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Symbol Visibility Macros
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Overview
========
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.
Visibility Macros
=================
**_LIBCPP_HIDDEN**
Mark a symbol as hidden so it will not be exported from shared libraries.
**_LIBCPP_FUNC_VIS**
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.
**_LIBCPP_OVERRIDABLE_FUNC_VIS**
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.
**_LIBCPP_INLINE_VISIBILITY**
Mark a function as hidden and force inlining whenever possible.
**_LIBCPP_ALWAYS_INLINE**
A synonym for `_LIBCPP_INLINE_VISIBILITY`
**_LIBCPP_TYPE_VIS**
Mark a type's typeinfo and vtable as having default visibility.
`_LIBCPP_TYPE_VIS`. This macro has no effect on the visibility of the
type's member functions. This attribute cannot be used on class templates.
**GCC Behavior**: GCC does not support Clang's `type_visibility(...)`
attribute. With GCC the `visibility(...)` attribute is used and member
functions are affected.
**_LIBCPP_TEMPLATE_ONLY**
The same as `_LIBCPP_TYPE_VIS` except that it may be applied to class
templates.
**Windows Behavior**: DLLs do not support dllimport/export on class templates.
The macro has an empty definition on this platform.
**_LIBCPP_ENUM_VIS**
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.
**_LIBCPP_EXTERN_TEMPLATE_TYPE_VIS**
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.
**_LIBCPP_CLASS_TEMPLATE_INSTANTIATION_VIS**
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.
**_LIBCPP_EXTERN_TEMPLATE_INLINE_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.
**_LIBCPP_EXCEPTION_ABI**
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++.
Links
=====
* `[cfe-dev] Visibility in libc++ - 1 <http://lists.llvm.org/pipermail/cfe-dev/2013-July/030610.html>`_
* `[cfe-dev] Visibility in libc++ - 2 <http://lists.llvm.org/pipermail/cfe-dev/2013-August/031195.html>`_
* `[libcxx] Visibility fixes for Windows <http://lists.llvm.org/pipermail/cfe-commits/Week-of-Mon-20130805/085461.html>`_