Disabling MSVC warnings 4251 and 4275

[martingalvan-volue]

Hi, I noticed that

spdlog/CMakeLists.txt

Line 165 in 5ebfc92

target_compile_options(spdlog PUBLIC $<$<AND:$<CXX_COMPILER_ID:MSVC>,$<NOT:$<COMPILE_LANGUAGE:CUDA>>>:/wd4251

disables the following MSVC linker warnings:

https://learn.microsoft.com/en-us/cpp/error-messages/compiler-warnings/compiler-warning-level-1-c4251
https://learn.microsoft.com/en-us/cpp/error-messages/compiler-warnings/compiler-warning-level-2-c4275

There's no explanation given in the code as to why these are disabled; however, looking at the description it seems like it's not correct to do so (4251 seems particularly concerning). Is there a reason why they're disabled instead of fixed?

[tt4g]

This option was added when Windows shared libraries were supported (#1467).
I do not see the need for this option. It could be removed if there is a clear reason why the option is not needed.

[martingalvan-volue]

Hi, I came back to this today and I think I know why these warnings might've been disabled. What happens is that spdlog has classes which are marked as dllexport when using spdlog as a DLL, but inherit from or use non-dllexport classes. An example of this is spdlog_ex, which inherits from std::exception and has a non-static data member of type std::string. According to the MSVC docs, these warnings are safe to disable if the non-dllexport class is part of the C++ standard library (which seems to be the case here).

I'm not sure, however, of how useful it is to disable these warnings when building spdlog itself as opposed to disabling them within the header files so they're not triggered in user code. In my case, I had to surround all inclusions of spdlog.h with #pragma warning(disable : 4251 4275) in order to be able to build my project. It would be nice if we could consider disabling these warnings within spdlog's own headers (as we do e.g. in os-onl.h with warning 4702).

[tt4g]

PR is welcome.