# std::strong_order

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Defined in header `<compare>` |  |
inline namespace /* unspecified */ { inline constexpr /* unspecified */ strong_order = /* unspecified */; } |  | (since C++20)
Call signature |  |
template< class T, class U > requires /* see below */ constexpr std::strong_ordering strong_order( T&& t, U&& u ) noexcept(/* see below */); |  |

Compares two values using 3-way comparison and produces a result of type `std::strong_ordering`.

Let t and u be expressions and `T` and `U` denote decltype((t)) and decltype((u)) respectively, std::strong_order(t, u) is expression-equivalent to:

- If std::is_same_v<std::decay_t<T>, std::decay_t<U>> is true:
  - std::strong_ordering(strong_order(t, u)), if it is a well-formed expression with overload resolution performed in a context that does not include a declaration of `std::strong_order`,
  - otherwise, if `T` is a floating-point type:
    - if std::numeric_limits<T>::is_iec559 is true, performs the ISO/IEC/IEEE 60559 totalOrder comparison of floating-point values and returns that result as a value of type std::strong_ordering (note: this comparison can distinguish between the positive and negative zero and between the NaNs with different representations),
    - otherwise, yields a value of type std::strong_ordering that is consistent with the ordering observed by `T`'s comparison operators,
  - otherwise, std::strong_ordering(std::compare_three_way()(t, u)) if it is well-formed.
- In all other cases, the expression 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::strong_order` 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 `__strong_order_fn`.

All instances of `__strong_order_fn` are equal. The effects of invoking different instances of type `__strong_order_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::strong_order` 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::strong_order` above, `__strong_order_fn` models

- std::invocable<__strong_order_fn, Args...>,
- std::invocable<const __strong_order_fn, Args...>,
- std::invocable<__strong_order_fn&, Args...>, and
- std::invocable<const __strong_order_fn&, Args...>.

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

#### Strict total order of IEEE floating-point types

Let x and y be values of same IEEE floating-point type, and total_order_less(x, y) be the boolean result indicating if x precedes y in the strict total order defined by totalOrder in ISO/IEC/IEEE 60559.

(total_order_less(x, y) || total_order_less(y, x)) == false if and only if x and y have the same bit pattern.

- if neither x nor y is NaN:
  - if x < y, then total_order_less(x, y) == true;
  - if x > y, then total_order_less(x, y) == false;
  - if x == y,
    - if x is negative zero and y is positive zero, total_order_less(x, y) == true,
    - if x is not zero and x's exponent field is less than y's, then total_order_less(x, y) == (x > 0) (only meaningful for decimal floating-point number);
- if either x or y is NaN:
  - if x is negative NaN and y is not negative NaN, then total_order_less(x, y) == true,
  - if x is not positive NaN and y is positive NaN, then total_order_less(x, y) == true,
  - if both x and y are NaNs with the same sign and x's mantissa field is less than y's, then total_order_less(x, y) == !std::signbit(x).

### Example

| This section is incomplete Reason: no example

### See also

strong_ordering (C++20) | the result type of 3-way comparison that supports all 6 operators and is substitutable (class)
weak_order (C++20) | performs 3-way comparison and produces a result of type `std::weak_ordering` (customization point object)
partial_order (C++20) | performs 3-way comparison and produces a result of type `std::partial_ordering` (customization point object)
compare_strong_order_fallback (C++20) | performs 3-way comparison and produces a result of type `std::strong_ordering`, even if operator<=> is unavailable (customization point object)

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