When is the use of std::ref necessary?

Question

Consider:

std::tuple<int , const A&> func (const A& a) 
{
  return std::make_tuple( 0 , std::ref(a) );
}

Is the std::ref required for writing correct and portable code? (It compiles fine without it)

Background:

If I remove std::ref my code builds fine without any warnings (g++-4.6 -Wall), but doesn't run correctly.

In case of interest the definition of A:

struct A {
  std::array<int,2> vec;
  typedef int type_t;

  template<typename... OPs,typename... VALs>
  A& operator=(const std::pair< std::tuple<VALs...> , std::tuple<OPs...> >& e) {
    for( int i = 0 ; i < vec.size() ; ++i ) {
      vec[i] = eval( extract(i,e.first) , e.second );
    }
  }
};

Answer 1

• make_tuple(0, a) makes a tuple<int, A>.
• make_tuple(0, ref(a)) makes a tuple<int, reference_wrapper<A>>.
• You can also say tuple<int, A&> t(0, a); for a tuple you can't make with make_tuple, or use std::tie.