When is the use of std::ref necessary?

Question

Consider:

std::tuple 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 vec;
  typedef int type_t;

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

user_288875 · Answer

Answering the question in the title (When is the use of std::ref necessary?): Another case where std::ref is useful is when looping over a list of references to objects and modify them:

std::vector v1, v2;
  
void test() {
  for (std::vector& vv : 
    // Compiles
    { std::ref(v1), std::ref(v2) } 
  
    // Compiler rejects this with:
    //   binding reference of type 'vector<...>' to value of 
    //   type 'const vector<...>' drops 'const' qualifier 
    // { v1, v2} 
  ) {
      vv.push_back(3);
  }
}

Without using std::ref in the list, the objects are treated as const and can't be modified (see also https://godbolt.org/z/Ta6YM31KM).

When is the use of std::ref necessary?

4 Answers