BOOST_SYMBOL_IMPORT should not be defined to nothing

[Flamefire]

For non-Win32 platforms BOOST_SYMBOL_IMPORT is defined to nothing which is wrong:

https://gcc.gnu.org/wiki/Visibility

Symbol visibility is "default" by default but if the linker encounters just one definition with it hidden - just one - that typeinfo symbol becomes permanently hidden (remember the C++ standard's ODR - one definition rule).

Hence (all being build with hidden visibility) a program using a boost shared library will not see e.g. the same typeinfo as the Boost lib which results in e.g. failure of being able to catch or dynamic_cast the class.

I observed this behavior on OSX, on Linux it seems harder to reproduce. See also https://stackoverflow.com/questions/65951466/why-can-a-thrown-exception-with-hidden-visibility-still-be-caught for more details

Solution: Define BOOST_SYMBOL_IMPORT to BOOST_SYMBOL_VISIBLE when BOOST_SYMBOL_EXPORT is too.

[pdimov]

Summary of what we know so far:

• when using hidden visibility, for exceptions thrown from .so to be caught, they need to be marked BOOST_SYMBOL_VISIBLE in both the library and the caller.
• BOOST_SYMBOL_EXPORT is defined to BOOST_SYMBOL_VISIBLE, but BOOST_SYMBOL_IMPORT is not.
• this causes exception classes marked as BOOST_LIBRARY_DECL to not work with libc++ (on both Linux and macOS).
• libstdc++ uses string comparison for exceptions, and hence masks the issue; exceptions work there regardless.
• the two ways to fix this issue are (1) mark all exception classes BOOST_SYMBOL_VISIBLE in addition to BOOST_LIBRARY_DECL, or (2) make BOOST_SYMBOL_IMPORT also expand to BOOST_SYMBOL_VISIBLE.
• (2) has the upside of making everything work, and the downside that class members inherit the visibility of their class; therefore, inline member functions of a class marked VISIBLE become VISIBLE too, unless -fvisibility-inlines-hidden is used. This isn't always desirable.
• CMake chooses to make _DECL expand to the equivalent of _VISIBLE for both export and import; they have chosen solution (2).

[pdimov]

In addition, it appears that the problem only affects exception classes that are marked _DECL but are entirely header-only. So at the moment I'm inclined to think that this is a problem with the specific library (ProgramOptions) rather than with the macros.

[pdimov]

boostorg/program_options#106

[Flamefire]

I'd like to add a third option: Add a BOOST_CLASS_EXPORT define which on Windows is the same as BOOST_SYMBOL_EXPORT but on platforms supporting visibility it should be BOOST_SYMBOL_VISIBLE
And maybe a BOOST_CLASS_IMPORT defined as BOOST_SYMBOL_IMPORT for clarity.

Because check this class:

struct Foo{
  virtual ~Foo();
  virtual int getAnswer() = 0;
  static int id;
};

How would you decorate this class? You need BOOST_SYMBOL_VISIBLE to make the virtual parts work on libc++ but BOOST_SYMBOL_EXPORT/IMPORT to handle the static data member defined in a cpp-file.

I realize (now) that BOOST_CLASS_EXPORT could be #define BOOST_CLASS_EXPORT BOOST_SYMBOL_VISIBLE BOOST_SYMBOL_EXPORT as it seems from boostorg/program_options#106 the duplicated attribute doesn't cause issues.

However for classes (especially exception classes) fully defined in the header I don't see the need for BOOST_SYMBOL_EXPORT as BOOST_SYMBOL_VISIBLE should be enough already and that is what is documented too.