< cpp‎ | utility
Utilities library
Language support
Type support (basic types, RTTI)
Library feature-test macros (C++20)
Dynamic memory management
Program utilities
Coroutine support (C++20)
Variadic functions
Debugging support
Three-way comparison
General utilities
Date and time
Function objects
Formatting library (C++20)
Relational operators (deprecated in C++20)
Integer comparison functions
Swap and type operations
Common vocabulary types
Elementary string conversions

Defined in header <compare>
inline namespace /* unspecified */ {

    inline constexpr /* unspecified */
        compare_strong_order_fallback = /* unspecified */;

(since C++20)
Call signature
template< class T, class U >

    requires /* see below */
constexpr std::strong_ordering

    compare_strong_order_fallback( T&& t, U&& u ) noexcept(/* see below */);
(since C++20)

Performs three-way comparison on subexpressions t and u and produces a result of type std::strong_ordering, even if the operator <=> is unavailable.

If std::decay_t<T> and std::decay_t<U> are the same type, std::compare_strong_order_fallback(t, u) is expression-equivalent to:

  • std::strong_order(t, u), if it is a well-formed expression;
  • otherwise, t == u ? std::strong_ordering::equal :
    t < u  ? std::strong_ordering::less :
if the expressions t == u and t < u are both well-formed and each of decltype(t == u) and decltype(t < u) models boolean-testable, except that t and u are evaluated only once.

In all other cases, std::compare_strong_order_fallback(t, u) is ill-formed, which can result in substitution failure when it appears in the immediate context of a template instantiation.


Customization point objects

The name std::compare_strong_order_fallback denotes a customization point object, which is a const function object of a literal semiregular class type. For exposition purposes, the cv-unqualified version of its type is denoted as __compare_strong_order_fallback_fn.

All instances of __compare_strong_order_fallback_fn are equal. The effects of invoking different instances of type __compare_strong_order_fallback_fn on the same arguments are equivalent, regardless of whether the expression denoting the instance is an lvalue or rvalue, and is const-qualified or not (however, a volatile-qualified instance is not required to be invocable). Thus, std::compare_strong_order_fallback can be copied freely and its copies can be used interchangeably.

Given a set of types Args..., if std::declval<Args>()... meet the requirements for arguments to std::compare_strong_order_fallback above, __compare_strong_order_fallback_fn models

Otherwise, no function call operator of __compare_strong_order_fallback_fn participates in overload resolution.

[edit] Example

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 2114
C++20 the fallback mechanism only required
return types to be convertible to bool
constraints strengthened

[edit] See also

performs 3-way comparison and produces a result of type std::strong_ordering
(customization point object)[edit]