1========================
2Symbol Visibility Macros
3========================
4
5.. contents::
6   :local:
7
8Overview
9========
10
11Libc++ uses various "visibility" macros in order to provide a stable ABI in
12both the library and the headers. These macros work by changing the
13visibility and inlining characteristics of the symbols they are applied to.
14
15Visibility Macros
16=================
17
18**_LIBCPP_HIDDEN**
19  Mark a symbol as hidden so it will not be exported from shared libraries.
20
21**_LIBCPP_FUNC_VIS**
22  Mark a symbol as being exported by the libc++ library. This attribute must
23  be applied to the declaration of all functions exported by the libc++ dylib.
24
25**_LIBCPP_OVERRIDABLE_FUNC_VIS**
26  Mark a symbol as being exported by the libc++ library, but allow it to be
27  overridden locally. On non-Windows, this is equivalent to `_LIBCPP_FUNC_VIS`.
28  This macro is applied to all `operator new` and `operator delete` overloads.
29
30  **Windows Behavior**: Any symbol marked `dllimport` cannot be overridden
31  locally, since `dllimport` indicates the symbol should be bound to a separate
32  DLL. All `operator new` and `operator delete` overloads are required to be
33  locally overridable, and therefore must not be marked `dllimport`. On Windows,
34  this macro therefore expands to `__declspec(dllexport)` when building the
35  library and has an empty definition otherwise.
36
37**_LIBCPP_INLINE_VISIBILITY**
38  Mark a function as hidden and force inlining whenever possible.
39
40**_LIBCPP_ALWAYS_INLINE**
41  A synonym for `_LIBCPP_INLINE_VISIBILITY`
42
43**_LIBCPP_TYPE_VIS**
44  Mark a type's typeinfo and vtable as having default visibility.
45  `_LIBCPP_TYPE_VIS`. This macro has no effect on the visibility of the
46  type's member functions. This attribute cannot be used on class templates.
47
48  **GCC Behavior**: GCC does not support Clang's `type_visibility(...)`
49  attribute. With GCC the `visibility(...)` attribute is used and member
50  functions are affected.
51
52**_LIBCPP_TYPE_VIS_ONLY**
53  The same as `_LIBCPP_TYPE_VIS` except that it may be applied to templates.
54
55  **Windows Behavior**: DLLs do not support dllimport/export on class templates.
56  The macro has an empty definition on this platform.
57
58  Note: This macro should be renamed `_LIBCPP_TEMPLATE_TYPE_VIS`.
59
60**_LIBCPP_ENUM_VIS**
61  Mark the typeinfo of an enum as having default visibility. This attribute
62  should be applied to all enum declarations.
63
64  **Windows Behavior**: DLLs do not support importing or exporting enumeration
65  typeinfo. The macro has an empty definition on this platform.
66
67  **GCC Behavior**: GCC un-hides the typeinfo for enumerations by default, even
68  if `-fvisibility=hidden` is specified. Additionally applying a visibility
69  attribute to an enum class results in a warning. The macro has an empty
70  definition with GCC.
71
72**_LIBCPP_EXTERN_TEMPLATE_TYPE_VIS**
73  Mark the member functions, typeinfo, and vtable of the type named in
74  a `_LIBCPP_EXTERN_TEMPLATE` declaration as being exported by the libc++ library.
75  This attribute must be specified on all extern class template declarations.
76
77  This macro is used to override the `_LIBCPP_TYPE_VIS_ONLY` attribute
78  specified on the primary template and to export the member functions produced
79  by the explicit instantiation in the dylib.
80
81  **GCC Behavior**: GCC ignores visibility attributes applied the type in
82  extern template declarations and applying an attribute results in a warning.
83  However since `_LIBCPP_TYPE_VIS_ONLY` is the same as `_LIBCPP_TYPE_VIS` the
84  visibility is already correct. The macro has an empty definition with GCC.
85
86  **Windows Behavior**: `extern template` and `dllexport` are fundamentally
87  incompatible *on a template class* on Windows; the former suppresses
88  instantiation, while the latter forces it. Specifying both on the same
89  declaration makes the template class be instantiated, which is not desirable
90  inside headers. This macro therefore expands to `dllimport` outside of libc++
91  but nothing inside of it (rather than expanding to `dllexport`); instead, the
92  explicit instantiations themselves are marked as exported. Note that this
93  applies *only* to extern template *classes*. Extern template *functions* obey
94  regular import/export semantics, and applying `dllexport` directly to the
95  extern template declaration is the correct thing to do for them.
96
97**_LIBCPP_CLASS_TEMPLATE_INSTANTIATION_VIS**
98  Mark the member functions, typeinfo, and vtable of an explicit instantiation
99  of a class template as being exported by the libc++ library. This attribute
100  must be specified on all template class explicit instantiations.
101
102  It is only necessary to mark the explicit instantiation itself (as opposed to
103  the extern template declaration) as exported on Windows, as discussed above.
104  On all other platforms, this macro has an empty definition.
105
106**_LIBCPP_EXTERN_TEMPLATE_INLINE_VISIBILITY**
107  Mark a member function of a class template as visible and always inline. This
108  macro should only be applied to member functions of class templates that are
109  externally instantiated. It is important that these symbols are not marked
110  as hidden as that will prevent the dylib definition from being found.
111
112  This macro is used to maintain ABI compatibility for symbols that have been
113  historically exported by the libc++ library but are now marked inline.
114
115**_LIBCPP_EXCEPTION_ABI**
116  Mark the member functions, typeinfo, and vtable of the type as being exported
117  by the libc++ library. This macro must be applied to all *exception types*.
118  Exception types should be defined directly in namespace `std` and not the
119  versioning namespace. This allows throwing and catching some exception types
120  between libc++ and libstdc++.
121
122Links
123=====
124
125* `[cfe-dev] Visibility in libc++ - 1 <http://lists.llvm.org/pipermail/cfe-dev/2013-July/030610.html>`_
126* `[cfe-dev] Visibility in libc++ - 2 <http://lists.llvm.org/pipermail/cfe-dev/2013-August/031195.html>`_
127* `[libcxx] Visibility fixes for Windows <http://lists.llvm.org/pipermail/cfe-commits/Week-of-Mon-20130805/085461.html>`_
128