Use SWIG for .cpp which depends on another .cpp - cmake

Question

I have a .cpp file which calls another .cpp file. I want to use SWIG to create a Python wrapper for this.
How do I make this using SWIG.
When I had a single .cpp file I was able to create a .so in the following manner:
//app.cpp
#include "app.hpp"
int p(int a)
{
std::cout<<"hello...SWIG runs fine!"<<std::endl;
return a;
}
//app.hpp
#include <iostream>
int p(int a);
//app.i
%module app
%{
#include "app.hpp"
%}
%include "app.hpp"
Commands run were:
swig -c++ -python app.i
g++ -Isrc -fPIC -I/../../../../usr/include/python3.6m
-I/../../../../usr/include/x86_64-linux-gnu/python3.6m -lpython3.6m -c app.cpp app_wrap.cxx
g++ -shared -fPIC -o _app.so app.o app_wrap.o
I was successful in generating a .so file with this method. However, now my app.cpp needs to use a function defined in another .cpp file (a function called int fn1(int x) in hello.cpp). How do I generate .so now? If anyone could provide a small example it'd be great!
EDIT:: Someone suggested I would have to use SWIG and CMAKE together. Is this true? If yes, how can I do so?

Answer 1

Solved it!
swig -c++ -python file1.i
g++ -Isrc -fPIC -I/../../../../usr/include/python3.6m
-I/../../../../usr/include/x86_64-linux-gnu/python3.6m -lpython3.6m -c file1.cpp file2.cpp file1_wrap.cxx
g++ -shared -fPIC -o _file1.pyd file1.o file2.o file1_wrap.o
Turns out we can use CMake to define dependencies and do this process automatically.
Also, SWIG can be used with CMake as mentioned on SWIG documentation Introduction_build_system