Deprecation Notices:

Building with libstdc++ was deprecated with Xcode 8 and is not
supported in Xcode 10 when targeting iOS. C++ projects must now
migrate to libc++ and are recommended to set a deployment target of
iOS 7 or later. Besides changing the C++ Standard Library build
setting,developers should audit hard-coded linker flags and target
dependencies to remove references to libstdc++ (including -lstdc++,
-lstdc++.6.0.9,libstdc++.6.0.9.tbd,and libstdc++.6.0.9.dylib). Project dependencies such as static archives that were built against
libstdc++ will also need to be rebuilt against libc++. (40885260)

资料来源：Release Notes of XCode Beta 2

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XCode 10正式发布其release note,它仍然如此：

Building with libstdc++ was deprecated with Xcode 8 and is not supported in Xcode 10 when targeting iOS. C++ projects must now migrate to libc++ and are recommended to set a deployment target of macOS 10.9 or later,or iOS 7 or later. Besides changing the C++ Standard Library build setting,developers should audit hard-coded linker flags and target dependencies to remove references to libstdc++ (including -lstdc++,-lstdc++.6.0.9,and libstdc++.6.0.9.dylib). Project dependencies such as static archives that were built against libstdc++ will also need to be rebuilt against libc++. (40885260)