you should have an active transaction.</p>
<p>This is equivalent to <a class="el" href="classWt_1_1Dbo_1_1weak__ptr.html#abd9ba3eca68c4de65ecfdd493e35aed4" title="Promotes to a ptr.">lock()</a> or the ptr<C> cast operator. </p>
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<a class="anchor" id="a3ab3b78fba6dcdb657688cce05abb622"></a><!-- doxytag: member="Wt::Dbo::weak_ptr::reset" ref="a3ab3b78fba6dcdb657688cce05abb622" args="(C *obj=0)" -->
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<td class="memname">void <a class="el" href="classWt_1_1Dbo_1_1weak__ptr.html">Wt::Dbo::weak_ptr</a>< C >::reset </td>
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<p>Sets the value. </p>
<p>This is assigns a new value, and: </p>
<div class="fragment"><pre class="fragment"> p.reset(obj);
</pre></div><p> is equivalent to: </p>
<div class="fragment"><pre class="fragment"> p = <a class="code" href="classWt_1_1Dbo_1_1ptr.html" title="A smart pointer for a database object.">Wt::Dbo::ptr<T></a>(obj);
</pre></div><p>Since this needs to query the previous value, you should have an active transaction. </p>
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</div>
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