# std::strong_order

< cpp‎ | utility

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 Defined in header  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>> == 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(t <=> u) if it is well-formed,
• other the expression is ill-formed, which can result in substitution failure when it appears in the immediate context of a template instantiation.
• Otherwise, std::strong_order(t, u) is ill-formed.

## Contents

### Expression-equivalent

Expression e is expression-equivalent to expression f, if e and f have the same effects, either are both potentially-throwing or are both not potentially-throwing (i.e. noexcept(e) == noexcept(f)), and either are both constant subexpressions or are both not constant subexpressions.

###  Customization point objects

The name std::strong_order denotes a customization point object, which is a function object of a literal semiregular class type (denoted, for exposition purposes, as strong_order_ftor). All instances of strong_order_ftor are equal. 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_ftor will satisfy std::invocable<const strong_order_ftor&, Args...>. Otherwise, no function call operator of strong_order_ftor participates in overload resolution.

### Notes

#### 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,
• 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

 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)  performs 3-way comparison and produces a result of type std::strong_ordering, even if operator<=> is unavailable (customization point object)