Is there any way to not have to write function declarations twice (headers) and still retain the same scalability in compiling, clarity in debugging, and flexibility in design when programming in C++?
Use Lzz. It takes a single file and automatically creates a .h and .cpp for you with all the declarations/definitions in the right place.
Lzz is really very powerful, and handles 99% of full C++ syntax, including templates, specializations etc etc etc.
Update 150120:
Newer C++ '11/14 syntax can only be used within Lzz function bodies.