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2 Symbol Visibility Macros
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11 Libc++ uses various "visibility" macros in order to provide a stable ABI in
12 both the library and the headers. These macros work by changing the
13 visibility and inlining characteristics of the symbols they are applied to.
19 Mark a symbol as hidden so it will not be exported from shared libraries.
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.
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.
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.
37 **_LIBCPP_INLINE_VISIBILITY**
38 Mark a function as hidden and force inlining whenever possible.
40 **_LIBCPP_ALWAYS_INLINE**
41 A synonym for `_LIBCPP_INLINE_VISIBILITY`
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.
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.
52 **_LIBCPP_TYPE_VIS_ONLY**
53 The same as `_LIBCPP_TYPE_VIS` except that it may be applied to templates.
55 **Windows Behavior**: DLLs do not support dllimport/export on class templates.
56 The macro has an empty definition on this platform.
58 Note: This macro should be renamed `_LIBCPP_TEMPLATE_TYPE_VIS`.
61 Mark the typeinfo of an enum as having default visibility. This attribute
62 should be applied to all enum declarations.
64 **Windows Behavior**: DLLs do not support importing or exporting enumeration
65 typeinfo. The macro has an empty definition on this platform.
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
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.
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.
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.
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.
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.
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.
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.
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.
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++.
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>`_