Compile error for (char based) STL (stream) containers in Visual Studio

Question

This is basically the same question as [SO]: C2491: \'std::numpunct<_Elem>::id\' : definition of dllimport static data member not allowed [closed], but considering the follow

Answer 1

The problem occurs because in the templated class std::numpunct, the public variable id is declared with _CRTIMP2_PURE_IMPORT. I haven't chased down the definition of this, but it presumably sets this variable to be imported from the C runtime dll.

The way to fix the problem is to specialize the std::numpunct class for the character type you wish to use, but declare the id variable without _CRTIMP2_PURE_IMPORT.

As the OP refers to a now out of date version of Visual Studio, I won't try and reference specific files or line numbers, as they may change from version to version. I also won't provide a specific specialization that can be used, as this probably depends upon exactly what the OP wants (it may be easy if they wish to just use their locale, or a bit more difficult if they wish to use other/any locale).

Answer 2

Start notes:

• I am using VStudio Community 2015 (v14.0.25431.01 Update 3). Version is important here, since standard header files might change across versions (and line numbers might differ)
• Created [MSDN]: Compile error for STL (stream) containers in Visual Studio

Approaches:

1. Quick (shallow) investigation

   On VStudio IDE double click, on the 2^nd note in the Output window (after attempting to compile the file), and from there repeated RClicks on relevant macros, and from the context menu choosing Go To Definition (F12):

   • xlocnum (#120): (comment is part of the original file/line)

     __PURE_APPDOMAIN_GLOBAL _CRTIMP2_PURE static locale::id id; // unique facet id
     

   • yvals.h: (#494):

          #define _CRTIMP2_PURE _CRTIMP2
     

   • crtdefs.h (#29+):

     #ifndef _CRTIMP2
         #if defined CRTDLL2 && defined _CRTBLD
             #define _CRTIMP2 __declspec(dllexport)
         #else
             #if defined _DLL && !defined _STATIC_CPPLIB
                 #define _CRTIMP2 __declspec(dllimport)  // @TODO - cfati: line #34: Here is the definition
             #else
                 #define _CRTIMP2
             #endif
         #endif
     #endif
     

   As seen, __declspec(dllimport) is defined on line #34. Repeating the process on the _DLL macro, yielded no result. Found on [MSDN]: Predefined Macros:

   _DLL Defined as 1 when the /MD or /MDd (Multithreaded DLL) compiler option is set. Otherwise, undefined.

   I thought of 2 possible ways to go on (both resulting in a successful build):

   • Use static version of CRT Runtime ([MSDN]: /MD, /MT, /LD (Use Run-Time Library)). I don't consider it a viable option, especially when the project consists of .dlls (and it does): bad things can happen (e.g. [SO]: Errors when linking to protobuf 3 on MSVC 2013, or even nastier ones can occur at runtime)
   • Manually #undef _DLL (in main.cpp, before any #include). This is a lame workaround (gainarie). It builds fine, but tampering with these things could (and most likely will) trigger Undefined Behavior at runtime

   None of these 2 options was fully satisfactory, so:
   

2. Going a (little) bit deeper

   Tried to simplify things even more (main.cpp):

   #include <sstream>
   
   
   //typedef unsigned short CharType;  // wchar_t  unsigned short
   #define CharType unsigned short
   
   
   int main() {
       std::basic_stringstream<CharType> stream;
       CharType c = 0x41;
       stream << c;
       return 0;
   }
   

   Notes:

   • Replaced typedef by #define (to strip out new type definition complexity)
   • Switched to unsigned short which is wchar_t's definition (/Zc:wchar_t-) to avoid any possible type size / alignment differences

   
   "Compiled" the above code with [MSDN]: /E (Preprocess to stdout) and [MSDN]: /EP (Preprocess to stdout Without #line Directives) (so that the warnings/errors only reference line numbers from current file):

   • Generated preprocessed files (using each flag froma bove): ~1MB+ (~56.5k lines)
   • The only difference in the files was the #define (wchar_t vs. unsigned short) somewhere at the very end
   • Compiling the files (shockingly :)) yielded the same result: the wchar_t one compiled while the unsigned short failed with the same error
   • Added some #pragma message statements (yes, they are handled by the preprocessor, but still) in the file that fails (before each warning/note), noticed some difference between the 2 #defines, but so far unable to figure out why ¹
   • While browsing the generated file(s), noticed a template<> struct char_traits<char32_t> definition, so I gave it a try, and it worked (at least the current program compiled) ¹ (and, as expected sizeof(char32_t) is 4). Then, found [MSDN]: char, wchar_t, char16_t, char32_t

   
   Notes:

   • Although this fixed my current problem (still don't know why), will have to give it a shot on the end goal
   • ¹ Although I looked over the file, I didn't see any template definitions targeting only the "privileged" types (e.g. I didn't see anything that would differentiate wchar_t, signed char or char32_t from unsigned short for example), so I don't know (yet) why it works for some types but not for others. This is an open topic, whenever I'll get new updates, I will share them

Bottom line:

As empirically discovered, the following types are allowed, when working with char based STL containers:

• char
• unsigned char
• signed char
• wchar_t
• char16_t
• char32_t
• unsigned short (/Zc:wchar_t- only )

Final note(s):

• I will incorporate anything useful (e.g. comments) in the answer

@EDIT0:

• Based on @IgorTandetnik's answer on [MSDN]: Compile error for STL (stream) containers in Visual Studio, although there is still a little bit of fog left on:

  • unsigned char and signed char
  • Difference between static and dynamic C++ RTLib

  
  I'm going to accept this as an answer.