2686. Why is std::hash specialized for error_code, but not error_condition?

Section: 19.5.2 [system.error.syn] Status: C++17 Submitter: Tim Song Opened: 2016-05-03 Last modified: 2021-06-06

Priority: 3

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Discussion:

Both error_code and error_condition have an operator< overload, enabling their use in associative containers without having to write a custom comparator.

However, only error_code has a std::hash specialization. So it's possible to have a set<error_code>, a set<error_condition>, an unordered_set<error_code>, but not an unordered_set<error_condition>. This seems...odd.

[2016-08 - Chicago]

Thurs AM: Moved to Tentatively Ready

Proposed resolution:

This wording is relative to N4582.

1. Edit [system_error.syn], header <system_error> synopsis, as indicated:

   namespace std {
       // ...
   
       // 19.5.6 Hash support
       template<class T> struct hash;
       template<> struct hash<error_code>;
       template<> struct hash<error_condition>;
   
      // ...
   }
   

2. Edit 19.5.7 [syserr.hash] as indicated:

   template <> struct hash<error_code>;
   template <> struct hash<error_condition>;
   

   -1- The template specializations shall meet the requirements of class template hash (20.12.14).