reference to 'byte' is ambiguous

[vangola-silver]

Versions:

• MSYS2: MSYS_NT-10.0-18363
• GCC: 10.2.0
• POCO: 1.10.1-2 (https://packages.msys2.org/package/mingw-w64-x86_64-poco)
• C++17

Hi guys,
I currently trying to port one of my Linux programs to Windows/MinGW. During the compilation I see some “strange” errors regarding a redefinition of ‘byte’.
This is the full error output:

C:/msys64/mingw32/i686-w64-mingw32/include/rpcndr.h:63:25: note: 'typedef unsigned char byte'
   63 |   typedef unsigned char byte;
      |                         ^~~~ 
In file included from C:/msys64/mingw32/i686-w64-mingw32/include/msxml.h:25,
                 from C:/msys64/mingw32/i686-w64-mingw32/include/urlmon.h:450,
                 from C:/msys64/mingw32/i686-w64-mingw32/include/objbase.h:163,
                 from C:/msys64/mingw32/i686-w64-mingw32/include/ole2.h:17,
                 from C:/msys64/mingw32/i686-w64-mingw32/include/naptypes.h:13,
                 from C:/msys64/mingw32/i686-w64-mingw32/include/ras.h:17,
                 from C:/msys64/mingw32/i686-w64-mingw32/include/mprapi.h:14,
                 from C:/msys64/mingw32/i686-w64-mingw32/include/iprtrmib.h:12,
                 from C:/msys64/mingw32/i686-w64-mingw32/include/Iphlpapi.h:15,
                 from C:/msys64/mingw32/include/Poco/UnWindows.h:33,
                 from C:/msys64/mingw32/include/Poco/Platform_WIN32.h:22,
                 from C:/msys64/mingw32/include/Poco/Foundation.h:100,
                 from C:/msys64/mingw32/include/Poco/XML/XML.h:23,
                 from C:/msys64/mingw32/include/Poco/SAX/ContentHandler.h:21,
C:/msys64/mingw32/i686-w64-mingw32/include/oaidl.h:579:5: error: reference to 'byte' is ambiguous
  579 |     byte *pRecord;
      |     ^~~~
In file included from C:/msys64/mingw32/include/c++/10.2.0/bits/stl_algobase.h:61,
                 from C:/msys64/mingw32/include/c++/10.2.0/bits/stl_tree.h:63,
                 from C:/msys64/mingw32/include/c++/10.2.0/map:60,
C:/msys64/mingw32/include/c++/10.2.0/bits/cpp_type_traits.h:404:30: note: candidates are: 'enum class std::byte'
  404 |   enum class byte : unsigned char;
      |                              ^~~~
In file included from C:/msys64/mingw32/i686-w64-mingw32/include/naptypes.h:8,
                 from C:/msys64/mingw32/i686-w64-mingw32/include/ras.h:17,
                 from C:/msys64/mingw32/i686-w64-mingw32/include/mprapi.h:14,
                 from C:/msys64/mingw32/i686-w64-mingw32/include/iprtrmib.h:12,
                 from C:/msys64/mingw32/i686-w64-mingw32/include/Iphlpapi.h:15,
                 from C:/msys64/mingw32/include/Poco/UnWindows.h:33,
                 from C:/msys64/mingw32/include/Poco/Platform_WIN32.h:22,
                 from C:/msys64/mingw32/include/Poco/Foundation.h:100,
                 from C:/msys64/mingw32/include/Poco/XML/XML.h:23,
                 from C:/msys64/mingw32/include/Poco/SAX/ContentHandler.h:21

Do you know a way how to fix this?
Any help will be appreciated.

[Biswa96]

This issue seems to be related to mingw-w64 headers. A minimal, simple, hello world like example would be helpful to identify the issue faster 😊

[vangola-silver]

Trying to reproduce the error using a simple example I was able to reduce it to a few lines of code and was able to find the solution (or at least a workaround).

Steps to reproduce:

1. Create source file with following content

#include <map>
using namespace std;
#include <Poco/SAX/ContentHandler.h>

2. Try to compile it with following (or similar) line

g++ -c -MMD -MP -std=gnu++17 -Wall -DPOCO_STATIC -DMINGW32 -D_WIN32 -O2 -I.  -ID:\sergej-work\ProSystems-git/INCLUDE -IC:\msys64\mingw32\include\Poco/Net/include -IC:\msys64\mingw32\include\Poco/Util/include -IC:\msys64\mingw32\include\Poco/JSON/include -IC:\msys64\mingw32\include\Poco/JWT/include -IC:\msys64\mingw32\include\Poco/XML/include -IC:\msys64\mingw32\include\Poco/Foundation/include  -o ../GnuDev/mingw32/rel/Test.o Test.cpp

3. Observe the error message as shown in my first post.

Steps to fix the issue:

1. Delete the line using namespace std;

2. Compile it again and observe that the issue is gone.

[Biswa96]

I can not reproduce the issue with your sample source code. Some suggestions:

1. Maybe there is something wrong with your code or building environment.
2. Place using namespace after all the includes.
3. Try not to use using namespace std;, at least not with STL. Instead use std:: like notation.
4. In the command, try not to add include /mingw64 directories. Those will be automatically included by compiler.

[vangola-silver]

To clarify the sample code was not take from my project as is, but was a result of including several headers. One of the header happend to have the using namespace std; line in it, which I did not see before combining the files by hand.
Your suggestion to use std:: like notation did the trick.
Now I know why the CppCoreGuideline says "Don’t write using namespace at global scope in a header file".

[Biswa96]

So, is this case closed? 🕵️

[vangola-silver]

Yeah, thanks for helping!

[ingvart]

I ran into the same issue. For me this means that in a project one can never "using namespace std" anywhere in the entire application/code if windows.h is included, which is a headache.

[Biswa96]

one can never "using namespace std" anywhere

One should not use "using namespace std" anywhere. See this little explanation https://isocpp.org/wiki/faq/coding-standards#using-namespace-std