operator<=> on the C++ Standard Library
Document Number: P0790R1 Date: 2018-08-06 Author: David Stone (firstname.lastname@example.org, email@example.com) Audience: LEWG, LWG
This paper lists (what are expected to be) non-controversial changes to the C++ standard library in response to P0515, which adds
operator<=> to the language. This is expected to be non-controversial because it tries to match existing behavior as much as possible. As a result, all proposed additions are either
strong_ordering, matching the existing comparison operators.
This document should contain a complete list of types or categories of types in C++.
R1: A much broader version of this paper was presented to LEWG at a previous meeting. What remains in this paper is everything which the group did not find controversial and which probably does not require significant justification. All controversial aspects will be submitted in separate papers.
operator<=> proposal was written such that the "generated" operators are equivalent to source code rewrites – there is no actual
operator== that a user could take the address of. Users are not allowed to form pointers to standard library member functions and are unable to form pointers to friend functions defined inline in the class. There are some cases where we do not specify how the operator was implemented, only that the expression
a @ b is valid; these cases are not broken by such a change because users could not have depended on it, anyway. In general, we accept changes that overload existing functions, which also has the effect of breaking code which takes the address of a free function.
Types that are not proposed to get
operator<=> in this paper
These types are not comparable now. This paper does not propose adding any new comparisons to any of these types.
- deprecated types
- exception types
- tag classes (
- arithmetic function objects (
- comparison function objects (
- logical function objects (
- bitwise function objects (
seed_seq(paper needed to add
enable_shared_from_this: It would be nice to give it a
strong_orderingto allow derived classes to
= default. However, this means that all classes that do not explicitly delete their comparison operator get an
operator<=>that compares only the
enable_shared_from_thisbase class, which is almost certainly wrong. Since this is intended to be used as a base class, we should not add
operator<=>to it. Moreover, classes which
enable_shared_from_thisare unlikely to be basic value classes so they do not lose much by not being able to default.
initializer_listis a reference type. It would be strange to give it reference semantics on copy but value semantics for comparison. It would also be surprising if two
initializer_listcontaining the same set of values compared as not equal. Therefore, I recommend not defining it for this type.
Types from C that are not proposed to get
operator<=> in this paper
Types that should get
operator<=>, no change from current comparisons
These types are all currently comparable.
strong_equality(paper would be needed to change this to
synchronized_pool_resource: (implicitly from
unsynchronized_pool_resource: (implicitly from
monotonic_buffer_resource: (implicitly from
nullptr_tonly (no homogenous operator)
strong_ordering, heterogeneous with durations of other representations and periods
strong_ordering, heterogeneous in the duration
Types that will get their
operator<=> from a conversion operator
These types will get
operator<=> if possible without any changes, just like they already have whatever comparison operators their underlying type has.
integral_constantand all types deriving from
operator T &)
This has the disadvantage that types which have a template comparison operator will not have their wrapper convertible. For instance,
std::reference_wrapper<std::string> is not currently comparable. This does not affect
bitset::reference, as it has a fixed conversion to
bool, but it does affect the other three.
Types that wrap another type
This turned out to be much more complicated than expected and will require its own paper.
operator<=> with these types requires more thought than this paper has room for, and thus will be discussed separately.
They contain state that is not observed in the comparison operators. Therefore, they will get their own paper.
Current comparison operators return a
valarray<bool>, giving you the result for each pair (with undefined behavior for differently-sized
valarray arguments). It might make sense to provide some sort of function that returns
valarray<comparison_category>, but that should not be named
operator<=>. This paper does not suggest adding
Types that have no comparisons now but are being proposed to get
operator<=> in another paper
This paper does not propose changing any of the following types -- they are here only for completeness.
strong_equality in the working draft. I will be writing a separate paper proposing
Not Updating Concepts That Provide Comparisons
This category includes things like
Compare. This is addressed in a separate paper.
Not Updating Concepts That Require Comparisons
This includes things like
EqualityComparable. This is addressed in a separate paper.
operator<=> should be
noexcept where possible, following the lead of the language feature and allowing
= default as an implementation strategy for some types.
When we list a result type as "unspecified" it is unspecified whether it has
operator<=>. There are not any unspecified result types for which we currently guarantee any comparison operators are present, so there is no extra work to do here.