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<!DOCTYPE concept
  PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">
<concept id="GUID-59ED6D14-0976-58AE-8DDC-73F051866406" xml:lang="en"><title>How
to construct a header</title><shortdesc>The header of a linked list needs to know the type of data for
each list element, and where in each element the pointer to the next element
can be found.</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-5BD8E773-5793-5C52-BF07-62FF1839F19A"><p>the <i>type</i> of object
which forms the elements.</p> </li>
<li id="GUID-9E43534B-D692-53CE-83D9-FE4F1B2DFC42"><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 singly linked list of <codeph>CMyClass</codeph> elements,
defined as:</p>
<codeblock id="GUID-8851C435-33CD-5F1C-95E3-60CEC2770D64" xml:space="preserve">class CMyClass : public CBase
    {
    ...
    TSglQueLink iSlink;
    ...
    };</codeblock>
<p>construct a <codeph>TSglQue</codeph> as follows:</p>
<codeblock id="GUID-D634E7AB-9240-5F0E-9D36-BE476B038842" xml:space="preserve">TSglQue&lt;CMyClass&gt; queHeader(_FOFF(CMyClass,iSlink));</codeblock>
<p>The <codeph>TSglQue</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>iSlink</codeph> data member
from the start of <codeph>CMyClass</codeph>. </p>
<fig id="GUID-3B9C78A4-02FC-5D63-AFD2-53996F9CAEC7">
<title>Links in singly linked lists</title>
<image href="GUID-FB654184-C256-546F-AF6C-E0FADA78D5D0_d0e214615_href.png" placement="inline"/>
</fig>
</conbody></concept>