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<!DOCTYPE concept
  PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">
<concept id="GUID-442F2E69-87B5-54E7-B62A-70B742432372" xml:lang="en"><title>How
to Commit and Revert </title><shortdesc>Changes to a store are not made permanent until they are committed
and until such changes are committed, they can be rolled back (i.e. reverted)</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>
<p>Transactions are supported by a commit and revert protocol.</p>
<p>In the following code fragment, <codeph>store</codeph> is a pointer to
an opened permanent file store. The function <codeph>doUpdateStoreL()</codeph> adds
a number of streams to this store and, if successful, establishes a new commit
point.</p>
<p>If a failure occurs or the call to <codeph>CommitL()</codeph> fails, then <codeph>doUpdateStoreL()</codeph> leaves;
the leave is trapped and any changes made to the store are rolled back, so
that the store is in the same state as it was before the changes were started.
The store, therefore, remains in a consistent state in the event of failure.</p>
<p><codeph>CItemArray</codeph> is a class that encapsulates an array of <codeph>TItem</codeph> objects;
the relevant part of the definition is:</p>
<codeblock id="GUID-0B5B04A7-AB4A-55CA-BCF0-9E19623F1554" xml:space="preserve">typedef TBuf&lt;100&gt; TItem;</codeblock>
<codeblock id="GUID-52A3FAC2-4E23-5574-A21C-DE087ADDB911" xml:space="preserve">class CItemArray : public CBase
    {
public:
    ...
    void AddItemL(const TItem&amp; anItem);
    void StoreL() const;
       void ExternalizeL(RWriteStream&amp; aStream) const;
    ...
private:
    CStreamStore&amp; iStore;
       ...
    CArrayFixFlat&lt;TStreamId&gt;* iArray;
    }</codeblock>
<p>The example also assumes that the store is one that allows streams to be
modified and replaced; for example, a permanent file store.</p>
<codeblock id="GUID-21AFD117-3D52-55BE-BEB7-430CF08D0429" xml:space="preserve">...
TRAPD(error,doUpdateStoreL(*store));
if (error!=KErrNone)
    {
    store-&gt;Revert();
    ...
    }
...</codeblock>
<codeblock id="GUID-EB07D2AA-ECAA-5014-84F0-85767A60E366" xml:space="preserve">LOCAL_C void doUpdateStoreL(CPersistentStore&amp; aStore)
    {
    _LIT(KTxtHello,"hello");
    _LIT(KtxtWorld," world!");
              // get the root stream into memory
    CItemArray* array=CItemArray::NewLC(aStore,aStore.Root());
    
              // Add some items
    TItem item;
    item = KTxtHello;
    array-&gt;AddItemL(item);
    item = KTxtWorld;
    array-&gt;AddItemL(item);
             // Re-write the root stream with new data
    array-&gt;StoreL();
            // commit all changes
    aStore.CommitL();
    ...
    }</codeblock>
<codeblock id="GUID-3E2184DA-7C74-58F0-925E-6AE1317732C9" xml:space="preserve">void CItemArray::AddItemL(const TItem&amp; anItem)
    {
    RStoreWriteStream outstream;
    TStreamId id=outstream.CreateLC(iStore);
    outstream&lt;&lt;anItem;
    outstream.CommitL();
    CleanupStack::PopAndDestroy();
    iArray-&gt;AppendL(id);
    }
</codeblock>
</conbody><related-links>
<link href="GUID-C9D8D913-C65F-5A69-A606-30F59BFB38E2.dita"><linktext>File Stores</linktext>
</link>
</related-links></concept>