Implementing +TKey derived classes

Constructors

The constructors are used to set the +following information:

the key offset
the type of comparison +to be made between keys
the length of the key +(for text type keys only).

For example:

TKeyDerived::TKeyDerived(TInt anOffset,TKeyCmpText aType) + : TKey(anOffset,aType) + {} + TKeyDerived::TKeyDerived(TInt anOffset,TKeyCmpText aType,TInt aLength) + : TKey(anOffset,aType,aLength) + {} + TKeyDerived::TKeyDerived(TInt anOffset,TKeyCmpNumeric aType) + : TKey(anOffset,aType) + {} +

Implementing the At() function

A typical implementation +of the virtual function At(), which gets a pointer to the +key of an element corresponding to a given index, might be structured:

TAny* TKeyDerived::At(TInt anIndex) const + { + if (anIndex==KIndexPtr) + { + return((TUint8 *)iPtr+iKeyOffset); + } + // + // code to return a pointer to the key in the appropriate element + // + } +

The derived class will need to have a pointer to the array +it represents. At the very least, the derived class will need a data member +for this pointer and, possibly, a member function to set it. This is needed +in order to implement the At() function.

TKey and +derived classes use untyped pointers, i.e. pointers of type TAny*. +It may be desirable to provide type safety by further deriving a templated +class.