Default c++ compiler version
You can find the default version by looking in the manual:

man clang


It states:


  The default C++ language standard is gnu++14.


This is basically the same as C++14, but with some GNU specific extensions.

The way to change the default setting when used "standalone" (i.e. without an Xcode project, Makefile or similar) is actually to make a shell alias. There's no clang configuration file where you can change the default - you'll need to recompile the compiler to change it.