Add nullptr output operator overload (2221)

[zoecarver]

Summary

This patch fixes LWG issue 2221 by adding a nullptr overload to basic_ostream.

[EricWF]

include/ostream

219

We externally instantiate the rest of these methods, but we can't do that here (for backwards compat reasons).

You need to add _LIBCPP_INLINE_VISIBILITY and maybe put it at the bottom of the group because it's unique in this way.

714

inline or just define the thing inside the class.

717

static_cast<const void*>(nullptr);

But I'm not sure we want to forward to the const void* overload. My understanding of that overload is that it exists to print "bool" values for types like ostream that used to implement if (stream) via a conversion to void*.

I think we may just want to print nill.

test/std/input.output/iostream.format/output.streams/ostream.formatted/ostream.inserters/streambuf.pass.cpp

74

I don't think this test is portable. Implementations can print null or nill or "hubbabalo" for that matter.
We should probably just make this a LIBCPP_ASSERT. And maybe we should also assert the string isn't empty (for when other STL's use our test suite).

[mclow.lists]

include/ostream

717

I suggested this approach to Zoe, because I thought that that was what GCC did. (print 0x0)
Turns out that I was wrong; GCC prints nullptr (at least that's what wandbox claims about gcc 9).

test/std/input.output/iostream.format/output.streams/ostream.formatted/ostream.inserters/streambuf.pass.cpp

74

Agreed.

[zoecarver]

include/ostream

219

Thanks, will do.

714

I'll do the later.

717

Alright. I like printing nullptr better anyway, so I will do that if it's OK with both of you.

test/std/input.output/iostream.format/output.streams/ostream.formatted/ostream.inserters/streambuf.pass.cpp

74

Will do.

[zoecarver]

• fix inlining
• fix visibility
• fix "nullptr" output value
• update tests

[mclow.lists]

Once you fix the nits, I think this is good to go.
Be sure to update www/cxx1z_status.html as well.

include/ostream

224

Don't need the \0; the compiler does that for you.
Nor the static cast; the type of a string literal is const char * already.
Just say:

{ return *this << "nullptr"; }

[zoecarver]

• update status
• fix operator implementation

include/ostream

224

🤦‍♂️but I liked the static_cast so much \s.

[mclow.lists]

Committed as revision 364802 (with a minor change; I updated the synopsis)

include/ostream

public:
     basic_ostream& operator<<(long __n);
     basic_ostream& operator<<(unsigned long __n);
     basic_ostream& operator<<(long long __n);
     basic_ostream& operator<<(unsigned long long __n);
     basic_ostream& operator<<(float __f);
     basic_ostream& operator<<(double __f);
     basic_ostream& operator<<(long double __f);
     basic_ostream& operator<<(const void* __p);
+    basic_ostream& operator<<(nullptr_t);
     basic_ostream& operator<<(basic_streambuf<char_type, traits_type>* __sb);
 
     // 27.7.2.7 Unformatted output:
     basic_ostream& put(char_type __c);
     basic_ostream& write(const char_type* __s, streamsize __n);
     basic_ostream& flush();
 
     // 27.7.2.5 seeks:
     inline _LIBCPP_HIDE_FROM_ABI_AFTER_V1
     pos_type tellp();
     inline _LIBCPP_HIDE_FROM_ABI_AFTER_V1
     basic_ostream& seekp(pos_type __pos);
     inline _LIBCPP_HIDE_FROM_ABI_AFTER_V1
#ifndef _LIBCPP_NO_EXCEPTIONS
     {
         this->__set_badbit_and_consider_rethrow();
     }
 #endif  // _LIBCPP_NO_EXCEPTIONS
     return *this;
 }
 
 template<class _CharT, class _Traits>
 basic_ostream<_CharT, _Traits>&
+basic_ostream<_CharT, _Traits>::operator<<(nullptr_t)
+{
+    return *this << static_cast<const void*>(0x0);
+}
+
+template<class _CharT, class _Traits>
+basic_ostream<_CharT, _Traits>&
 __put_character_sequence(basic_ostream<_CharT, _Traits>& __os,
                           const _CharT* __str, size_t __len)
 {
 #ifndef _LIBCPP_NO_EXCEPTIONS
     try
     {
 #endif  // _LIBCPP_NO_EXCEPTIONS
         typename basic_ostream<_CharT, _Traits>::sentry __s(__os);

test/std/input.output/iostream.format/output.streams/ostream.formatted/ostream.inserters/streambuf.pass.cpp

int main(int, char**)
     {
         testbuf<char> sb;
         std::ostream os(&sb);
         testbuf<char> sb2("testing...");
         assert(sb.str() == "");
         os << &sb2;
         assert(sb.str() == "testing...");
     }
+    { // LWG 2221 - nullptr
+        testbuf<char> sb;
+        std::ostream os(&sb);
+        os << nullptr;
+        assert(sb.str().c_str()[0] == '0');
+    }
 
   return 0;
 }