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   <TITLE>Create custom TCollection TCollectionItem classes</TITLE>

   <META NAME="Author" CONTENT="Harold Howe">

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<H3>

Create custom <TT>TCollection</TT>/<TT>TCollectionItem</TT> classes

</H3>

<P>

The VCL uses <TT>TCollection</TT>'s in many of its classes. For example, the <TT>Items</TT>

property of <TT>TListView</TT> is a <TT>TCollection</TT>. This FAQ describes how you can

create a custom collection in your code. The FAQ describes how to create a

<TT>TCollection</TT> that contains an array of <TT>DWORD</TT>'s.

</P>

<B>Step 1: Derive a class from <TT>TCollectionItem</TT></B>

<P>

Derive a class from <TT>TCollectionItem</TT> that will store the data that you

want to keep in the collection. The collection will store an array of objects

for the class that you derive. The class shown here stores an internal <TT>DWORD</TT>, and

provides a property to manipulate the data. Your derived class should provide a

constructor that takes a <TT>TCollection</TT> pointer as an argument.

</P>

<pre>

<b>class</b> TDWORDItem <b>:</b> <b>public</b> TCollectionItem

<b>{</b>

    <b>private</b><b>:</b>

        DWORD FValue<b>;</b>

    <b>public</b> <b>:</b>

        <b>__fastcall</b> TDWORDItem<b>(</b>TCollection <b>*</b>Collection<b>)</b><b>;</b>

        <b>__property</b> DWORD Value <b>=</b> <b>{</b>read <b>=</b> FValue<b>,</b> write <b>=</b> FValue<b>}</b><b>;</b>

<b>}</b><b>;</b>

</pre>

<B>Step 2: Derive a class from <TT>TCollection</TT></B>

<P>

Your <TT>TCollection</TT> derivative will host a collection of

<TT>TCollectionItem</TT> derived objects. The <TT>TCollection</TT> subclass

must provide methods for adding, deleting, and manipulating the collection. The

base class <TT>TCollection</TT> already contains some functionality for manipulating

the collection, and in fact, your <TT>TCollection</TT> subclass could get by without adding

any new functionality. However, I have chosen to add functions and properties that

make the <TT>DWORD</TT> collection easier to maintain from outside the class.

</P>

<P>

The class shown here contains a array property called <TT>Items</TT> that allows

read and write access to the collection. Notice that the property

is an array of <TT>DWORD</TT>'s, and not an array of <TT>TDWORDItem</TT>'s or

<TT>TCollectionItem</TT>'s. The <TT>TCollection</TT> class already contains an

<TT>Items</TT> array property that provides direct access to the

<TT>TCollectionItem</TT> pointers, but in my case, an array of <TT>DWORD</TT>'s is

more useful than an array of <TT>TDWORDItem</TT>'s. In your code, you may want to

just stick with the <TT>Items</TT> property that is provided by <TT>TCollection</TT>.

Likewise <TT>TCollection</TT> also provides an <TT>Add</TT> method that returns a

newly constructed <TT>TCollectionItem</TT>. You initialize the item after calling

<TT>Add</TT> (see the FAQ on

<A TARGET=_top HREF="faq15.htm">Adding items to a ListView from code at runtime</A>

to see what I mean)</TT>. I decided to provide a new <TT>Add</TT> method that

takes a <TT>DWORD</TT> as an argument. As you will see in the next step, my

<TT>Add</TT> function merely calls the base class <TT>Add</TT> function, and

then initializes the item. You could omit the <TT>Add</TT> method in your class

if you prefer to initialize the collection item outside of the collection class (just

like <TT>TListView</TT>).

</P>

<pre>

<b>class</b> TDWORDList <b>:</b> <b>public</b> TCollection

<b>{</b>

    <b>private</b><b>:</b>

        DWORD <b>__fastcall</b> GetItem<b>(</b><b>int</b> Index<b>)</b><b>;</b>

        <b>void</b> <b>__fastcall</b> SetItem<b>(</b><b>int</b> Index<b>,</b>DWORD AValue<b>)</b><b>;</b>

    <b>public</b><b>:</b>

        <b>__fastcall</b> TDWORDList<b>(</b><b>)</b><b>;</b>

        <b>void</b> <b>__fastcall</b> Add<b>(</b>DWORD AValue<b>)</b><b>;</b>

        <b>void</b> <b>__fastcall</b> Delete<b>(</b><b>int</b> Index<b>)</b><b>;</b>

        <b>__property</b> DWORD Items<b>[</b><b>int</b> Index<b>]</b> <b>=</b> <b>{</b>read <b>=</b> GetItem<b>,</b> write <b>=</b> SetItem<b>}</b><b>;</b>

<b>}</b><b>;</b>

</pre>

<B>Step 3: Code the functions for both classes</B>

<P>

The block of code below provides the implementation for the <TT>TDWORDItem</TT> and

<TT>TDWORDList</TT> classes. <TT>TDWORDItem</TT> only has one function, its constructor.

<TT>TDWORDList</TT> has a constructor, an <TT>Add</TT> function, a <TT>Delete</TT>

function, and read and write methods for its <TT>Items</TT> array property.

</P>

<pre>

<font color="navy">// The only function for TDWORDItem is it's constructor. Pass the</font>

<font color="navy">// TCollection pointer to the base class, and then initialize FValue.</font>

<b>__fastcall</b> TDWORDItem<b>:</b><b>:</b>TDWORDItem<b>(</b>TCollection <b>*</b>Collection<b>)</b>

  <b>:</b> TCollectionItem<b>(</b>Collection<b>)</b><b>,</b>

    FValue<b>(</b><font color="blue">0</font><b>)</b>

<b>{</b>

<b>}</b>



<font color="navy">// The TDWORDList must pass the name of the item class to the base class</font>

<font color="navy">// TCollection constructor. This is done using BCB's __classid keyword</font>

<b>__fastcall</b> TDWORDList<b>:</b><b>:</b>TDWORDList<b>(</b><b>)</b>

  <b>:</b> TCollection<b>(</b><b>__classid</b><b>(</b>TDWORDItem<b>)</b><b>)</b>

<b>{</b>

<b>}</b>



<font color="navy">// Add should create a new TDWORDItem and add it to the collection.</font>

<font color="navy">// The base class function TCollection::Add can do this for us.</font>

<font color="navy">// After adding the item, assign its Value property. Observe that my</font>

<font color="navy">// custom Add function merely moves the statement Item->Value = AValue</font>

<font color="navy">// inside the collection class.</font>

<b>void</b> <b>__fastcall</b> TDWORDList<b>:</b><b>:</b>Add<b>(</b>DWORD AValue<b>)</b>

<b>{</b>

    TDWORDItem <b>*</b>Item <b>=</b> <b>(</b>TDWORDItem <b>*</b><b>)</b> TCollection<b>:</b><b>:</b>Add<b>(</b><b>)</b><b>;</b>

    Item<b>-></b>Value <b>=</b> AValue<b>;</b>

<b>}</b>



<font color="navy">// To implement a Delete function, you must code the function by hand.</font>

<font color="navy">// TCollection won't help you out with the Delete function. You can code</font>

<font color="navy">// Delete however you want. I create a copy of the old list, clear out</font>

<font color="navy">// all items, and then add the old items back in, skipping the index that</font>

<font color="navy">// should be removed. This could be probably be improved somewhat.</font>

<b>void</b> <b>__fastcall</b> TDWORDList<b>:</b><b>:</b>Delete<b>(</b><b>int</b> Index<b>)</b>

<b>{</b>

    DWORD <b>*</b>OldList <b>=</b> <b>new</b> DWORD<b>[</b>Count<b>]</b><b>;</b>

    <b>int</b> OldCount <b>=</b> Count<b>;</b>



    <font color="navy">// create a copy of the old values</font>

    <b>for</b> <b>(</b><b>int</b> j<b>=</b><font color="blue">0</font><b>;</b> j &lt; OldCount<b>;</b> j<b>++</b><b>)</b>

        OldList<b>[</b>j<b>]</b> <b>=</b> Items<b>[</b>j<b>]</b><b>;</b>



    <font color="navy">// clear the entire list</font>

    Clear<b>(</b><b>)</b><b>;</b>



    <font color="navy">// add the old values back in, but skip the index</font>

    <font color="navy">// that is being deleted</font>

    <b>for</b> <b>(</b><b>int</b> j<b>=</b><font color="blue">0</font><b>;</b> j &lt; OldCount<b>;</b> j<b>++</b><b>)</b>

    <b>{</b>

        <b>if</b><b>(</b>j<b>!=</b>Index<b>)</b>

            Add<b>(</b>OldList<b>[</b>j<b>]</b><b>)</b><b>;</b>

    <b>}</b>

    <b>delete</b> <b>[</b><b>]</b>OldList<b>;</b>

<b>}</b>



<font color="navy">// GetItem is the read method for the Items array property that</font>

<font color="navy">// returns a DWORD. TCollection::GetItem can return the correct</font>

<font color="navy">// TDWORDItem * based on the Index argument. Once we have the</font>

<font color="navy">// pointer, we can return the Value contained by that item.</font>

DWORD <b>__fastcall</b> TDWORDList<b>:</b><b>:</b>GetItem<b>(</b><b>int</b> Index<b>)</b>

<b>{</b>

    TDWORDItem <b>*</b>Item <b>=</b> <b>(</b>TDWORDItem <b>*</b><b>)</b> TCollection<b>:</b><b>:</b>GetItem<b>(</b>Index<b>)</b><b>;</b>

    <b>return</b> Item<b>-></b>Value<b>;</b>

<b>}</b>



<font color="navy">// SetItem is the write method for the Items array property. SetItem</font>

<font color="navy">// calls TCollection::GetItem to return the TDWORDItem for the given</font>

<font color="navy">// Index. SetItem then assigns the Value property of that item.</font>

<b>void</b> <b>__fastcall</b> TDWORDList<b>:</b><b>:</b>SetItem<b>(</b><b>int</b> Index<b>,</b>DWORD AValue<b>)</b>

<b>{</b>

    TDWORDItem <b>*</b>Item <b>=</b> <b>(</b>TDWORDItem <b>*</b><b>)</b> TCollection<b>:</b><b>:</b>GetItem<b>(</b>Index<b>)</b><b>;</b>

    Item<b>-></b>Value <b>=</b> AValue<b>;</b>

<b>}</b>

</pre>

<B>Step 5: Use the collection in you code</B>

<P>

</P>

<pre>

  TDWORDList <b>*</b>list <b>=</b> <b>new</b> TDWORDList<b>;</b>

  list<b>-></b>Add<b>(</b><font color="blue">502</font><b>)</b><b>;</b>

  list<b>-></b>Add<b>(</b><font color="blue">29</font><b>)</b><b>;</b>

  list<b>-></b>Add<b>(</b><font color="blue">100</font><b>)</b><b>;</b>

  list<b>-></b>Add<b>(</b><font color="blue">0</font><b>)</b><b>;</b>

  list<b>-></b>Add<b>(</b><font color="blue">30</font><b>)</b><b>;</b>

  list<b>-></b>Items<b>[</b><font color="blue">3</font><b>]</b> <b>=</b> list<b>-></b>Items<b>[</b><font color="blue">0</font><b>]</b> <b>*</b> <font color="blue">10</font><b>;</b>

  list<b>-></b>Delete<b>(</b><font color="blue">2</font><b>)</b><b>;</b>



  <font color="navy">// The code below shows what the code above would look like if</font>

  <font color="navy">// TDWORDList did not contain a customized Add method or Items</font>

  <font color="navy">// property.</font>

  TDWORDList <b>*</b>list <b>=</b> <b>new</b> TDWORDList<b>;</b>

  TDWORDItem <b>*</b>item<b>;</b>

  item <b>=</b> <b>(</b>TDWORDItem <b>*</b><b>)</b>list<b>-></b>Add<b>(</b><b>)</b><b>;</b>

  item<b>-></b>Value <b>=</b> <font color="blue">502</font><b>;</b>

  item <b>=</b> <b>(</b>TDWORDItem <b>*</b><b>)</b>list<b>-></b>Add<b>(</b><b>)</b><b>;</b>

  item<b>-></b>Value <b>=</b> <font color="blue">29</font><b>;</b>

  item <b>=</b> <b>(</b>TDWORDItem <b>*</b><b>)</b>list<b>-></b>Add<b>(</b><b>)</b><b>;</b>

  item<b>-></b>Value <b>=</b> <font color="blue">100</font><b>;</b>

  item <b>=</b> <b>(</b>TDWORDItem <b>*</b><b>)</b>list<b>-></b>Add<b>(</b><b>)</b><b>;</b>

  item<b>-></b>Value <b>=</b> <font color="blue">0</font><b>;</b>

  item <b>=</b> <b>(</b>TDWORDItem <b>*</b><b>)</b>list<b>-></b>Add<b>(</b><b>)</b><b>;</b>

  item<b>-></b>Value <b>=</b> <font color="blue">30</font><b>;</b>

  <b>(</b><b>(</b>TDWORDItem <b>*</b><b>)</b>list<b>-></b>Items<b>[</b><font color="blue">3</font><b>]</b><b>)</b><b>-></b>Value <b>=</b>

     <b>(</b><b>(</b>TDWORDItem <b>*</b><b>)</b>list<b>-></b>Items<b>[</b><font color="blue">0</font><b>]</b><b>)</b><b>-></b>Value<b>*</b><font color="blue">10</font><b>;</b>

</pre>

<P>

<B>Note:</B> The <TT>TCollection</TT> destructor calls the <TT>Clear</TT> function if

the collection contains any items. Clear automatically deletes the array of

<TT>TCollectionItem</TT>'s. As long as you properly delete your <TT>TCollection</TT>

object, you don't need to worry about deleting the collection items.

</P>

<P>

<B>Note:</B> You should consider using the STL <TT>vector</TT> or <TT>list</TT>

templates instead of using <TT>TCollection</TT>. The code from this FAQ could be

duplicated using the <TT>vector</TT> template without deriving any new classes.

STL templates are more portable than the <TT>TCollection</TT> class. You should

consider using STL templates whenever your collection doesn't need to be

published in the form of a property. A collection based on <TT>TCollection</TT>

might be preferrable when you are designing a component that needs a collection

as a property. The code below shows how the <TT>vector</TT> template could

have been used in this FAQ.

</P>

<pre>

<font color="green">#include &lt;vector></font>

<b>using</b> <b>namespace</b> std<b>;</b>



vector &lt; DWORD <b>></b> dVector<b>;</b>

dVector<b>.</b>push_back<b>(</b><font color="blue">502</font><b>)</b><b>;</b>

dVector<b>.</b>push_back<b>(</b><font color="blue">29</font><b>)</b><b>;</b>

dVector<b>.</b>push_back<b>(</b><font color="blue">100</font><b>)</b><b>;</b>

dVector<b>.</b>push_back<b>(</b><font color="blue">0</font><b>)</b><b>;</b>

dVector<b>.</b>push_back<b>(</b><font color="blue">30</font><b>)</b><b>;</b>

dVector<b>[</b><font color="blue">3</font><b>]</b> <b>=</b> dVector<b>[</b><font color="blue">0</font><b>]</b> <b>*</b> <font color="blue">10</font><b>;</b>

</pre>





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