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3162. system_error::system_error(error_code ec) not explicit

Section: 19.5.8.2 [syserr.syserr.members] Status: NAD Submitter: Peter Dimov Opened: 2018-10-02 Last modified: 2026-06-10

Priority: 3

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Discussion:

The constructor for std::system_error taking a single argument of type std::error_code is not marked explicit, which allows implicit conversions from error_code to system_error. I think that this is an oversight and not intentional, and that we should make this constructor explicit.

[2020-04-07 Issue Prioritization]

Priority to 3 after reflector discussion.

[Brno 2026-06-10; Status changed: New → NAD.]

Too late. Submitter suggested NAD in 2023 and LWG agreed, but failed to close it at the time.

Proposed resolution:

This wording is relative to N4762.

1. Change 19.5.8.1 [syserr.syserr.overview] p2, class system_error synopsis, as indicated

   namespace std {
     class system_error : public runtime_error {
     public:
       system_error(error_code ec, const string& what_arg);
       system_error(error_code ec, const char* what_arg);
       explicit system_error(error_code ec);
       system_error(int ev, const error_category& ecat, const string& what_arg);
       system_error(int ev, const error_category& ecat, const char* what_arg);
       system_error(int ev, const error_category& ecat);
       const error_code& code() const noexcept;
       const char* what() const noexcept override;
     };
   }
   

2. Change 19.5.8.2 [syserr.syserr.members] as indicated

   explicit system_error(error_code ec);
   

   -5- Effects: […]

   -6- Ensures: […]