address-of-function-parameter.cc

/* Not an exercise from the Pohl book, but fun with Scott Meyers' assertion that
   "rhs is an lvalue even though it has an rvalue reference type" on p.3 of
   __Effective Modern C++_.  "By similar reasoning, all [function] parameters
   are lvalues" (p.3) where (p. 4) "In a function call, the expressions passed
   at the call site are the function's arguments.  The arguments are used to
   initialize the function's parameters."  Also, p. 158, "a parameter is always
   an lvalue, even though its type is rvalue-reference-to-Widget."
 */

#include <array>
#include <cassert>

#include <utility>

using namespace std;

class Widget {
public:
  Widget() : me(this) {}
  // p. 2: "A useful heuristic to determine whether an expression is an lvalue
  // is to ask if you can take its address."
  Widget(Widget &&rhs) : me(&rhs) {}

  const Widget *wheres_this() const { return me; }

private:
  const Widget *me;
};

int main(void) {
  Widget awidget;
  const Widget *widgetp = awidget.wheres_this();
  Widget another(std::move(awidget));
  assert(widgetp == another.wheres_this());

  Widget bwidget;
  const Widget *widgetpp = bwidget.wheres_this();
  std::array<Widget, 3> yetmore{
      {std::move(bwidget), std::move(bwidget), std::move(bwidget)}};
  for (auto &&w : yetmore) {
    assert(widgetpp == w.wheres_this());
  }
}