[Solved] Why am I getting an undefined reference error while using separate .cpp and .h files for a class?

Question

In your second and final step, you didn’t instruct the compiler (linker more exactly) to take into account Adder.o, so your final executable still doesn’t know the implementation of Adder::add
Try, after getting Adder.o, to run g++ main.cpp Adder.o

Also, this may be relevant : Difference between compiling with object and source files

Also, if that is the complete code, as others have pointed out, in the Adder.cpp, you are just defining a simple function, not the one from the Adder class.

Accepted Answer

In your second and final step, you didn’t instruct the compiler (linker more exactly) to take into account Adder.o, so your final executable still doesn’t know the implementation of Adder::add
Try, after getting Adder.o, to run g++ main.cpp Adder.o

Also, this may be relevant : Difference between compiling with object and source files

Also, if that is the complete code, as others have pointed out, in the Adder.cpp, you are just defining a simple function, not the one from the Adder class.