This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 116a. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2024-12-19

391. Require direct binding of short-lived references to rvalues

[Voted into WP at October 2005 meeting.]

After some email exchanges with Rani Sharoni, I've come up with the following proposal to allow reference binding to non-copyable rvalues in some cases. Rationale and some background appear afterwards.

---- proposal ----

Replace the section of 9.4.4 [dcl.init.ref] paragraph 5 that begins "If the initializer expression is an rvalue" with the following:

• If the initializer expression is an rvalue, with T2 a class type, and ``cv1 T1'' is reference-compatible with ``cv2 T2,'' the reference is bound as follows:
  • If the lifetime of the reference does not extend beyond the end of the full expression containing the initializer expression, the reference is bound to the object represented by the rvalue (see 7.2.1 [basic.lval]) or to a sub-object within that object.
  • otherwise, the reference is bound in one of the following ways (the choice is implementation-defined):
    • [... continues as before - the original wording applies unchanged to longer-lived references]

---- rationale ----

1. The intention of the current wording is to provide the implementation freedom to construct an rvalue of class type at an arbitrary location and copy it zero or more times before binding any reference to it.
2. The standard allows code to call a member function on an rvalue of class type (in 7.6.1.5 [expr.ref], I guess). This means that the implementation can be forced to bind the reference directly, with no freedom to create any temporary copies. e.g.

      class nc {
        nc (nc const &);  // private, nowhere defined
      public:
        nc ();
        nc const &by_ref () const { return *this; }
      };
   
      void f () {
        void g (nc const &);
   
        g (nc());          // Ill-formed
        g (nc().by_ref()); // Ok - binds directly to rvalue
      }
   

   Forcing a direct binding in this way is possible wherever the lifetime of the reference does not extend beyond the containing full expression, since the reference returned by the member function remains valid for this long.
3. As demonstrated above, existing implementations must already be capable of constructing an rvalue of class type in the "right" place the first time. Some compilers already silently allow the direct binding of references to non-copyable rvalues.
4. The change will not break any portable user code. It would break any platform-specific user code that relies on copies being performed by the particular implementation.

---- background ----

The proposal is based on a recent discussion in this group. I originally wanted to leave the implementation free to copy the rvalue if there was a callable copy constructor, and only have to bind directly if none was callable. Unfortunately, a traditional compiler can't always tell whether a function is callable or not, e.g. if the copy constructor is declared but not defined. Rani pointed this out in an example, and suggested that maybe trivial copy constructors should still be allowed (by extension, maybe wherever the compiler can determine callability). I've gone with this version because it's simpler, and I also figure the "as if" rule gives the compiler some freedom with POD types anyway.

Notes from April 2003 meeting:

We agreed generally with the proposal. We were unsure about the need for the restriction regarding long-lived references. We will check with the proposer about that.

Jason Merrill points out that the test case in issue 86 may be a case where we do not want to require direct binding.

Further information from Rani Sharoni (April 2003):

I wasn't aware about the latest suggestion of Raoul as it appears in core issue 391. In our discussions we tried to formulate a different proposal.

The rational, as we understood, behind the implementation freedom to make an extra copying (8.5.3/5/2/12) of the rvalue is to allow return values in registers which on some architectures are not addressable. The example that Raoul and I presented shows that this implementation freedom is not always possible since we can "force" the rvalue to be addressable using additional member function (by_ref). The example only works for short lived rvalues and this is probably why Raoul narrow the suggestion.

I had different rational which was related to the implementation of conditional operator in VC. It seems that when conditional operator is involved VC does use an extra copying when the lifetime of the temporary is extended:

  struct A { /* ctor with side effect */};

  void f(A& x) {
    A const& r = cond ? A(1) : x; // VC actually make an extra copy of
                                  // the rvalue A(1)
  }

I don't know what the consideration behind the VC implementation was (I saw open bug on this issue) but it convinced me to narrow the suggestion.

IMHO such limitation seems to be too strict because it might limit the optimizer since returning class rvalues in registers might be useful (although I'm not aware about any implementation that actually does it). My suggestion was to forbid the extra copying if the ctor is not viable (e.g. A::A(A&) ). In this case the implementation "freedom" doesn't exist (since the code might not compile) and only limits the programmer freedom (e.g. Move Constructors - http://www.cuj.com/experts/2102/alexandr.htm [Note: URL is now defunct; observed March,2019.]).

Core issue 291 is strongly related to the above issue and I personally prefer to see it resolved first. It seems that VC already supports the resolution I prefer.

Notes from October 2003 meeting:

We ended up feeling that this is just one of a number of cases of optimizations that are widely done by compilers and allowed but not required by the standard. We don't see any strong reason to require compilers to do this particular optimization.

Notes from the March 2004 meeting:

After discussing issue 450, we found ourselves reconsidering this, and we are now inclined to make a change to require the direct binding in all cases, with no restriction on long-lived references. Note that such a change would eliminate the need for a change for issue 291.

Proposed resolution (October, 2004):

Change 9.4.4 [dcl.init.ref] bullet 5.2 sub-bullet 1 as follows:

If the initializer expression is an rvalue, with T2 a class type, and "cv1 T1" is reference-compatible with "cv2 T2", the reference is bound to the object represented by the rvalue (see 7.2.1 [basic.lval]) or to a sub-object within that object. in one of the following ways (the choice is implementation-defined):

• The reference is bound to the object represented by the rvalue (see 7.2.1 [basic.lval]) or to a sub-object within that object.
• A temporary of type "cv1 T2" [sic] is created, and a constructor is called to copy the entire rvalue object into the temporary. The reference is bound to the temporary or to a sub-object within the temporary.

The constructor that would be used to make the copy shall be callable whether or not the copy is actually done. [Example:

  struct A { };
  struct B : public A { } b;
  extern B f();
  const A& rca = f ();  // Bound Either bound to the A sub-object of the B rvalue,
                        // or the entire B object is copied and the reference
                        // is bound to the A sub-object of the copy

—end example]

[This resolution also resolves issue 291.]