Passing optional parameter by reference in c++

Why can’t you use function overloading? Surely it’s the easiest solution to your problem?

void foo(double &bar, double &foobar) 
{ 
   bar = 100; 
   foobar = 150; 
}

void foo(double &bar) 
{ 
   double foobar = 0.0;
   foo(bar, foobar);
}