std::predicate

 

C++

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Freestanding and hosted

Language

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

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Iterators library

Ranges library (C++20)

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Numerics library

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Filesystem library (C++17)

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Technical specifications

Symbols index

External libraries

 

Concepts library

Core language concepts same_as derived_from convertible_to common_reference_with common_with integral signed_integral unsigned_integral floating_point swappableswappable_with destructible constructible_from default_initializable move_constructible copy_constructible assignable_from

Comparison concepts equality_comparableequality_comparable_with totally_orderedtotally_ordered_with Object concepts movable copyable semiregular regular Callable concepts invocableregular_invocable predicate relation equivalence_relation strict_weak_order

 

Defined in header <concepts>
template < class F, class... Args > concept predicate =   std::regular_invocable<F, Args...> &&   boolean-testable<std::invoke_result_t<F, Args...>>;		(since C++20)

The concept predicate<F, Args...> specifies that F is a predicate that accepts arguments whose types and value categories are encoded by Args..., i.e., it can be invoked with these arguments to produce a boolean-testable result.

Note that regular_invocable requires the invocation to not modify either the callable object or the arguments and be equality-preserving.

Equality preservation

An expression is equality preserving if it results in equal outputs given equal inputs.

• The inputs to an expression consist of its operands.
• The outputs of an expression consist of its result and all operands modified by the expression (if any).

In specification of standard concepts, operands are defined as the largest subexpressions that include only:

• an id-expression, and
• invocations of std::move, std::forward, and std::declval.

The cv-qualification and value category of each operand is determined by assuming that each template type parameter denotes a cv-unqualified complete non-array object type.

Every expression required to be equality preserving is further required to be stable: two evaluations of such an expression with the same input objects must have equal outputs absent any explicit intervening modification of those input objects.