a.cpp

#include "header_a.h"

bool A::even(int n) {
  return n == 0 || B().odd(n - 1);
}

b.cpp

#include "header_b.h"

bool B::odd(int n) {
  return !(n == 0 || !A().even(n - 1));
}

header_a.h

#pragma once
#include "header_b.h"

class A {
public:
  bool even(int n);
};

header_b.h

#pragma once
#include "header_a.h"

class B {
public:
  bool odd(int n);
};

main.cpp

#include <cassert>
#include "header_a.h"
#include "header_b.h"

int main() {
  assert(A().even(0));
  assert(A().even(2));
  assert(B().odd(1));
  assert(B().odd(3));
  return 0;
}

Makefile

test: main.o a.o b.o
	g++ $^ -o $@