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    10 <!DOCTYPE concept

    11   PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">

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

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

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

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

    16 needed:</p>

    17 <ul>

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

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

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

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

    22 </ul>

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

    24 defined as:</p>

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

    26     {

    27     ...

    28     TDblQueLink iDlink;

    29     ...

    30     };</codeblock>

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

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

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

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

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

    36 list.</p>

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

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

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

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

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

    42 <title/>

    43 <image href="GUID-29E7942D-A00D-5771-9782-59260965C687_d0e217648_href.png" placement="inline"/>

    44 </fig>

    45 </conbody></concept>