It's much less of your own code if you use TCL (THINK Class Library), which shipped with THINK C 4.0 (and THINK Pascal) in mid 1989.

Your System 6.0.8 is from April 1991, so TCL was well established by then and the C/C++ version in THINK C 5 even used proper C++ features instead of the hand-rolled "OOP in C" (nested structs with function pointers) used by TCL in THINK C 4.

I used TCL for smaller projects, mostly with THINK Pascal which was a bit more natural using Object Pascal, and helped other people use it and transition their own programs that previously used the Toolbox directly, but my more serious programs used MacApp which was released for Object Pascal in 1985, and for C++ in 1991.