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<concept xml:lang="en" id="GUID-8E363123-A85E-521C-BC10-96C59130E45C"><title>How to walk the heap</title><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>