How to construct and manipulate a singly linked list

The following code fragments show how a singly linked list can be constructed and manipulated. The list consists of instances of an example class, CItem, which form items on a stack implemented as a singly linked list using the iSlink data member as the link object. In this example, a CItem object can contain an item of text implemented as an HBufC.

The class is declared as:

class CItem : public CBase
    {
public :
    static CItem* NewL(const TDesC& aText);
    static CItem* NewLC(const TDesC& aText);
                  CItem();
    virtual ~CItem();
    const HBufC* GetText();
public :
    static const TInt iOffset;
private :
    void ConstructL(const TDesC& aText);
private :
    TSglQueLink iSlink;
    HBufC* iText;
    friend class CStack;
    };

The CItem member functions are implemented as:

const TInt CItem::iOffset = _FOFF(CItem,iSlink);

CItem* CItem::NewLC(const TDesC& aText)
    {
    CItem* self = new (ELeave) CItem();
    CleanupStack::PushL(self);
    self->ConstructL(aText);
    return self;
    }

CItem* CItem::NewL(const TDesC& aText)
    {
    CItem* self = CItem::NewLC(aText);
    CleanupStack::Pop();
    return self;
    }

void CItem::ConstructL(const TDesC& aText)
    {
    iText = aText.AllocL();
    }

CItem::CItem()
    {}

CItem::~CItem()
    {
    delete iText;
    }

const HBufC* CItem::GetText()
    {
    return (iText);
    }

As part of the construction process, a CItem constructs an HBufC of the correct length and copies the content of the descriptor parameter into it.

The stack is implemented by an instance of the example class CStack. This maintains the stack by adding CItem objects onto the end of the list and removing them from the end of the list. When removing them from the end of the list, a pointer to the removed CItem object is returned.

In this example, the list header, iStack, and the iterator, iStackIter, are declared as data members of the class and are constructed when the CStack object is constructed. A C++ constructor must be supplied so that iStackIter can be constructed. (TSglQueIter has no default constructor).

AddToStack() takes a CItem object and adds it to the end of the singly linked list.

RemoveFromStack() takes the CItem object at the end of the singly linked list, removes it from the list and returns a pointer to it.

The CStack class is declared as:

class CStack : public CBase
    {
public :
    static CStack* NewL();
    static CStack* NewLC();
                   CStack();
    void Construct();
    virtual ~CStack();
    CItem* RemoveFromStack();
    void AddToStack(CItem& anItem);
private :
    TSglQue<CItem> iStack;
    TSglQueIter<CItem> iStackIter;
    };

The CStack member functions are implemented as:

CStack* CStack::NewLC()
    {
    CStack* self = CStack::NewL();
    CleanupStack::PushL(self);
    return self;
    }

CStack* CStack::NewL()
    {
    CStack* self = new (ELeave) CStack;
    return self;
    }

CStack::CStack()
    : iStack(CItem::iOffset),iStackIter(iStack)
    {}

The C++ constructor is needed so that the list header (iStack) and the list iterator (iStackIter) can be properly constructed.

Before destroying a CStack object, the list is destroyed. This is achieved using the iterator (iStackIter). The iterator pointer is set to point to each element in turn, removing that element from the list before destroying it.

Once the iterator has reached the end of the list, the operator++ returns NULL.

The destruction process is safe if the list is empty; the statement iStackIter.SetToFirst() is harmless, the operator++ returns NULL and execution of the body of the while loop never happens.

CStack::~CStack()
    {
    CItem* item;
         iStackIter.SetToFirst();
    while ((item = iStackIter++) != NULL)
        {
        iStack.Remove(*item);
        delete item;
        };
    }

Adding an element to the stack simply involves adding the CItem object to the end of the list.

void CStack::AddToStack(CItem& anItem)
    {
    iStack.AddLast(anItem);
    }

The RemoveFromStack() function returns NULL, if the list is empty, otherwise it just uses the Last() member function to return a pointer to the last element in the list before removing it.

CItem* CStack::RemoveFromStack()
    {
    CItem* lastitem;
         if (iStack.IsEmpty())
        return NULL;
         lastitem = iStack.Last();
    iStack.Remove(*lastitem);
         return (lastitem);
    }

Executing the code results in a singly linked list of CItem objects each containing a pointer to an HBufC descriptor each of which, in turn, contains the text “8”, “89”, and so on through to “89ABCDEF”:

{
    CStack* stack;
    CItem* item;
    TBuf<16> buffer;
         TRAPD(leavecode,stack = CStack::NewL());
    if (leavecode != KErrNone)
        {
        // Cannot create stack
        return;
        }

for (TUint jj = 8; jj < 16; jj++)
        {
        buffer.AppendNumUC(jj,EHex);
        TRAPD(leavecode,item = CItem::NewL(buffer));
        if (leavecode != KErrNone)
            {
            // Cannot create item
            delete stack;
            return;
            }
        stack->AddToStack(*item);
        }

as the following shows:

The following code removes each CItem element from the list, starting with the last and working through to the first until the list is empty.

while ((item = stack->RemoveFromStack()) != NULL)
        {
        // item->GetText());can be used to access the text.
        delete item;
        };

delete stack;

Note that unlike doubly linked lists, elements can only be added to the start or the end of a singly linked list. Elements cannot be added to the middle of the list.