CMapStringToString vs std::map

Question

Are there any real benefits for me to migrate my useage of CMapStringToString over to std::map<CString, CString> in my application?

I don't understand, I tried to change the code and now it won't compile:

7>------ Build started: Project: Meeting Schedule Assistant, Configuration: Release x64 ------
7>CalendarSettingsOutlookPage.cpp
7>C:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.30.30705\include\xhash(112,53): error C2064: term does not evaluate to a function taking 1 arguments
7>C:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.30.30705\include\xhash(135): message : see reference to variable template 'const bool _Nothrow_hash<std::hash<ATL::CStringT<wchar_t,StrTraitMFC_DLL<wchar_t,ATL::ChTraitsCRT<wchar_t> > > >,ATL::CStringT<wchar_t,StrTraitMFC_DLL<wchar_t,ATL::ChTraitsCRT<wchar_t> > > >' being compiled
7>C:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.30.30705\include\xhash(135): message : while compiling class template member function 'size_t std::_Uhash_compare<_Kty,_Hasher,_Keyeq>::operator ()<_Kty>(const _Keyty &) noexcept(<expr>) const'
7>        with
7>        [
7>            _Kty=CString,
7>            _Hasher=std::hash<CString>,
7>            _Keyeq=std::equal_to<CString>,
7>            _Keyty=ATL::CStringT<wchar_t,StrTraitMFC_DLL<wchar_t,ATL::ChTraitsCRT<wchar_t>>>
7>        ]
7>C:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.30.30705\include\xhash(1109): message : see reference to variable template 'const bool _Nothrow_hash<std::_Umap_traits<ATL::CStringT<wchar_t,StrTraitMFC_DLL<wchar_t,ATL::ChTraitsCRT<wchar_t> > >,ATL::CStringT<wchar_t,StrTraitMFC_DLL<wchar_t,ATL::ChTraitsCRT<wchar_t> > >,std::_Uhash_compare<ATL::CStringT<wchar_t,StrTraitMFC_DLL<wchar_t,ATL::ChTraitsCRT<wchar_t> > >,std::hash<ATL::CStringT<wchar_t,StrTraitMFC_DLL<wchar_t,ATL::ChTraitsCRT<wchar_t> > > >,std::equal_to<ATL::CStringT<wchar_t,StrTraitMFC_DLL<wchar_t,ATL::ChTraitsCRT<wchar_t> > > > >,std::allocator<std::pair<ATL::CStringT<wchar_t,StrTraitMFC_DLL<wchar_t,ATL::ChTraitsCRT<wchar_t> > > const ,ATL::CStringT<wchar_t,StrTraitMFC_DLL<wchar_t,ATL::ChTraitsCRT<wchar_t> > > > >,0>,ATL::CStringT<wchar_t,StrTraitMFC_DLL<wchar_t,ATL::ChTraitsCRT<wchar_t> > > >' being compiled
7>C:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.30.30705\include\xhash(1096): message : while compiling class template member function 'void std::_Hash<std::_Umap_traits<_Kty,_Ty,std::_Uhash_compare<_Kty,_Hasher,_Keyeq>,_Alloc,false>>::clear(void) noexcept'
7>        with
7>        [
7>            _Kty=CString,
7>            _Ty=CString,
7>            _Hasher=std::hash<CString>,
7>            _Keyeq=std::equal_to<CString>,
7>            _Alloc=std::allocator<std::pair<const CString,CString>>
7>        ]
7>D:\My Programs\2022\MeetSchedAssist\Meeting Schedule Assistant\CalendarSettingsOutlookPage.cpp(156): message : see reference to function template instantiation 'void std::_Hash<std::_Umap_traits<_Kty,_Ty,std::_Uhash_compare<_Kty,_Hasher,_Keyeq>,_Alloc,false>>::clear(void) noexcept' being compiled
7>        with
7>        [
7>            _Kty=CString,
7>            _Ty=CString,
7>            _Hasher=std::hash<CString>,
7>            _Keyeq=std::equal_to<CString>,
7>            _Alloc=std::allocator<std::pair<const CString,CString>>
7>        ]
7>C:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.30.30705\include\unordered_map(67): message : see reference to class template instantiation 'std::_Hash<std::_Umap_traits<_Kty,_Ty,std::_Uhash_compare<_Kty,_Hasher,_Keyeq>,_Alloc,false>>' being compiled
7>        with
7>        [
7>            _Kty=CString,
7>            _Ty=CString,
7>            _Hasher=std::hash<CString>,
7>            _Keyeq=std::equal_to<CString>,
7>            _Alloc=std::allocator<std::pair<const CString,CString>>
7>        ]
7>D:\My Programs\2022\MeetSchedAssist\Meeting Schedule Assistant\CalendarSettingsOutlookPage.h(59): message : see reference to class template instantiation 'std::unordered_map<CString,CString,std::hash<CString>,std::equal_to<CString>,std::allocator<std::pair<const CString,CString>>>' being compiled
7>C:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.30.30705\include\xhash(111,44): error C2056: illegal expression
7>Done building project "Meeting Schedule Assistant.vcxproj" -- FAILED.

If I use std::map<CString, CString> it works but std::unordered_map<CString, CString> throws out the errors above. My usage calls:

if (m_mapCalendarList.find(strCalendarName) != m_mapCalendarList.end())
if (strCalendarId == m_mapCalendarList[strCalendarName])
m_mapCalendarList.clear();

The variable is defined in the header.

One benefit is that you could now write `for (auto const& entry : my_map) { ... }`. Whether that is a real benefit to your application is not something we can answer. Also, I don't know whether MFC's dictionary types are sorted. If they aren't, a closer equivalent would be [`unordered_map`](https://en.cppreference.com/w/cpp/container/unordered_map). — IInspectable, Nov 10 '21 at 08:40
@IInspectable They are not sorted. And valid point about `for` loops. In this particular case I am building the map once and then using it to lookup values to see if they exist. Eg: `(m_mapCalendarList.Lookup(strCalendarName, strCalendarId)`. I don't actual iterate the collection. — Andrew Truckle, Nov 10 '21 at 08:48
@AlexF Thanks for observations. For this application I use `CString` heavily so felt it best to retain it for consistency at this time. — Andrew Truckle, Nov 10 '21 at 08:55
@IInspectable If I use `std::map` it compiles. If I use `std::unordered_map` it does not compile. — Andrew Truckle, Nov 10 '21 at 09:15
`std::map` only requires that `std::less` is implemented (which, for `CString` it is). `std::unordered_map`, on the other hand, needs a hashing function. You would need to provide that yourself (see [`std::hash`](https://en.cppreference.com/w/cpp/utility/hash) for details). Since you are populating the dictionary only once, using a `std::map` doesn't hurt, though. — IInspectable, Nov 10 '21 at 09:30
If you want to migrate your app to anything non MFC, yes! We are in a era where non-Windows platforms development is being asked, and MFC is not supported there! I am surprised of your comparison being between ` CMapStringToString` vs `std::map` instead of sthg like `CMap` vs `std::map`! — sergiol, Nov 10 '21 at 15:37
BTW, even Microsoft recommends to stay away from their own archaic MFC collections, and to use the `std` ones instead. https://stackoverflow.com/a/1348138/383779 — sergiol, Nov 10 '21 at 15:46
@sergiol I never used the CMap container in my app. That’s why. — Andrew Truckle, Nov 10 '21 at 16:15
@sergiol `CMapStringToString` is older than the `CMap`. I'm also working on code that still contains some `CMapStringToString` — Jabberwocky, Nov 12 '21 at 08:51
@Jabberwocky: I know, but I am surprised people nowadays is still using it. We are two steps ahead of hard-coded `CMapSOMETHINGToSOMEWHAT` very ungeneric classes, 1. is `CMap`, 2. is `std::map`. — sergiol, Nov 12 '21 at 14:21

score 4 · Accepted Answer · edited Nov 12 '21 at 18:41

MFC hash maps have some drawbacks, except from just being not C++ Standard and not portable:

Do not recalculate hash table automatically. Not even able to resize hash table when populated. Default size of hash table may be suboptimal for you.
As noted in comments, they don't support range-based for, or ranges algorithm
Not copyable and not movable by themselves. Moving a container may be useful
Don't support move semantic for element insertion, this involves some overhead

For std::unordered_map, you need to implement hash function, and pass it as the corresponding template parameter. Or specialize standard hash for CString. But I prefer the former option.

And would be good to pass also comparison function, that performs locale-independent comparison. Since with a hash that is not aware of locale, you can't have locale-aware map anyway, but CString::operator== is locale-aware.

As there's HashKey that can take LPCSTR and LPCWSTR that is used by CMap, implementation of own STL-compatible hashes is trivial.

You can completely avoid dealing with hashes, and use std::unordered_map<std::string, std::string>. But transition to std::string will cause way more changes I guess.

CMapStringToString vs std::map

1 Answers1