Question

void function(typeA* ptr_to_A) {
  if (!ptr_to_A) {
    typeB B; // typeB is a derived class of typeA
    ptr_to_A = &B;
  }
  do_stuff_to_ptr_to_A(ptr_to_A);
  // my hope is that B is still in scope here so that this function will operate on my B object (which is on the stack) which only gets created if ptr_to_A was initially NULL
}

Will this function do what I think it does (what I want it to do)? That is, only allocate B on the stack if the argument was a null pointer?

Answer 1

Will this function do what I think it does?

No, it’s undefined behavior because B goes out of scope. Since this is undefined behavior anything can happen, and therefore you can’t predict the results. You want to keep B in at least the same scope as the function call, so just move it to the top of the method:

void function(typeA* ptr_to_A) {
   typeB B; // typeB is a derived class of typeA
   if (!ptr_to_A) {
      ptr_to_A = &B;
   }
   do_stuff_to_ptr_to_A(ptr_to_A);
}

But if you only want to allocate a typeB if ptr_to_A is null then you could do this:

void function(typeA* ptr_to_A) {
  if (!ptr_to_A) {
    typeB B; // typeB is a derived class of typeA
    do_stuff_to_ptr_to_A(&B);
  } else {
    do_stuff_to_ptr_to_A(ptr_to_A);
  }
}