A sample of extern

.gitignore

### https://raw.github.com/github/gitignore/f2ce448f2ba7a092da05482ceca99209127c0884/c++.gitignore

# Prerequisites
*.d

# Compiled Object files
*.slo
*.lo
*.o
*.obj

# Precompiled Headers
*.gch
*.pch

# Compiled Dynamic libraries
*.so
*.dylib
*.dll

# Fortran module files
*.mod
*.smod

# Compiled Static libraries
*.lai
*.la
*.a
*.lib

# Executables
*.exe
*.out
*.app

README

a.cpp

#include "a.hpp"

A::A() : a(0), b(a) {}

int A::c = 10;
const int &A::d = A::c;

int N = 100;

a.hpp

#pragma once

struct A {
	int a;
	const int &b;
	static int c;
	static const int &d;
	A();
};

b.cpp

#include <iostream>

#include "a.hpp"

extern const int N;

using namespace std;

int main() {
	A x;
	x.a = 1;
	cout << x.b << endl;
	cout << x.c << endl;
	cout << x.d << endl;
	x.c = 20;
	cout << x.d << endl;
	cout << N << endl;
}

Makefile

CC = $(CXX)
%.out:
	$(LINK.o) $^ $(LOADLIBES) $(LDLIBS) -o $@
main.out: b.o a.o