Xcode 提供 -std=c++20 但命令行 clang 不提供

Question

I have the Xcode command line tools installed and can use clang++ fine for versions up to C++17. In Xcode itself, I can select C++20 in the build settings:

But when I try to compile using clang++ from the command line with this option:

error: invalid value 'c++20' in '-std=c++20'
note: use 'c++17' for 'ISO C++ 2017 with amendments' standard
note: use 'gnu++17' for 'ISO C++ 2017 with amendments and GNU extensions' standard
note: use 'c++2a' for 'Working draft for ISO C++ 2020' standard
note: use 'gnu++2a' for 'Working draft for ISO C++ 2020 with GNU extensions' standard

What would be the reason for this inconsistency? In case it matters:

~/ which clang++
/usr/bin/clang++
~/ clang++ -v
Apple clang version 11.0.3 (clang-1103.0.32.29)
Target: x86_64-apple-darwin20.6.0
Thread model: posix
InstalledDir: /Library/Developer/CommandLineTools/usr/bin

(I also tried clang -x c++ ).

Answer 1

Like you have noticed, Xcode and system uses different instance of Command Line Tools. Xcode uses the one that is embedded in Xcode, and the system will use a copy located in Library/Developer/ , which is not automatically upgraded along with Xcode.

Support of c++20 flag is only added for later versions of Apple Clang , which seems like the clang embedded within your Xcode does support it, but the clang install in Library/Developer does not.

To force them be the same version, you might need to download it yourself from https://developer.apple.com/download/all/?q=command%20line%20tools , and choose the version that matches your Xcode.