#include <stdio.h>

void Func(bool& param = *(bool*)nullptr)
{
    if (&param == nullptr)
        std::printf("%s", "no passed value (nullptr)\n");
    else
        std::printf("%s %s", "passed value:", param ? "True\n" : "False\n");
}

int main()
{
    bool var = false;

    Func(var);
    Func();
    var = true;
    Func(var);

    return 0;
}

//Result:
/*
passed value: False
no passed value (nullptr)
passed value: True
*/