//============= file A.h =============
#pragma once

class B;

// Make sure A is fully declared but no implementations are defined yet
class A
{
	B createB();
}

#include "B.h"

A::createB() { return B{}; } // Implementations MUST come after header include


//============= file B.h =============
#pragma once

class A;

class B
{
	A createA();
}

#include "A.h"

B::createA() { return A{}; }