Explicit ref and out keywords for arguments in C++11

explicit-ref-out-keywords.cpp

#include <utility>
#include <functional>
#include <stdio.h>

template <typename U>
struct Out {
  explicit Out (U& u_) : u (u_) {}
  Out (const Out<U>& other) = default;
  Out (Out<U>&& other) = default;
  Out<U>& operator= (const Out<U>&) = delete;
  
  const Out<U>& operator= (const U& u_) const {
    u = u_;
    return *this;
  }

  const Out<U>& operator= (U&& u_) const {
    using std::move;
    u = move (u_);
    return *this;
  }

private:
  U& u;
};

template <typename U>
struct Ref {
  explicit Ref (U& u_) : u (u_) {}
  Ref (const Ref<U>& other) = default;
  Ref (Ref<U>&& other) = default; 
  Ref (const std::reference_wrapper<U>& other) : u (&other.get ()) {}
  Ref<U>& operator= (const Ref<U>&) = delete;
  
  const Ref<U>& operator= (const U& u_) const {
    u = u_;
    return *this;
  }

  const Ref<U>& operator= (U&& u_) const {
    using std::move;
    u = move (u_);
    return *this;
  }
  
  operator U& () const {
    return u;
  }

private:
  U& u;
};

template<typename U>
inline
Out<U> out (U& u)
{
  return Out<U>(u);
}

template<typename U>
inline
Out<U> out (U& u)
{
  return Out<U>(u);
}

void
foo (Ref<int> x, Out<int> z)
{
  x = 1;
  int y = x; // int y = z is a compile-time error
  printf ("%d\n", y); 
  z = 3;
  x += 1; // z += 1 is a compile-time error
}

int
main ()
{
  using std::ref;
  int x = 0;
  int z = 0;
  foo (ref (x), out(z));
  printf ("%d %d\n", x, z);
  return 0;
}