best C++ compiler for mac.

Xcode works fine offline..

But I think you can download the XCode developer tools which will install the compiler without the IDE.

And then there is this:

https://solarianprogrammer.com/2017/05/21/compiling-gcc-macos/

You would struggle to find a better C/C++ compiler than the clang/clang++ technology found in Xcode, or separately in the Command-Line Tools (macOS 10.14) for Xcode 10.1 download — provided that you have a free developers account.

Apologies for any redundancy here from Keith Barkley's post.

You can always use g++ from the terminal window... First time you use it, a window might pop up that asks if you want to install the command-line developer tools -- obviously click on "install" when that window pops up.