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.
- Mark a symbol as hidden so it will not be exported from shared libraries.
- Mark a symbol as being exported by the libc++ library. This attribute must be applied to the declaration of all functions exported by the libc++ dylib.
- Mark a symbol as being exported by the libc++ library. This attribute may only be applied to objects defined in the libc++ library. On Windows this macro applies dllimport/dllexport to the symbol. On all other platforms this macro has no effect.
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 hidden and force inlining whenever possible.
- A synonym for _LIBCPP_INLINE_VISIBILITY
- Mark a type’s typeinfo, vtable and members as having default visibility. This attribute cannot be used on class templates.
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 typeinfo of an enum as having default visibility. This attribute should be applied to all enum declarations.
Windows Behavior: DLLs do not support importing or exporting enumeration typeinfo. The macro has an empty definition on this platform.
GCC Behavior: GCC un-hides the typeinfo for enumerations by default, even if -fvisibility=hidden is specified. Additionally applying a visibility attribute to an enum class results in a warning. The macro has an empty definition with GCC.
Mark the member functions, typeinfo, and vtable of the type named in a _LIBCPP_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.
GCC Behavior: GCC ignores visibility attributes applied the type in extern template declarations and applying an attribute results in a warning. However since _LIBCPP_TEMPLATE_VIS is the same as __attribute__((visibility(“default”)) the visibility is already correct. The macro has an empty definition with GCC.
Windows Behavior: extern template and dllexport are fundamentally incompatible on a template class on Windows; the former suppresses instantiation, while the latter forces it. Specifying both on the same declaration makes the template class 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 template classes. Extern template functions obey regular import/export semantics, and applying dllexport directly to the extern template declaration 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 template class 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.
Mark a member function of a class template as visible and always inline. This macro should only be applied to member functions of class templates that are externally instantiated. It is important that these symbols are not marked as hidden as that will prevent the dylib definition from being found.
This macro is used to maintain ABI compatibility for symbols that have been historically exported by the libc++ library but are now marked inline.
- Mark the member functions, typeinfo, and vtable of the type as being exported by the libc++ library. This macro must be applied to all exception types. Exception types should be defined directly in namespace std and not the versioning namespace. This allows throwing and catching some exception types between libc++ and libstdc++.