CString inside CMap

[udiraz]

When I wrote: typedef CMap MyType; MyType myType; I got the following error: error C2440: 'type cast' : cannot convert from 'CString' to 'DWORD_PTR' do you know why ? Thanks in advance Udi Raz -- modified at 11:03 Sunday 11th December, 2005

[RChin]

Your declaration is invalid, probably because you have tried to use the <> signs without ticking the 'Ignore HTML tags in this message'.

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I Dream of Absolute Zero

[udiraz]

OK, Thanks, fixed it.. Udi Raz

[RChin]

This is just a guess, but I think the message means it cannot convert your CString class to a unique hash key. If you try: CMap<CString*, CString*&, int, int&> Then that compiles. You just have to remember to delete your allocated references. It probably uses the object's address as the hash key. Personally, would use the stl's map class.

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I Dream of Absolute Zero

[JonEngle]

you need to use CMap MyType;

[Christian Graus]

I think the second parameter needs to be a CString*. However, you'd do better to use std::map, CMap is hideous. Christian Graus - Microsoft MVP - C++

[Jack Puppy]

If you look at the definition of CMap, you'll see the following: CMap<KEY, ARG_KEY, VALUE, ARG_VALUE> The KEY and VALUE are the data types you're mapping. In your case, KEY = CString and VALUE = int. The ARG_ types are the types used in argument passing within CMap. The code that calculates the hash used for lookups is found in the GetAssocAt function of the CMap template: CMap<KEY, ARG_KEY, VALUE, ARG_VALUE>::GetAssocAt(ARG_KEY key, UINT& nHashBucket, UINT& nHashValue) const The exact line of code that calcuates the hash is here: nHashValue = HashKey<ARG_KEY>(key); With your map declaration, the line translates to: nHashValue = HashKey<CString>(key); If you search for HashKey in the Visual Studio folder, you'll see that Microsoft has defined a handful of HashKey functions for various data types: UINT HashKey(WORD key) const; UINT HashKey(void* key) const; UINT HashKey(void* key) const; UINT HashKey(WORD key) const; UINT HashKey(LPCTSTR key) const; UINT HashKey(LPCTSTR key) const; UINT HashKey(LPCTSTR key) const; etc... You'll also notice that there isn't a HashKey defined for CString. This is why you're getting the 'cannot convert' error. The compiler does not know how to convert from a CString to a DWORD_PTR, which is the default implementation of HashKey. In the case of CString, there are two ways to solve the problem: 1) Define your own custom HashKey routine for CString, as described in the article http://support.microsoft.com/default.aspx?scid=kb;en-us;158541 2) The simple solution - use LPCTSTR (which has a HashKey provided by Microsoft, and has a casting operator defined in the CString class) as your argument key in your CMap declaration: CMap<CString, LPCTSTR, int, int> myMap; Bernie (Boom Boom) Geoffrion worked Atlanta Flames games in the 1970s with the splendid Jiggs McDonald. One night, Geoffrion said, "Jiggs, there are only three things to hockey: shooting and skating." McDonald said, "Right, Boomer. And what's the third?" The exasperated Geoffrion replied," Jiggs, that's the three. Shooting. And. Skating."

[ThatsAlok]

udiraz wrote:

typedef CMap MyType; MyType myType;

thats why i prefer STL:: MAP

"Opinions are neither right nor wrong. I cannot change your opinion. I can, however, change what influences your opinion." - David Crow

cheers, Alok Gupta VC Forum Q&A :- I/ IV