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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_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_TYPE_VIS_ONLY**
  The same as `_LIBCPP_TYPE_VIS` except that it may be applied to templates.

  **Windows Behavior**: DLLs do not support dllimport/export on class templates.
  The macro has an empty definition on this platform.

  Note: This macro should be renamed `_LIBCPP_TEMPLATE_TYPE_VIS`.

**_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_TYPE_VIS_ONLY` 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_TYPE_VIS_ONLY` is the same as `_LIBCPP_TYPE_VIS` the
  visibility is already correct. The macro has an empty definition with GCC.

**_LIBCPP_EXTERN_TEMPLATE_INLINE_VISIBILITY**
  Mark a member function of a class template as hidden and inline except when
  building the libc++ library where it marks the symbol as being exported by
  the library.

  This macro is used to maintain ABI compatibility for symbols that have been
  historically exported by the libc++ library but are now marked inline. It
  should only be applied to member functions of class templates that are
  externally instantiated.

**_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>`_