Symbol Visibility Macros¶
Libc++ uses various “visibility” macros in order to provide a stable ABI in both the library and the headers. These macros work by changing the visibility and inlining characteristics of the symbols they are applied to.
The std namespace also has visibility attributes applied to avoid having to add visibility macros in as many places. Namespace std has default type_visibility to export RTTI and other type-specific information. Note that type_visibility is only supported by Clang, so this doesn’t replace type-specific attributes. The only exception are enums, which GCC always gives default visibility, thus removing the need for any annotations.
Mark a symbol as hidden so it will not be exported from shared libraries.
Mark a symbol as being part of our ABI. This includes functions that are part of the libc++ library, type information and other symbols. On Windows, this macro applies dllimport/dllexport to the symbol, and on other platforms it gives the symbol default visibility.
Mark a symbol as being exported by the libc++ library, but allow it to be overridden locally. On non-Windows, this is equivalent to _LIBCPP_FUNC_VIS. This macro is applied to all operator new and operator delete overloads.
Windows Behavior: Any symbol marked dllimport cannot be overridden locally, since dllimport indicates the symbol should be bound to a separate DLL. All operator new and operator delete overloads are required to be locally overridable, and therefore must not be marked dllimport. On Windows, this macro therefore expands to __declspec(dllexport) when building the library and has an empty definition otherwise.
Mark a function as not being part of the ABI of any final linked image that uses it.
Mark a function as being hidden from the ABI (per _LIBCPP_HIDE_FROM_ABI) when libc++ is built with an ABI version after ABI v1. This macro is used to maintain ABI compatibility for symbols that have been historically exported by libc++ in v1 of the ABI, but that we don’t want to export in the future.
This macro works as follows. When we build libc++, we either hide the symbol from the ABI (if the symbol is not part of the ABI in the version we’re building), or we leave it included. From user code (i.e. when we’re not building libc++), the macro always marks symbols as internal so that programs built using new libc++ headers stop relying on symbols that are removed from the ABI in a future version. Each time we release a new stable version of the ABI, we should create a new _LIBCPP_HIDE_FROM_ABI_AFTER_XXX macro, and we can use it to start removing symbols from the ABI after that stable version.
Mark a type’s typeinfo and vtable as having default visibility. This macro has no effect on the visibility of the type’s member functions.
GCC Behavior: GCC does not support Clang’s type_visibility(…) attribute. With GCC the visibility(…) attribute is used and member functions are affected.
Windows Behavior: DLLs do not support dllimport/export on class templates. The macro has an empty definition on this platform.
Mark the member functions, typeinfo, and vtable of the type named in an extern template declaration as being exported by the libc++ library. This attribute must be specified on all extern class template declarations.
This macro is used to override the _LIBCPP_TEMPLATE_VIS attribute specified on the primary template and to export the member functions produced by the explicit instantiation in the dylib.
Windows Behavior: extern template and dllexport are fundamentally incompatible on a class template on Windows; the former suppresses instantiation, while the latter forces it. Specifying both on the same declaration makes the class template be instantiated, which is not desirable inside headers. This macro therefore expands to dllimport outside of libc++ but nothing inside of it (rather than expanding to dllexport); instead, the explicit instantiations themselves are marked as exported. Note that this applies only to extern class templates. Extern function templates obey regular import/export semantics, and applying dllexport directly to the extern template declaration (i.e. using _LIBCPP_FUNC_VIS) is the correct thing to do for them.
Mark the member functions, typeinfo, and vtable of an explicit instantiation of a class template as being exported by the libc++ library. This attribute must be specified on all class template explicit instantiations.
It is only necessary to mark the explicit instantiation itself (as opposed to the extern template declaration) as exported on Windows, as discussed above. On all other platforms, this macro has an empty definition.
Mark a symbol as hidden so it will not be exported from shared libraries. This is intended specifically for method templates of either classes marked with _LIBCPP_TYPE_VIS or classes with an extern template instantiation declaration marked with _LIBCPP_EXTERN_TEMPLATE_TYPE_VIS.
When building libc++ with hidden visibility, we want explicit template instantiations to export members, which is consistent with existing Windows behavior. We also want classes annotated with _LIBCPP_TYPE_VIS to export their members, which is again consistent with existing Windows behavior. Both these changes are necessary for clients to be able to link against a libc++ DSO built with hidden visibility without encountering missing symbols.
An unfortunate side effect, however, is that method templates of classes either marked _LIBCPP_TYPE_VIS or with extern template instantiation declarations marked with _LIBCPP_EXTERN_TEMPLATE_TYPE_VIS also get default visibility when instantiated. These methods are often implicitly instantiated inside other libraries which use the libc++ headers, and will therefore end up being exported from those libraries, since those implicit instantiations will receive default visibility. This is not acceptable for libraries that wish to control their visibility, and led to PR30642.
Consequently, all such problematic method templates are explicitly marked either hidden (via this macro) or inline, so that they don’t leak into client libraries. The problematic methods were found by running bad-visibility-finder against the libc++ headers after making _LIBCPP_TYPE_VIS and _LIBCPP_EXTERN_TEMPLATE_TYPE_VIS expand to default visibility.