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<concept id="GUID-6163197B-7983-5FB7-9B92-02E490A900FB" xml:lang="en"><title>How to

construct a header</title><shortdesc>This document describes how to create a header for a doubly linked

list.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>

<p>In order to construct a header, two important pieces of information are

needed:</p>

<ul>

<li id="GUID-2E721D77-E010-5211-BABE-94578AECEEE4"><p>the <i>type</i> of object

which forms the elements.</p> </li>

<li id="GUID-71FC8FBF-803F-55BC-AC52-D672B1DA36BA"><p>the <i>offset</i> of

the link object from the start of the element.</p> </li>

</ul>

<p>To create a header for a doubly linked list of <codeph>CMyClass</codeph> elements,

defined as:</p>

<codeblock id="GUID-D995F2F7-6A38-5EE3-9210-1302E1662759" xml:space="preserve">class CMyClass : public CBase

    {

    ...

    TDblQueLink iDlink;

    ...

    };</codeblock>

<p>construct a <codeph>TDblQue</codeph> as follows:</p>

<codeblock id="GUID-D13C33D2-EE7E-5915-92E2-F3EC5C772B4D" xml:space="preserve">TDblQue&lt;CMyClass&gt; queHeader(_FOFF(CMyClass,iDlink));</codeblock>

<p>The <codeph>TDblQue</codeph> class takes a class template parameter which

defines the type of object which is to form an element of the list. In this

example, objects of type <codeph>CMyClass</codeph> form the elements of the

list.</p>

<p>The <codeph>_FOFF</codeph> macro (i.e. the "field offset" macro) calculates

the offset of the component link object within the element. In this example,

the required value is the offset of the <codeph>iDlink</codeph> data member

from the start of <codeph>CMyClass</codeph>. </p>

<fig id="GUID-9C102C34-EFFA-530A-AA1E-C6AE08FDFA37">

<title/>

<image href="GUID-29E7942D-A00D-5771-9782-59260965C687_d0e213911_href.png" placement="inline"/>

</fig>