Boost C++ Libraries

On compilers that do not conform to Standard C++ rules of reference binding, operations on optional references might give adverse results: rather than binding a reference to a designated object they may create an unexpected temporary and bind to it. Compilers known to have these deficiencies include GCC versions 4.2, 4.3, 4.4, 4.5; QCC 4.4.2; MSVC versions 8.0, 9.0, 10.0, 11.0, 12.0. On these compilers prefer using direct-initialization and copy assignment of optional references to copy-initialization and assignment from T&:

const int i = 0;
optional<const int&> or1;
optional<const int&> or2 = i;  // not portable
or1 = i;                       // not portable

optional<const int&> or3(i);   // portable
or1 = optional<const int&>(i); // portable

In order to check if your compiler correctly implements reference binding use this test program.

#include <cassert>

const int global_i = 0;

struct TestingReferenceBinding
{
  TestingReferenceBinding(const int& ii)
  {
    assert(&ii == &global_i);
  }

  void operator=(const int& ii)
  {
    assert(&ii == &global_i);
  }

  void operator=(int&&) // remove this if your compiler doesn't have rvalue refs
  {
    assert(false);
  }
};

int main()
{
  const int& iref = global_i;
  assert(&iref == &global_i);

  TestingReferenceBinding ttt = global_i;
  ttt = global_i;

  TestingReferenceBinding ttt2 = iref;
  ttt2 = iref;
}