Issue 2342: User conversion to wchar_t const* or to wchar

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2342. User conversion to `wchar_t const*` or to `wchar_t` not invoked for `operator<<`

Section: 31.7.6.2 [ostream] Status: New Submitter: Alf P. Steinbach Opened: 2013-10-29 Last modified: 2016-01-28

Priority: 4

View all other issues in [ostream].

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Discussion:

For wide streams argument types wchar_t const* and wchar_t are supported only as template parameters. User defined conversions are not considered for template parameter matching. Hence inappropriate overloads of operator<< are selected when an implicit conversion is required for the argument, which is inconsistent with the behavior for char const* and char, is unexpected, and is a useless result.

Demonstration:

#include <iostream>

struct Byte_string
{ 
  operator char const*() const { return "Hurray, it works!"; } 
};

struct Wide_string
{ 
  operator wchar_t const*() const { return L"Hurray, it works!"; } 
};

struct Byte_ch
{ 
  operator char() const { return 'X'; } 
};

struct Wide_ch
{ 
  operator wchar_t() const { return L'X'; } 
};

auto main() -> int
{
  using namespace std;
  wcout << "'X' as char value   : " << Byte_ch() << endl;
  wcout << "'X' as wchar_t value: " << Wide_ch() << endl;
  wcout << "Byte string pointer : " << Byte_string() << endl;
  wcout << "Wide string pointer : " << Wide_string() << endl;
}

Example output:

'X' as char value   : X
'X' as wchar_t value: 88
Byte string pointer : Hurray, it works!
Wide string pointer : 000803C8

Proposed resolution:

This wording is relative to N3797.

Modify 31.7.6.2 [ostream], class template basic_ostream synopsis, as indicated:

namespace std {
[…]

// 27.7.3.6.4 character inserters
template<class charT, class traits>
  basic_ostream<charT,traits>& operator<<(basic_ostream<charT,traits>&,
                                          charT);
template<class charT, class traits>
  basic_ostream<charT,traits>& operator<<(basic_ostream<charT,traits>&,
                                          char);
template<class traits>
  basic_ostream<char,traits>& operator<<(basic_ostream<char,traits>&,
                                         char);
template<class traits>
  basic_ostream<wchar_t,traits>& operator<<(basic_ostream<wchar_t,traits>&,
                                            wchar_t);
[…]

template<class charT, class traits>
  basic_ostream<charT,traits>& operator<<(basic_ostream<charT,traits>&,
                                          const charT*);
template<class charT, class traits>
  basic_ostream<charT,traits>& operator<<(basic_ostream<charT,traits>&,
                                          const char*);
template<class traits>
  basic_ostream<char,traits>& operator<<(basic_ostream<char,traits>&,
                                         const char*);
template<class traits>
  basic_ostream<wchar_t,traits>& operator<<(basic_ostream<wchar_t,traits>&,
                                            const wchar_t*);
[…]
}

Modify 31.7.6.3.4 [ostream.inserters.character] as indicated: [Drafting note: The replacement of os by out in p1 and the insertion of "out." in p4 just fix two obvious typos — end drafting note]

template<class charT, class traits>
  basic_ostream<charT,traits>& operator<<(basic_ostream<charT,traits>& out,
                                          charT c);
template<class charT, class traits>
  basic_ostream<charT,traits>& operator<<(basic_ostream<charT,traits>& out,
                                          char c);
// specialization
template<class traits>
  basic_ostream<char,traits>& operator<<(basic_ostream<char,traits>& out,
                                         char c);
template<class traits>
  basic_ostream<wchar_t,traits>& operator<<(basic_ostream<wchar_t,traits>& out,
                                            wchar_t c);

// signed and unsigned
template<class traits>
  basic_ostream<char,traits>& operator<<(basic_ostream<char,traits>& out,
                                          signed char c);
template<class traits>
  basic_ostream<char,traits>& operator<<(basic_ostream<char,traits>& out,
                                          unsigned char c);
-1- Effects: Behaves as a formatted output function (31.7.6.3.1 [ostream.formatted.reqmts]) of out. Constructs a character sequence seq. If c has type char and the character type of the stream is not char, then seq consists of out.widen(c); otherwise seq consists of c. Determines padding for seq as described in 31.7.6.3.1 [ostream.formatted.reqmts]. Inserts seq into out. Calls osout.width(0).
-2- Returns: out.
template<class charT, class traits>
  basic_ostream<charT,traits>& operator<<(basic_ostream<charT,traits>& out,
                                          const charT* s);
template<class charT, class traits>
  basic_ostream<charT,traits>& operator<<(basic_ostream<charT,traits>& out,
                                          const char* s);
template<class traits>
  basic_ostream<char,traits>& operator<<(basic_ostream<char,traits>& out,
                                         const char* s);
template<class traits>
  basic_ostream<wchar_t,traits>& operator<<(basic_ostream<wchar_t,traits>& out,
                                            const wchar_t* s);
											
template<class traits>
  basic_ostream<char,traits>& operator<<(basic_ostream<char,traits>& out,
                                         const signed char* s);
template<class traits>
  basic_ostream<char,traits>& operator<<(basic_ostream<char,traits>& out,
                                         const unsigned char* s);
-3- Requires: s shall not be a null pointer.
-4- Effects: Behaves like a formatted inserter (as described in 31.7.6.3.1 [ostream.formatted.reqmts]) of out. Creates a character sequence seq of n characters starting at s, each widened using out.widen() (27.5.5.3), where n is the number that would be computed as if by:

traits::length(s) for the following overloads:

where the first argument is of type basic_ostream<charT, traits>& and the second is of type const charT*,

~~and also for the overload~~ where the first argument is of type basic_ostream<char, traits>& and the second is of type const char*,

where the first argument is of type basic_ostream<wchar_t, traits>& and the second is of type const wchar_t*,

std::char_traits<char>::length(s) for the overload where the first argument is of type basic_ostream<charT, traits>& and the second is of type const char*,

traits::length(reinterpret_cast<const char*>(s)) for the other two overloads.

Determines padding for seq as described in 31.7.6.3.1 [ostream.formatted.reqmts]. Inserts seq into out. Calls out.width(0).
-5- Returns: out.

2342. User conversion to wchar_t const* or to wchar_t not invoked for operator<<

2342. User conversion to `wchar_t const*` or to `wchar_t` not invoked for `operator<<`