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<concept id="GUID-8E363123-A85E-521C-BC10-96C59130E45C" xml:lang="en"><title>How
to walk the heap</title><shortdesc>The <codeph>THeapWalk</codeph> class provides behaviour for walking
the heap. This class is pure virtual and developers must provide a derived
class in order to use it.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>
<p>A heap can be checked to make sure that its allocated and free cells are
in a consistent state and that the heap is not corrupt.</p>
<p>The <codeph>THeapWalk</codeph> class provides the behaviour for doing this.
This class is pure virtual and developers must provide a derived class in
order to use it.</p>
<p>Simply construct a <codeph>THeapWalk</codeph> derived object, passing a
reference to the heap which is to be checked in its constructor. Walking the
heap is initiated by calling <codeph>Walk()</codeph>. This function follows
the list of allocated and free cells in the heap. The function calls the virtual
function <codeph>Info()</codeph> for every good cell it finds, passing the
cell's address and length.</p>
<p><codeph>Info()</codeph> is a pure virtual function and a derived class
must provide an implementation for it.</p>
<p><codeph>Walk()</codeph> terminates when it has successfully followed the
entire list of free and allocated cells or until it finds the first bad cell.</p>
<p>As a minimum, a class derived from <codeph>THeapWalk</codeph> might be:</p>
<codeblock id="GUID-A3AC2314-4C1E-5F1F-9BCB-694893AB47FB" xml:space="preserve">class TMyClass : public THeapWalk
    {</codeblock>
<codeblock id="GUID-C45FDA92-5BEE-57A3-BB1B-F4AB203EC0EC" xml:space="preserve">public :
    TMyClass(RHeap&amp; aHeap);
    void Info(TCellType aType,TAny *aBase,TInt aLength);
    }</codeblock>
<p>where the constructor would be implemented as:</p>
<codeblock id="GUID-F7B66153-EAFE-570C-997F-42BDBB05DE01" xml:space="preserve">TMyClass::TMyClass(RHeap&amp; aHeap)
    : THeapWalk(aHeap)
    {}</codeblock>
<p>Typically, the <codeph>Info()</codeph> function might consist of a switch
statement based on the value of <codeph>aType</codeph>:</p>
<codeblock id="GUID-379F76EF-8FC0-5085-BB05-373E2B64508C" xml:space="preserve">TMyClass::Info(TCellType aType,TAny *aBase,TInt aLength)
    {
    switch(aType)
        {
    case EGoodAllocatedCell:
        ...
        break;
    case EGoodFreeCell:
        ...
        break;
    case EBadFreeCellAddress:
        ...
        break;
    default:
        ...
        }
    }</codeblock>
</conbody></concept>