Linker error LNK2005 when compiling (x64) json-3.10.0.zip with Visual Studio 2019 16.11.1

[Andreas-Schniertshauer]

What is the issue you have?

Linker error LNK2005:

unit-regression2.obj : error LNK2005: "public: static class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > * sax_no_exception::error_string" (?error_string@sax_no_exception@@2PEAV?$basic_string@DU?$char_traits@D@std@@V?$allo
cator@D@2@@std@@EA) already defined in unit-disabled_exceptions.obj [build\json\test\json_unit.vcxproj]
build\json\test\Release\json_unit.exe : fatal error LNK1169: one or more multiply defined symbols found [build\json\test\json_unit.vcxproj]

Can you provide a small but working solution / fix?

Add a namespace in both files json\test\src\unit-disabled_exceptions.cpp and json\test\src\unit-regression2.cpp:

namespace
{
  class sax_no_exception : public nlohmann::detail::json_sax_dom_parser<json>
  {
  ...
  };
  std::string* sax_no_exception::error_string = nullptr;
}  // namespace

What is the expected behavior?

No link error

And what is the actual behavior instead?

LNK2005 error

Which compiler and operating system are you using?

• Compiler: Visual Studio 2019 16.11.1 Professional
• Operating system: Windows 10 21H1

Which version of the library did you use?

• latest release version 3.10.0 (zip file)
• other release - please state the version: ___
• the develop branch

If you experience a compilation error: can you compile and run the unit tests?

• yes
• no - please copy/paste the error message below

[nlohmann]

Very strange. We are using these versions in the CI without problems:

Can you share the build settings? What are you building? I am confused that unit-disabled_exceptions.cpp and unit-regression2.cpp should affect each other.

[nlohmann]

Alright, I can reproduce what you're doing. You're building the binary json_unit which links all tests. The unit tests are documented to be build with

$ mkdir build
$ cd build
$ cmake .. -DJSON_BuildTests=On
$ cmake --build .
$ ctest --output-on-failure

which still works. I'll check if it makes sense to fix the target json_unit or to remove it altogether - it's an artifact from earlier times when the test suite was not yet built with CMake.

[Andreas-Schniertshauer]

I am building in the x64 Developer Command Prompt of Visual Studio 2019 16.11.1 with cmake 3.20.21032501-MSVC_2 (Visual Studio) and msbuild 16.11.0.36601 with a lot of parameters passed to cmake, but you can reproduce it very easily with this command:

cmake ..\..\json .

I use normally these flags for cmake from json:
-D JSON_Install:BOOL=OFF -D JSON_ImplicitConversions:BOOL=OFF -D JSON_BuildTests:BOOL=ON

[nlohmann]

But what do you build then?

[nlohmann]

(The binary json_unit is marked EXCLUDE_FROM_ALL, so I don't think it was built with the test suite.)