Dangerous Linkage

A.cpp

#include <iostream>

struct S
{
	int i;
	int f(int j) // Multiply.
	{
		return i*j;
	}
};

int square(int i)
{
	S s{i};
	return s.f(i);
}

int times_2(int i);

int main()
{
	std::cout << "square(10) = " << square(10) << std::endl;
	std::cout << "times_2(10) = " << times_2(10) << std::endl;
}

B.cpp

struct S
{
	int i;
	int f(int j) // Add.
	{
		return i+j;
	}
};
	
int times_2(int i)
{
	S s{i};
	return s.f(i);
}