I’ve tried everything (I think) trying to get SM 4.3 to work with my iMac on 10.14 including different versions of XCode ranging from 6.2 to 10.3 where the SDK for 10.14 is clearly there. So, why is it not working even with manually entering C++ compiler options? I removed all compiler flags except libc++
SystemModeler 4.3 is quite old and the support libraries that simulations are linked with was at that time built with libstdc++, so you cannot change to link with libc++ (that’s why you get link errors).
What you need is an Xcode old enough to ship with libstdc++ and my googling tells me that Xcode 9 or older should do that. I verified on my machine that 4.3 works with Xcode 9.2 with the default compiler flags (-m64 -stdlib=libstdc++ -fPIC).
How did you download XCode 9.2? I went to the official Apple Software Downloads (Sign In - Apple) but after downloading versions XCode_9.2.xip (or XCode_9.3.xip) and trying to expand them, I get a message “The archive “XCode_9.2.xip” does not come from Apple”. Not sure how to proceed, unless you know a way of getting .dmg file for these.