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VC++ MFC Tutorial: A simple intorduction to using the MFC collections CArray, CList and CMap, Free Source Code Download

 
 John McTainsh 

Introduction.

This turorial will demonstrate the the MFC collection classes CList, CArray and CMap. These classes are an excillent way to manage dynamic data in a type safe manner. They are very easy to use and mostly encourage solid code.

Quick Start.
Collection Data Types.
Using CArray.
Using CList.
Using CMap.
Using pointers to objects.

A Quick Start.

Lets just use a simple List collection to start with. This example demonstartes a que of shoe sizes held as doubles. Items are added to the Tail of the que and removed from the Head for processing.

#include <afxtempl.h>       // MFC suport for Collections
...
    //Declare the que object
    CList<double, double> m_lstShoeSize;

    //Add items to the que
    m_lstShoeSize.AddTail( 10.5 );
    m_lstShoeSize.AddTail(  8.0 );
    m_lstShoeSize.AddTail(  9.5 );
    m_lstShoeSize.AddTail(  9.0 );

    //Process (show) the items in the list.
    for( POSITION pos = m_lstShoeSize.GetHeadPosition(); pos != NULL; )
    {
        _tprintf( _T("Shoe size is %4.1lf\n"), m_lstShoeSize.GetNext( pos ) );
    }

    //Process (remove) the items from the que
    while( !m_lstShoeSize.IsEmpty() )
    {
        double dRemovedShoe = m_lstShoeSize.RemoveHead();
        _tprintf( _T("Removing Shoe Size(%4.1lf)\n"), dRemovedShoe );
    }

Collection data types.

Jump ahead to using CArray if you just want to skip the theory. In the previous example you would note the use of a template . The two parameters are used to define how data is stored and retrieved from the collection.

TYPE.

This is the type of data that is used to hold the elements internally and RETURNED from the collection. Get returns this data type.

ARG_TYPE.

This type is use to specify the type of data used to write (STORE) data to the collection. Set uses this data type. Often this type is a reference to the TYPE value. Some examples follow;

    CList< PERSON*, PERSON* > m_lstPeople;     //1 List of pointers to struct
    CList< CString, CString > m_lstNames;      //2 List of CStrings
    CList< CString, CString&> m_lstNames;      //3 .. same using references
    CList< CString, LPCSTR  > m_lstNames;      //4 .. using constant pointers

Note 1: With regard sample 1, the list contains pointer not objects, so the objects must be created with new and deleted when no longer required.

Note 2: With regard sample 3, the ARG_TYPE parameter is used to indicate how the value will be passed into the collection, ie in the Add() method. It does NOT indicate a collection of referances.

Using CArray.

CArray is a collection that is best used for data that is to be accessed in a random or non sequensial manner. The array can dynamically shrink and grow as necessary. Array indexes always start at position 0. You can decide whether to fix the upper bound or allow the array to expand when you add elements past the current bound. Memory is allocated contiguously to the upper bound, even if some elements are null.

The following example adds two CTime objects to the array and then displays the contents of the entire array. The key functions are SetAtGrow which adds an item and increases the array size and the [] operator which is used to retrieve data from the array.

#include <afxtempl.h>       // MFC suport for Collections
...
   CArray<CTime, CTime&> m_aryTime;

    m_aryTime.SetAtGrow( 3, CTime::GetCurrentTime() );
    m_aryTime.SetAtGrow( 5, CTime( 1999, 6, 12 ) );
    
    for( int nCnt = 0; nCnt < m_aryTime.GetSize(); nCnt++ )
    {
        if( m_aryTime[nCnt].GetTime() != 0 )
        { 
            _tprintf( _T("Time is %s\n"), 
            m_aryTime[nCnt].Format( _T("%d/%b/%y %H:%M") ) );
        }
        else
        {
            _tprintf( _T("Invalid Time\n") );
        }
    }

Using CList.

Lists are simular to arrays but are optimised for data that is read in a more sequensial manner such as ques and lists. See the example in Quick Start. earlier for a simple list example. Note items are added at the head or tail of the list. Retrival is from the head or tail of the list and via an iterative process.

#include <afxtempl.h>       // MFC suport for Collections
...
    //Declare the que object
    CList<int, int> m_lstDepth;

    //Add items to the que
    m_lstDepth.AddTail( 100 );
    m_lstDepth.AddTail(  85 );
    m_lstDepth.AddTail(  95 );
    m_lstDepth.AddTail(  90 );

    //Process (show) the items in the list.
    for( POSITION pos = m_lstDepth.GetHeadPosition(); pos != NULL; )
    {
        _tprintf( _T("Dive depth is %4d\n"), m_lstDepth.GetNext( pos ) );
    }

Using CMap.

The CMap object is an simple collection, but unlike arrays and lists, which index and order the data they store, maps associate keys and values. To access a value stored in a map, specifying the value˘Î associated key. This is simular to a hash table.

#include <afxtempl.h>       // MFC suport for Collections
...
    CMap<CString,LPCSTR,CString,CString&> m_mapAddress;

    m_mapAddress[_T( "10.1.1.102" )] = _T("BILL");
    m_mapAddress[_T( "10.1.1.108" )] = _T("MAILSERVER");
    m_mapAddress[_T( "10.1.1.112" )] = _T("DevLaptop01");
    m_mapAddress[_T( "10.1.1.10" )]  = _T("PALEALE");
    m_mapAddress[_T( "10.1.3.1" )]   = _T("PTRAK");

    CString sMachine;
    CString sUnknownIP =  _T( "?0.?.?.112" );
    sUnknownIP.Replace( _T("?"), _T("1") );
    m_mapAddress.Lookup( sUnknownIP, sMachine );

    _tprintf( _T("Machine at IP %s is %s\n"), sUnknownIP, sMachine );

Using pointers to objects.

In all the previous examples the items in the list all supported the assigment = operator. This makes it possible to add and retrieve data from the collection. Items that do not support the assignment operator can still be saved to a collection but must be created and destroyed manualy. The objects must be dynamically allocated using the new operator, ie the "free store" or "heap" this ensures they are not deleted once they go out of scope. Therefore when removed from the list they must be manually deleted. A destructor is often good place to perform this task. The following example shows a list of PERSON structures that are created, displayed and deleted.

NOTE: Unlike other collections, pointers return the actual object rather than a copy, so changes made to the returned object are perminent.

#include <afxtempl.h>                            // MFC suport for Collections
...
//Person structure
#define LEN_CLOTH_SIZE    (5)                    //Max cloth size length
typedef struct _PERSON 
{
    int                nHeight;                  //in cm
    CString            sFullName;                //Name
    COleDateTime    tmBirthDate;                 //Birthday
    TCHAR            szShirt[LEN_CLOTH_SIZE];    //Shirt size    
 } PERSON, *LPPERSON;
 ...
   
    CList< LPPERSON, LPPERSON > m_lstPeople;

    //Bilbos details
    LPPERSON lpPerson = new PERSON;
    lpPerson->nHeight          = 95;
    lpPerson->sFullName        = _T("Bilbo Baggins");
    _tcscpy( lpPerson->szShirt, _T("XOS") );
    lpPerson->tmBirthDate      = COleDateTime( 1965, 6, 22, 3, 0, 0 );
    m_lstPeople.AddTail( lpPerson );

    //Fredo details
    lpPerson = new PERSON;
    lpPerson->nHeight          = 49;
    lpPerson->sFullName        = _T("Fredo Frog");
    _tcscpy( lpPerson->szShirt, _T("OS") );
    lpPerson->tmBirthDate      = COleDateTime( 1965, 1, 5, 18, 0, 0 );
    m_lstPeople.AddTail( lpPerson );

    //Display the People in the que
    POSITION posPerson = m_lstPeople.GetHeadPosition();
    while( posPerson != NULL )
    {
        LPPERSON lpDisplayPerson = m_lstPeople.GetNext( posPerson );
        _tprintf( _T("Name .........%s\n"), lpDisplayPerson->sFullName );
        _tprintf( _T("Height        %d\n"), lpDisplayPerson->nHeight  );
        _tprintf( _T("Shirt size    %s\n"), lpDisplayPerson->szShirt );
        _tprintf( _T("Birthday      %s\n\n"), lpDisplayPerson->tmBirthDate.
            Format( _T("%d/%b/%y %H:%M") ) );
    }

    //Free out the list
    while( !m_lstPeople.IsEmpty() )
    {
       LPPERSON lpLostPerson = m_lstPeople.RemoveHead();
       delete lpLostPerson;
    }

Note:

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