COM Collection

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Introduction

Visual Basic has a very nice object called Collection, which is part of VBA type library. In VC++ we do not have a similar object, which is a shame (in my opinion).

I searched MSDN and it turned out there is a sample called ATLCollections. This project has 2 collections. First is a collection of BSTRs and second one is a VARIANT collection which is using CComBSTRa as keys. I wanted to use only VARIANTs to be comparable with scripting languages so neither one was an option.

Next step was to search the Web. I found one on www.widgetware.com written by Chris Brooks (clbrooks@micron.com) and another one by Chris Sells (www.sellsbrothers.com). Both were template-based and I did not want to use the templates. So I decided to write a collection object.

What is a collection?

According to MSDN a Collection object is an ordered set of items that can be referred to as a unit. The Collection object provides a convenient way to refer to a related group of items as a single object. The items, or members, in a collection need only be related by the fact that they exist in the collection. Members of a collection don't have to share the same data type.

A collection can be created the same way other objects are created. For example:

Dim X As New Collection

Once a collection is created, members can be added using the Add method and removed using the Remove method. Specific members can be returned from the collection using the Item method, while the entire collection can be iterated using the For Each...Next statement.

It means that a collection is a COM object that has 3 properties and 2 methods: Add, Remove, Count, Item and _NewEnum, though in technical literature you may find that only Count and _NewEnum are necessary for the object to be called the collection.

My collection defines the interface IXCollection implemented by the class CXCollection.

The IDL looks like this:

interface IXCollection : IDispatch
{
[id(1), helpstring("Adds a member to a collection")] 
HRESULT Add([in] VARIANT vItem, [in, optional] VARIANT vKey);

[id(2), helpstring("Removes a member from a collection")]
HRESULT Remove([in] VARIANT vIndex);

[propget, id(3), 
    helpstring("Returns the number of members in a collection")] 
HRESULT Count([out, retval] long * plNumber);

[propget, id(DISPID_VALUE), 
    helpstring("Returns a specific member of a collection "
    "either by position or key")] 
HRESULT Item([in] VARIANT vIndex, [out, retval] VARIANT * pvItem);

[propget, restricted, id(DISPID_NEWENUM), helpstring("Method _NewEnum")] 
HRESULT _NewEnum([out, retval] IUnknown ** ppUnk);
} 

Now lets talk a little about each method and property.

Add

Adds items to the collection. You can add items to the collection using the optional vKey parameter. If key is used the individual item can be accessed using this key.

Remove

Removes items from the collection either by the index or by the key.

Count

Returns the number of items in the collection.

Item

Returns an individual item either by the index or by the key. If the index is a number it’s 1-based. This property is given the standard DISPID DISPID_VALUE to make it as the "default" property to simplify its use in Visual Basic and scripting languages.

_NewEnum

Returns an enumerator object that contains collection items allowing the client sequential access. This property is assigned the standard DISPID DISPID_NEWENUM, this DISPID is used by Visual Basic to implement its For-Each syntax.

The _NewEnum property returns an enumerator object that supports standard IEnumVARIANT interface. The IEnumVARIANT interface is a collection of Variants. It allows clients to enumerate heterogeneous collections of objects and intrinsic types when the clients cannot or do not know the specific type(s) of elements in the collection.

Source Code

The source code contains ShoppingCart project that implements the collection as well as C++ and VBScript test applications.

Environment

• VC++ 6.0 SP4, Win2000 SP2.

For detailed coverage of the collections and enumerators I refer you to “ATL Internals” by B. Rector, C. Sells and “Professional ATL COM Programming” by R. Grimes.

THIS CODE AND INFORMATION IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESSED OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE IMPLIED WARRANTIES OF MERCHANTABILITY AND/OR FITNESS FOR A PARTICULAR PURPOSE.