# std::partial_order

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

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

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

- If std::is_same_v<std::decay_t<T>, std::decay_t<U>> is true:
  - std::partial_ordering(partial_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::partial_order`,
  - otherwise, std::partial_ordering(std::compare_three_way()(t, u)), if it is well-formed,
  - otherwise, std::partial_ordering(std::weak_order(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::partial_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 `__partial_order_fn`.

All instances of `__partial_order_fn` are equal. The effects of invoking different instances of type `__partial_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::partial_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::partial_order` above, `__partial_order_fn` models

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

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

### Example

| This section is incomplete Reason: no example

### See also

partial_ordering (C++20) | the result type of 3-way comparison that supports all 6 operators, is not substitutable, and allows incomparable values (class)
strong_order (C++20) | performs 3-way comparison and produces a result of type `std::strong_ordering` (customization point object)
weak_order (C++20) | performs 3-way comparison and produces a result of type `std::weak_ordering` (customization point object)
compare_partial_order_fallback (C++20) | performs 3-way comparison and produces a result of type `std::partial_ordering`, even if operator<=> is unavailable (customization point object)

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