Private data member with automatic public read-only through an accessor.

readonly_member.cpp

#include <memory>

template <class Parent, typename T> class readable final {
  friend Parent;
  T value_;
  operator T& () { return value_; }
 public:
  readable(T const& v) : value_(v) {}
  readable(T&& v) : value_(std::move(v)) {}
  T const& operator () () { return value_; }
};

struct test {
  template <typename T> using attr = readable<test, T>;
  attr<int> v = 0;
  void inc() { ++v; }
};

int main() {
  test t;
  t.inc();
  //t.v++;  // Wont compile, though the argument is public !
  return t.v();
}