Determine if function was called without setting global variable

What is a good technique to determine if a function was called in C++ without setting a global variable?

#include <iostream>

bool var = false;

void X ()
{
    if (var) {std::cout<<" Y called "<< std::endl;}
}

void Y ()
{var = true;}

int main()
{
Y();
X();
}

I would really like an example of a way to do this without setting a global variable.