c++ - "class _LIBCPP_TEMPLATE_VIS vector"中宏_LIBCPP_TEMPLATE_VIS的用法是什么

Question

像标准库类vector，声明总是这样“类 _LIBCPP_TEMPLATE_VIS vector ”。

https://github.com/llvm-mirror/libcxx/blob/7102892bf3dfffb3f988aa9a0a29f58171a508d7/include/__config#L717

不知道为什么在类关键字和类名之间插入宏。

Answer 1

这是一个符号可见性宏。它存在的原因是确保 -fvisibility=hidden 编译标志不会影响 std::vector ABI。使用 Clang，它扩展为 __attribute__((__visibility__("default")))。该宏允许它适应不同的编译器/平台，如 documentation 中所述。 :

Libc++ uses various “visibility” macros in order to provide a stableABI in both the library and the headers. These macros work by changingthe visibility and inlining characteristics of the symbols they areapplied to.

_LIBCPP_TEMPLATE_VIS

Mark a type’s typeinfo and vtable as having default visibility. Thismacro 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 memberfunctions are affected.

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

另见 GCC visibility文档。