Does this cause undefined behavior?

Question

Address sanitizer is complaining.

struct X
{
  iterator begin();
  iterator end();
};

X foo();

const X& bar(const X& x)
{
  return x;
}

BOOST_FOREACH(const auto& xitem, bar(foo()))
{
  //use xitem
}

Answer 1

Yes. The FOREACH is happening on a reference to a struct which has been created by the foo call, and gone out of scope after being passed through to bar()

Answer 2

Yes, this causes undefined behavior.

When you bind a const reference to a temporary, the lifetime of the temporary is extended to the scope of the bound reference. In your case, this is the function bar. Thus, you cannot access the temporary after the bar function exits.

Edit:

Looked it up in the standard and actually the lifetime of the temporary is extended to the full expression containing bar. So it depends on how BOOST_FOREACH is implemented whether your code is UB or not.

From N3337 12.2.5

A temporary bound to a reference parameter in a function call (5.2.2) persists until the completion of the full expression containing the call.