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3673. §[locale.cons] Ambiguous argument in Throws for locale+name+category constructor

Section: 28.3.3.1.3 [locale.cons] Status: Resolved Submitter: Hubert Tong Opened: 2022-02-12 Last modified: 2023-03-22

Priority: 3

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Discussion:

locale(const locale& other, const char* std_name, category);

has

Throws: runtime_error if the argument is not valid, or is null.

There being three arguments, the statement is rather problematic. It looks like a copy/paste from

explicit locale(const char* std_name);

The conclusion, assuming that "the argument" is also std_name in the problem case, seems to be that the statement should be changed to read:

Throws: runtime_error if std_name is not valid, or is null.

However there is implementation divergence over whether or not values for the category argument not explicitly described as valid by 28.3.3.1.2.1 [locale.category] result in runtime_error.

libc++ does not throw. libstdc++ does.

Code:

#include <locale>
#include <stdio.h>
#include <exception>
int main(void) {
  std::locale Generic("C");
  try {
    std::locale Abomination(Generic, Generic, 0x7fff'ffff);
  } catch (std::runtime_error&) {
    fprintf(stderr, "Threw\n");
  }
}

Compiler Explorer link.

[2022-03-04; Reflector poll]

Set priority to 3 after reflector poll.

[2022-11-01; Jonathan comments]

The proposed resolution of 2295⁽ⁱ⁾ would resolve this too.

The implementation divergence is not a problem. 28.3.3.1.2.1 [locale.category] p2 makes invalid category values undefined, so silently ignoring them or throwing exceptions are both valid.

[2023-03-22 LWG 2295 was approved in Issaquah. Status changed: New → Resolved.]

Proposed resolution:

Preconditions: The category argument is a valid value 28.3.3.1.2.1 [locale.category].

Throws: runtime_error if the argumentstd_name is not valid, or is null.