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3266. to_chars(bool) should be deleted

Section: 28.2.1 [charconv.syn] Status: C++20 Submitter: Jens Maurer Opened: 2019-08-23 Last modified: 2021-02-25

Priority: 0

View all other issues in [charconv.syn].

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Discussion:

28.2.2 [charconv.to.chars] does not present an overload for bool (because it is neither a signed nor unsigned integer type), so an attempt to call to_chars with a bool argument would promote it to int and unambiguously call the int overload of to_chars.

This was not intended, since it is not obvious that the correct textual representation of a bool is 0/1 (as opposed to, say, "true"/"false").

The user should cast explicitly if he wants the 0/1 behavior. (Correspondingly, there is no bool overload for from_chars in the status quo, and conversions do not apply there because of the reference parameter.)

[2019-09-14 Issue Prioritization]

Status to Tentatively Ready and priority to 0 after eight positive votes on the reflector.

Proposed resolution:

This wording is relative to N4830.

1. Modify 28.2.1 [charconv.syn], header <charconv> synopsis, as indicated:

   […]
   // 28.2.2 [charconv.to.chars], primitive numerical output conversion
   struct to_chars_result {
     char* ptr;
     errc ec;
     friend bool operator==(const to_chars_result&, const to_chars_result&) = default;
   };
   
   to_chars_result to_chars(char* first, char* last, see below value, int base = 10);
   to_chars_result to_chars(char* first, char* last, bool value, int base = 10) = delete;
   to_chars_result to_chars(char* first, char* last, float value);
   […]