CMap在用CString做key類型時，ARG_KEY要選LPCTSTR?
文章來源:http://blog.csdn.net/flyingxu/archive/2005/12/26/562852.aspx

遇到好幾個人說CMap在用CString做key類型時有問題，說用int和DWORD就可以，用CString就不行。因此很多人推薦使用MFC中的CMapStringToPtr之類。

看下面的代碼：
//.h
????CMap<CString, LPCTSTR, int, int> typeMap;
//.cpp
????typeMap.SetAt(_T("ONE"),1);
????typeMap.SetAt(_T("TWO"),2);

????int nValue = 0;
????BOOL ret = typeMap.Lookup(_T("ONE"), nValue);
????ret = typeMap.Lookup(_T("THREE"), nValue);
????ret = typeMap.Lookup(_T("TWO"), nValue);
我的代碼運行的很好，我覺得關鍵是ARG_KEY要選LPCTSTR

原因: ?referencehttp://www.codeproject.com/KB/architecture/cmap_howto.aspx

Introduction

Programmers like me, who learnt?STL::map?before?CMap, always think?CMap?is difficult to use, and always try to useCMap?in the way as a?STL::map. In this article, I will explain about?CMap?and what you should do to use it for your own custom classes. And at the end of this article, I will show an example of how to use?CMap?correctly withCString*?(note, I mean?CString?pointer and not?CString?:>)

CMap Internal

The first thing to be noted is that?CMap?is actually a hash map, and not a tree map (and usually a Red-black tree) asSTL::map. Shown below is the internal structure of a?CMap.

How to declare a CMap

Many people get confused about?CMap's declaration?CMap<KEY, ARG_KEY, VALUE, ARG_VALUE>, why not justCMap<KEY, VALUE>?

In fact, the ultimate data container in?CMap?is?CPair, and the internal of?CPair?is?{KEY, VALUE}. Therefore,?CMapwill really store a?KEY, and not?ARG_KEY. However, if you check with the MFC source code, almost all the internal parameters passing within?CMap?itself is called with?ARG_KEY?and?ARG_VALUE, therefore, using?KEY&?as?ARG_KEYseems always a correct thing, except when:

You are using primitive date types like?int,?char, where pass-by-value makes no difference (may be even faster) with pass-by-reference.

If you use?CString?as?KEY, you should use?LPCTSTR?as?ARG_KEY?and not?CString&, we will talk more about this later.

So what should I do to make CMap work with my ClassX

Well, as I mentioned earlier,?CMap?is a hash map, a hash map will try to get the "hash value" -- a?UINT?-- from the key, and use that hash value as the index in the hash table (well, actually it is hash value % hash table size). If more then one key have the same hash value, they will be linked in a linked list. Therefore, the first thing you have to do is to provide a hash function.

CMap?will call a templated function?HashKey()?to do the hashing. The default implementation and specialized version for?LPCSTR?and?LPCWSTR?are listed as follows:

?Collapse|?Copy Code // inside <afxtemp.h> template<class ARG_KEY> AFX_INLINE UINT AFXAPI HashKey(ARG_KEY key) {// default identity hash - works for most primitive values return (DWORD)(((DWORD_PTR)key)>>4); }// inside <strcore.cpp> // specialized implementation for LPCWSTR #if _MSC_VER >= 1100 template<> UINT AFXAPI HashKey<LPCWSTR> (LPCWSTR key) #else UINT AFXAPI HashKey(LPCWSTR key) #endif {UINT nHash = 0;while (*key)nHash = (nHash<<5) + nHash + *key++;return nHash; }// specialized implementation for LPCSTR #if _MSC_VER >= 1100 template<> UINT AFXAPI HashKey<LPCSTR> (LPCSTR key) #else UINT AFXAPI HashKey(LPCSTR key) #endif {UINT nHash = 0;while (*key)nHash = (nHash<<5) + nHash + *key++;return nHash; }

As you can see, the default behavior is to "assume" that the key is a pointer, and convert it to?DWORD, and that's why you will get?"error C2440: 'type cast': cannot convert from 'ClassXXX' to 'DWORD_PTR'"?if you don't provide a specialized?HashKey()?for your?ClassX.

And because MFC only has specialized implementations for the?LPCSTR?and?LPCWSTR, and not for?CStringA?norCStringW, if you want to use?CString?in?CMap, you have to declare?CMap<CString, LPCTSTR....>.

OK, now you know how?CMap?calculates the hash value, but since more than one key may have the same hash value,CMap?needs to traverse the whole linked list to find the one with exactly the same key "content", not only with the same "hash value". And when?CMap?does the matching, it will call?CompareElements(), another templated function.

?Collapse|?Copy Code // inside <afxtemp.h> // noted: when called from CMap, // TYPE=KEY, ARG_TYPE=ARG_TYPE // and note pElement1 is TYPE*, not TYPE template<class TYPE, class ARG_TYPE> BOOL AFXAPI CompareElements(const TYPE* pElement1, const ARG_TYPE* pElement2) {ASSERT(AfxIsValidAddress(pElement1, sizeof(TYPE), FALSE));ASSERT(AfxIsValidAddress(pElement2, sizeof(ARG_TYPE), FALSE));// for CMap<CString, LPCTSTR...> // we are comparing CString == LPCTSTR return *pElement1 == *pElement2; }

Therefore, if you want to use?CMap?with your own custom?ClassX, you will have to provide a specialized implementation for?HashKey()?and?CompareElements().

Example: CMap with CString*

Provided as an example, below is what you need to do to make?CMap?work with?CString*, and of course, using the string content as the key, and not the address of the pointer.

?Collapse|?Copy Code template<> UINT AFXAPI HashKey<CString*> (CString* key) {return (NULL == key) ? 0 : HashKey((LPCTSTR)(*key)); }// I don't know why, but CompareElements can't work with CString* // have to define this typedef CString* LPCString;template<> BOOL AFXAPI CompareElements<LPCString, LPCString> (const LPCString* pElement1, const LPCString* pElement2) {if ( *pElement1 == *pElement2 ) {// true even if pE1==pE2==NULL return true;} else if ( NULL != *pElement1 && NULL != *pElement2 ) {// both are not NULL return **pElement1 == **pElement2;} else {// either one is NULL return false;} }

And the main program is as simple as:

?Collapse|?Copy Code int _tmain(int argc, TCHAR* argv[], TCHAR* envp[]) {CMap<CString*, CString*, int, int> map;CString name1 = "Microsoft";CString name2 = "Microsoft";map[&name1] = 100;int x = map[&name2];printf("%s = %d\n", (LPCTSTR)name1, x);*/return 0; } ?Collapse|?Copy Code --------- console output --------- Microsoft = 100

Please note that the program can compile without error even without the specialized?HashKey()?andCompareElements(), but of course, the output will then be 0, probably not what you want.

My final note about CMap

CMap?is a hash map and?STL::map?is a tree map, there is no meaning comparing the two for performance (it would be like comparing apples and oranges!). But if you will retrieve the keys in sorted order, then you will have to use?STL::map.

The design of?HashKey()?is critical to the overall performance. You should provide a?HashKey()?that has a low collision rate (different keys generally would have different hash values)?and?is easy to calculate (not a MD5 of the string, etc..). And we have to note that -- at least for some of the classes -- this is not easy.

When using?CMap?(as well as?STL::hash_map), always beware of the hash table size. As quoted in MSDN, "the hash table size should be a prime number. To minimize collisions, the size should be roughly 20 percent larger than the largest anticipated data set". By default,?CMap?hash table size is 17, which should be okay for around 10 keys. You can change the hash table size with?InitHashTable(UINT nHashSize), and can only do so before the first element is added to the map. You can find more prime numbers?here. (And don't mix-up with?CMap(UINT nBlockSize),?nBlockSize?is to acquire more than one?CAssoc?to speed up the creation of a new node.)

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