docs/DesignDocs/VisibilityMacros.rst - llvm-project/libcxx - Git at Google

 ========================
 Symbol Visibility Macros
 ========================

 .. contents::
    :local:

 .. _visibility-macros:

 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.

 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.

 Visibility Macros
 =================

 **_LIBCPP_HIDDEN**
   Mark a symbol as hidden so it will not be exported from shared libraries.

 **_LIBCPP_EXPORTED_FROM_ABI**
   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.

 **_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_HIDE_FROM_ABI**
   Mark a function as not being part of the ABI of any final linked image that
   uses it.

 **_LIBCPP_HIDE_FROM_ABI_AFTER_V1**
   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.

 **_LIBCPP_TEMPLATE_VIS**
   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.

 **_LIBCPP_EXTERN_TEMPLATE_TYPE_VIS**
   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.

 **_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 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.

 **_LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS**
   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 <https://github.com/smeenai/bad-visibility-finder>`_
   against the libc++ headers after making `_LIBCPP_TYPE_VIS` and
   `_LIBCPP_EXTERN_TEMPLATE_TYPE_VIS` expand to default visibility.

 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>`_
	========================
	Symbol Visibility Macros
	========================

	.. contents::
	:local:

	.. _visibility-macros:

	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.

	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.

	Visibility Macros
	=================

	_LIBCPP_HIDDEN
	Mark a symbol as hidden so it will not be exported from shared libraries.

	_LIBCPP_EXPORTED_FROM_ABI
	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.

	_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_HIDE_FROM_ABI
	Mark a function as not being part of the ABI of any final linked image that
	uses it.

	_LIBCPP_HIDE_FROM_ABI_AFTER_V1
	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.

	_LIBCPP_TEMPLATE_VIS
	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.

	_LIBCPP_EXTERN_TEMPLATE_TYPE_VIS
	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.

	_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 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.

	_LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
	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 <https://github.com/smeenai/bad-visibility-finder>`_
	against the libc++ headers after making `_LIBCPP_TYPE_VIS` and
	`_LIBCPP_EXTERN_TEMPLATE_TYPE_VIS` expand to default visibility.

	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>`_