今天更新到iOS12 所碰到的问题，于是去Google了一下
下面是苹果的回答：
https://forums.developer.apple.com/message/314346#315386
Correct. It is also not present in the iOS Simulator runtime. If you want to hack around this, you will need to copy the libstdc++.*dylib from the iOS 11.4 simulator runtime to the iOS 12.0 simulator runtime in addition to copying the libstdc++.tbd between the SDKs.
Note that at some point the store will stop accepting apps linked against libstdc++, so it behouves you to actually update your project to use libc++ (which has been preferred for the past ~5 years or so).
大致意思就是这个库在Xcode10中被废弃.建议尽快进行更新.当前的解决方法是从Xcode9中将这个文件拷贝到Xcode10的对应目录下
Finder commond + shift + G 前往
操作方法：
libstdc++
Xcode10中删除的libstdc++库
其中文件夹 1、2、3、4 中的文件分别对应复制到Xcode10中的以下4个目录中即可：
/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/Library/CoreSimulator/Profiles/Runtimes/iOS.simruntime/Contents/Resources/RuntimeRoot/usr/lib/
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/lib/
/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS.sdk/usr/lib/
/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator.sdk/usr/lib/
