Isn't adding declarations to std undefined behavior? Yes. This project is meant as a proof-of-concept for a proposal to standardize a forward declaration header for std. I omitted classes that are ...

clang++ -std=c++0x main.cpp clang++ -std=c++0x -c src.cpp clang++ main.o src.o -o output.out or g++ -std=c++0x -c main.cpp g++ -std=c++0x -c src.cpp g++ main.o src.o ...