Boost C++ Libraries

A number of compilers incorrectly treat const lvalues of integral type as rvalues, and create an illegal temporary when binding to an lvalue reference to const in some expressions. This could result in creating an optional lvalue reference that is in fact bound to an unexpected temporary rather than to the intended object. In order to prevent hard to find run-time bugs, this library performs compile-time checks to prevent expressions that would otherwise bind an optional reference to an unexpected temporary. As a consequence, on certain compilers certain pieces of functionality in optional references are missing. In order to maintain a portability of your code across diferent compilers, it is recommended that you only stick to the minimum portable interface of optional references: prefer 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;  // caution: not portable
or1 = i;                       // caution: not portable

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

Compilers known to have these deficiencies include GCC versions 4.2, 4.3, 4.4, 4.5, 5.1, 5.2; QCC 4.4.2; MSVC versions 8.0, 9.0, 10.0, 11.0, 12.0. 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;
}