Question about how to compile multiple files with g++

[Yen]

Hi, 
I have a problem to compile multiple files together, so please everybody give 
me a help, thanks!

We assume there are five files:

main.cpp : the main function
a.h      : the header file of class A
a.cpp    : the definition of class A
b.h      : the header file of class B
b.cpp    : the definition of class B

In the program, main.cpp uses class A (thus including a.h) and class A uses 
class B (thus including b.h).

So, how do I compile this program to generate the objective file?

I tried to use the following command, but it feedbacked the error "undefined 
reference to ~(objects in class A and B)".

ps. running platform: FreeBSD/i386 system
    compiler: gcc version 3.2.1

Thank you much!