2221. No formatted output operator for nullptr

Section: 30.7.5 [output.streams] Status: C++17 Submitter: Matt Austern Opened: 2012-12-07 Last modified: 2017-07-30

Priority: 3

View all issues with C++17 status.

Discussion:

When I write

std::cout << nullptr << std::endl;

I get a compilation error, "ambiguous overload for 'operator<<' in 'std::cout << nullptr'". As far as I can tell, the compiler is right to issue that error. There are inserters for const void*, const char*, const signed char*, and const unsigned char*, and none for nullptr_t, so the expression really is ambiguous.

Proposed wording:

The obvious library solution is to add a nullptr_t overload, which would be defined something like

template<class C, class T>
basic_ostream<C, T>& operator<<(basic_ostream<C, T>& os, nullptr_t) 
{ 
  return os << (void*) nullptr; 
}

We might also consider addressing this at a core level: add a special-case language rule that addresses all cases where you write f(nullptr) and f is overloaded on multiple pointer types. (Perhaps a tiebreaker saying that void* is preferred in such cases.)

[2016-01-18, comments from Mike and Ville collected by Walter Brown]

Mike Miller: "Changing overload resolution sounds like something that should be considered by EWG before CWG […]"

Ville: "Agreed, such a change would be Evolutionary. Personally, I think it would also be wrong, because I don't see how void* is the right choice to prefer in the case of code that is currently ambiguous. Sure, it would solve this particular library issue, but it seemingly has wider repercussions. If LWG really wants to, EWG can certainly discuss this issue, but I would recommend solving it on the LWG side (which doesn't mean that the standard necessarily needs to change, I wouldn't call it far-fetched to NAD it)."

[2016-08 Chicago]

Zhihao recommends NAD:

nullptr is printable if being treated as void*, but causes UB if being treated as char cv*. Capturing this ambigurity at compile time and avoid a runtime UB is a good thing.

[2016-08 Chicago]

Tues PM: General agreement on providing the overload; discussion on what it should say.

Polls:
Matt's suggestion (in the issue): 2/0/6/2/2/
Unspecified output: 3/2/5/0/1
Specified output: 1/1/6/3/0

Move to Open

[2016-08 Chicago]

The group consensus is that we only output nullptr because it is of a fundamental type, causing problems in functions doing forwarding, and we don't want to read it back.

Fri PM: Move to Tentatively Ready

Proposed resolution:

This wording is relative to N4606

  1. Insert the signature into 30.7.5.1 [ostream], class template basic_ostream synopsis, as follows:

    [Drafting notes: Why member? Don't want to define a new category of inserters just for this.]

    namespace std {
      template <class charT, class traits = char_traits<charT> >
      class basic_ostream
        : virtual public basic_ios<charT, traits> {
      public:
        […]
        basic_ostream<charT, traits>& operator<<(const void* p);
        basic_ostream<charT, traits>& operator<<(nullptr_t);
        basic_ostream<charT, traits>& operator<<(
          basic_streambuf<char_type, traits>* sb);
        […]
      };
    
  2. Append the following new paragraphs to 30.7.5.2.3 [ostream.inserters]:

    basic_ostream<charT, traits>& operator<<
      (basic_streambuf<charT, traits>* sb);
    

    […]

    -10- Returns: *this.

    basic_ostream<charT, traits>& operator<<(nullptr_t);
    

    -??- Effects: Equivalent to return *this << s; where s is an implementation-defined NTCTS.