DLL interface classes have data members from C++ standard library causing C4251 when building DLL

[abdes]

Many classes in the yaml-cpp API such as in binary.h, ostream-wrapper.h etc... are exported as part of the DLL interface while having members of types from the STL. This causes multiple C4251 warnings on MSVC.

Example:

yaml-cpp\include\yaml-cpp\node\detail\node_data.h(122): warning C4251: 'YAML::detail::node_data::m_undefinedPairs': class 'std::list<YAML::detail::node_data::kv_pair,std::allocator<_Ty>>' needs to have dll-interface to be used by clients of class 'YAML::detail::node_data' [yaml-cpp\yaml-cpp.vcxproj]
          with
          [
              _Ty=std::pair<YAML::detail::node *,YAML::detail::node *>
          ]
yaml-cpp\include\yaml-cpp\node\detail\node_data.h(121): note: see declaration of 'std::list<YAML::detail::node_data::kv_pair,std::allocator<_Ty>>'
          with
          [
              _Ty=std::pair<YAML::detail::node *,YAML::detail::node *>
          ]

STL code is not DLL ABI friendly and it is not recommended to have DLL interface classes having members of types from STL as the behavior is not predictable.

[Zingam]

C:\SDKs\vcpkg\installed\x64-windows\include\yaml-cpp\parser.h(81): warning C4251: 'YAML::Parser::m_pScanner': class 'std::unique_ptr<YAML::Scanner,std::default_delete<_Ty>>' needs to have dll-interface to be used by clients of class 'YAML::Parser' with [ _Ty=YAML::Scanner ] C:\SDKs\vcpkg\installed\x64-windows\include\yaml-cpp/parser.h(81): note: see declaration of 'std::unique_ptr<YAML::Scanner,std::default_delete<_Ty>>' with [ _Ty=YAML::Scanner ] C:\SDKs\vcpkg\installed\x64-windows\include\yaml-cpp\parser.h(82): warning C4251: 'YAML::Parser::m_pDirectives': class 'std::unique_ptr<YAML::Directives,std::default_delete<_Ty>>' needs to have dll-interface to be used by clients of class 'YAML::Parser' with [ _Ty=YAML::Directives ] C:\SDKs\vcpkg\installed\x64-windows\include\yaml-cpp/parser.h(82): note: see declaration of 'std::unique_ptr<YAML::Directives,std::default_delete<_Ty>>' with [ _Ty=YAML::Directives ] C:\SDKs\vcpkg\installed\x64-windows\include\yaml-cpp\binary.h(61): warning C4251: 'YAML::Binary::m_data': class 'std::vector<uint8_t,std::allocator<_Ty>>' needs to have dll-interface to be used by clients of class 'YAML::Binary' with [ _Ty=uint8_t ] C:\SDKs\vcpkg\installed\x64-windows\include\nlohmann/json.hpp(18807): note: see declaration of 'std::vector<uint8_t,std::allocator<_Ty>>' with [ _Ty=uint8_t ] C:\SDKs\vcpkg\installed\x64-windows\include\yaml-cpp\ostream_wrapper.h(45): warning C4251: 'YAML::ostream_wrapper::m_buffer': class 'std::vector<char,std::allocator<char>>' needs to have dll-interface to be used by clients of class 'YAML::ostream_wrapper' C:\SDKs\vcpkg\installed\x64-windows\include\nlohmann/json.hpp(4056): note: see declaration of 'std::vector<char,std::allocator<char>>' C:\SDKs\vcpkg\installed\x64-windows\include\yaml-cpp\emitter.h(125): warning C4251: 'YAML::Emitter::m_pState': class 'std::unique_ptr<YAML::EmitterState,std::default_delete<_Ty>>' needs to have dll-interface to be used by clients of class 'YAML::Emitter' with [ _Ty=YAML::EmitterState ] C:\SDKs\vcpkg\installed\x64-windows\include\yaml-cpp/emitter.h(125): note: see declaration of 'std::unique_ptr<YAML::EmitterState,std::default_delete<_Ty>>' with [ _Ty=YAML::EmitterState ]

[newohrim]

Stucked at this problem on Release linkage. Previously defined YAML_CPP_STATIC_DEFINE in one of the header files hoping and totally forgot about this stuff. So other files who individually included yaml-cpp/yaml.h knew nothing about the definition. Had to define YAML_CPP_STATIC_DEFINE in the build file to avoid such failures in the future.

So if anyone gets such errors during linkage, here is a possible solution: define YAML_CPP_STATIC_DEFINE in yours build system or just pass it as a command line argument :)