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<concept xml:lang="en" id="GUID-497930CE-4D61-50EE-A63B-3656158EE29C"><title>Thin Templates</title><prolog><metadata><keywords/></metadata></prolog><conbody><p>Symbian platform C++ uses C++ templates in an efficient way called the thin template idiom. </p> <p>Templates are particularly useful for collection classes, such as arrays. The class declaration: </p> <codeblock id="GUID-ACAAAE20-4A81-5C4F-930B-31097FBAD432" xml:space="preserve">class CArrayFixFlat&lt;T&gt; ...</codeblock> <p>specifies a family of array classes, which may contain any type. The template may be instantiated, for instance, as </p> <codeblock id="GUID-D02E8C13-85FA-5DA2-8A5E-5F1C15F4038C" xml:space="preserve">CArrayFixFlat&lt;CCoeControl*&gt;* iControls;</codeblock> <p>which declares an array of <codeph>CCoeControl*</codeph> pointers. Other collection classes use the same idea, for instance <codeph>TPckgBuf&lt;T&gt;</codeph>, <codeph>TSglQue&lt;T&gt;</codeph> etc. </p> <p>Templates may also use other parameters, such as an integer: </p> <codeblock id="GUID-209E8594-EFC2-5F55-AF45-51CE538C1336" xml:space="preserve">class TBufC&lt;TInt S&gt; ...;</codeblock> <p>This type of template class may be instantiated also: </p> <codeblock id="GUID-5ABEBD1D-1B0A-5C57-B35E-260180F31978" xml:space="preserve">TBufC&lt;20&gt; name; </codeblock> <p>The functionality provided by templates is powerful, and highly desirable. Without templates, collection classes usually use <codeph>void*</codeph> pointers: as a result, they are not typesafe. </p> <p>Templates can be difficult to manage. A template really defines a whole family of classes. Each member of that family that is instantiated requires its own object code. Avoidance of object code duplication is a difficult issue in a C++ implementation. For the Symbian platform, object code duplication must be avoided at all costs. The solution used is the <i>thin template idiom</i>. </p> <p>The thin template idiom begins with a base class, code in terms of <codeph>TAny*</codeph> parameters: </p> <codeblock id="GUID-C6ADB9B7-4290-5A6B-B1B4-7D9C88E90B1F" xml:space="preserve">class CArrayFixBase ... {

    IMPORT_C const TAny* At(TInt aIndex) const;</codeblock> <p>This base class has the real code, just once. The code resides in a single DLL and is exported from that DLL. The base class may be fat: it may contain an arbitrary amount of code. </p> <p>Then, a derived class is used as follows: </p> <codeblock id="GUID-58361D84-0D13-5E9B-936F-1D4724A71282" xml:space="preserve">class CArrayFix&lt;T&gt; : public CArrayFixBase {

    inline const T& At(TInt aIndex) const