Environment: WinNT, VC 6.0
This article describes three enhanced collection template classes,
CArrayEx,
CMapEx and
CSortedArray.
CArrayEx and
CMapEx each provide an assignment operator and class
copy constructor that allow us to easily craft two dimensional arrays and mappings.
In addition, CMapEx provides the ability to
map CStrings, by providing a template hash function.
CSortedArray is a dynamic array template class that
provides “sort after insertion” functionality.
Motivation for CArrayEx, CMapEx
Many of us have tried to create a 2 dimensional array using MFC’s standard
CArray class, only to encounter the
following error:
error C2582: 'operator =' function is unavailable
Example 1:
typedef CArray <int,int> CIntArray;
CArray<CIntArray, CIntArray &> a2D;
CIntArray aInt;
a2D.Add (aInt); // error C2582 ****
Example 2:
typedef CMap<int,int,int,int> CIntMap;
CMap<int,int,CIntMap,CIntMap &> m2D;
CIntMap mInt;
m2D.SetAt (1, mInt); // error C2582 ****
The error occurs because CArray doesn’t have copy a
constructor and an assignment operator. The need for these is obvious. If we
create array of pointers [CArray<void,void>],
we need to copy not only the pointer, but also the values. Microsoft refers us to
their knowledge base articles “Collection Class Helpers” and “Collections: How to Make a
Type-Safe Collection” for more information.
The problem is solved using CArrayEx:
typedef CArrayEx<int,int> CIntArray;
CArrayEx<CIntArray, CIntArray &> a2D;
CIntArray aInt;
a2D.Add (aInt); // OK
and by using CMapEx:
typedef CMapEx<int,int,int,int> CIntMap;
CMapEx<int,int,CIntMap,CIntMap &> m2D;
CIntMap mInt;
m2D.SetAt (1, mInt); // OK
Another motivation for CMapEx
If you’ve tried to create a mapping (using CMap)
with a CString key, you’d have encountered the following error:
CMap<CString,CString,int,int> a;
a.SetAt (CString(“a”), 1);
// error: cannot convert from ‘class CString’ to ‘unsigned long’
This happens because there’s no function to hash a CString to an unsigned long.
CMapEx addresses this problem by providing a
HashKey() function.
template<> inline UINT AFXAPI HashKey<CString> (CString strKey)
{
LPCSTR key = strKey;
UINT nHash = 0;
while (*key)
nHash = (nHash<<5) + nHash + *key++;
return nHash;
}
Motivation for CSortedArray
CSortedArray allows you to easily find data (by
doing a binary search) because it maintains itself in sorted order.