How can I add a (.a) static library to a C++ program?

How can I use a static library in a C++ program that I have created?

For example, lets say I preform the following actions:

1. First I create file: foo.h, and add the following code:

  int foo(int a);

2. In turn I create foo.cpp, and I add the following:

  #include foo.h

  int foo(int a)
  {
    return a+1;
  }

3. Then I use GNU's C++ compiler GCC (except I use the G++ version) which looks something like the following:

3a.   g++ foo.cpp
3b.   ar rc libfoo.a foo.o

4. I create my program's main file, main.cpp

File: "main.cpp"

#include "foo.h"

int main()
{ 
    int i = foo(2);
    return i;
}

Here is where I am getting an error. How can I compile my .a static library with main.cpp?

What I am attempting to do is the following:

g++ -L. -lfoo prog.cpp

but get an error because the function foo would not be found