Unifying the many ways to compare

Date: 2018-10-07
Author: David Stone (davidmstone@google.com, david@doublewise.net)
Audience: Library Evolution Working Group (LEWG)

Dependencies

• P0790: Effect of operator<=> on the C++ Standard Library
• P1190: I did not order this

Background

We currently have several facilities to compare things (mostly strings). We should unify on "the one true way". For reference, the currently existing functionality relevant to this paper (other than existing comparison operators) is:

• char_traits::eq (returns bool)
• char_traits::eq_int_type (returns bool)
• char_traits::lt (returns bool)
• char_traits::compare (returns int)
• basic_string::compare (returns int)
• basic_string_view::compare (returns int)
• sub_match::compare (returns int)
• istreambuf_iterator::equal (returns bool)
• filesystem::path::compare (returns int)
• filesystem::equivalent (returns bool, provides the weak equality of whether two paths resolve to the same file)

Note that for char_traits, we also have a "character traits" 'concept' with the same requirements as the concrete char_traits type.

Proposal

• Add char_traits::cmp, which returns std::strong_ordering for all built-in character types.
• Require all character traits classes to
  • have a static member function cmp that returns a comparison category or
  • have lt and compare (as they do now).
• Deprecate support for character traits classes that do not have static member cmp.
• Remove the requirement that character traits classes provide lt and compare.
• Deprecate char_traits::lt and char_traits::compare.
  • Note: we may also deprecate eq and eq_int_type, depending on the outcome of P1190: I did not order this.
• Deprecate basic_string::compare basic_string_view::compare, sub_match::compare, and filesystem::path::compare in favor of operator<=>.
• Deprecate istreambuf_iterator::equal in favor of operator==.
• Deprecate filesystem::equivalent (operating on filesystem::path objects) in favor of an overload of std::weak_equality?