in Xcode LLVM C++ standard library with c++11 support

[CoderAndCat]

in Xcode LLVM C++ standard library with c++11 support and c++ 17 Language.
Which tag is suitable for use in xcode？

[webern]

Are you using the checked-in Xcode project? I think it might be broken currently by an update in Xcode #119.

But you should be able to generate a working Xcode project using CMake.
cmake -G Xcode .
https://stackoverflow.com/a/41830798/2779792

Edit: this might answer your question more directly... C++17 features are needed because I used std::optional.

[CoderAndCat]

I want to us mx in my iOS project as static library. but wrong!

[webern]

Are you using an Objective C++ file as a bridge to swift? I found this which suggests that the problem could be that you are using Objective-C instead of Objective-C++. https://stackoverflow.com/a/54498903

I don't know how to bridge between C++ and Swift, but it seems like that is the source of the problem. The header file you pictured is being evaluated in C instead of C++.