using MFC CMap for CString int pairs

Question

I'm currently working on a DLL that needs to convert back and forth between the friendly name for a value and the value itself. As this code is used in many places throughout the codebase, I want to try and keep it simple and in a single function or object so I only have to declare them once.

From my reading it looks like CMap is the tool for the job, but I can't seem to discover any combination of template arguments that compiles without errors.

My values are CString and int. I tried the following definition:

CMap<int, int, CString, CString> encodermap;

which compiles, but when I try to add a value:

encodermap["Encoder 1"] = 0;

I get the following compiler errors:

error C2057: expected constant expression
error C2466: cannot allocate an array of constant size 0
error C2501: 'encodermap' : missing storage-class or type specifiers
error C2040: 'encodermap' : 'int []' differs in levels of indirection from 'CMap<KEY,ARG_KEY,VALUE,ARG_VALUE>'

I've tried changing the CMap to this:

CMap<CString, CString, int, int> encodermap;

but I get the same four errors.

I'm sure I must be missing something but I'm at a loss as to what.

Because of the SDK being used for this work I'm require VS2003

Answer 1

The problem

I think you've inverted the key type and the valye type.

Your original declaration defines int as being the key to be searched for with operator[]. So encodermap[0] = "Encoder 1"; would work.

But when your compiler sees encodermap["Encoder 1"] = 0;, he tries to find an operator[] which takes char* (or something to which char * can be converted to) and returns an int. The last error message tells you that he couldn't find such an operator for your map.

With MSVC 2015, the error message is more concise: C2679.

The solution

You should define your CMap with a CString key and an int value. The trick to know, is that for a CString KEY, the the ARG_KEY should be LPCWSTR . So the right definition would be:

CMap<CString, LPCWSTR, int, int> encodermap;

This permits to use CString as key in the map's operator[].

Now if you use MFC on windows, you probably use UNICODE and wide chars (therefore the LPCWSTR instead of LPCSTR). When calling the operator you then have either to use a CString or a wide literal:

        encodermap[L"Encoder 1"] = 0;
        encodermap[CString("Encoder 2")] = 1;

Answer 2

Try this:

CMap<CString, LPCTSTR, int, int> encodermap;

This CodeProject article CMap How-to may be of some help.

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

In fact, the ultimate data container in CMap is CPair, and the internal of CPair is {KEY, VALUE}. Therefore, CMap will 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_KEY seems 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.

Edit: Cristophe, another option for the assignment is encodermap[_T("Encoder 1")] = 0;, which will work for single-byte, multi-byte or Unicode with the LPCTSTR typedef. You will also need to #include <tchar.h>.