C++ named requirements: Callable

 

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C++ named requirements

Basic DefaultConstructible MoveConstructible (C++11) CopyConstructible CopyAssignable MoveAssignable (C++11) Destructible Type properties ScalarType PODType TriviallyCopyable (C++11) TrivialType (C++11) StandardLayoutType (C++11)   ImplicitLifetimeType Library-wide BooleanTestable EqualityComparable LessThanComparable Swappable ValueSwappable (C++11) NullablePointer (C++11) Hash (C++11) Allocator FunctionObject Callable Predicate BinaryPredicate Compare

Container Container ReversibleContainer AllocatorAwareContainer SequenceContainer ContiguousContainer (C++17) AssociativeContainer UnorderedAssociativeContainer (C++11)   Container element DefaultInsertable (C++11) CopyInsertable (C++11) MoveInsertable (C++11) EmplaceConstructible (C++11) Erasable (C++11) Iterator LegacyIterator LegacyInputIterator LegacyOutputIterator LegacyForwardIterator LegacyBidirectionalIterator LegacyRandomAccessIterator LegacyContiguousIterator (C++17) ConstexprIterator (C++20) Stream I/O FormattedInputFunction UnformattedInputFunction FormattedOutputFunction UnformattedOutputFunction Formatters BasicFormatter (C++20) Formatter (C++20)

Random Numbers SeedSequence (C++11) RandomNumberEngine (C++11) RandomNumberDistribution (C++11) UniformRandomBitGenerator (C++11) RandomNumberEngineAdaptor (C++11) Concurrency BasicLockable (C++11) Lockable (C++11) TimedLockable (C++11) SharedLockable (C++14) SharedTimedLockable (C++14) Mutex (C++11) TimedMutex (C++11) SharedMutex (C++17) SharedTimedMutex (C++14) Ranges RangeAdaptorObject (C++20) RangeAdaptorClosureObject (C++20) Multidimensional View LayoutMapping (C++23) LayoutMappingPolicy (C++23) AccessorPolicy (C++23) Other CharTraits RegexTraits (C++11)   BitmaskType LiteralType (C++11) NumericType UnaryTypeTrait (C++11) BinaryTypeTrait (C++11) TransformationTrait (C++11) Clock (C++11) TrivialClock (C++11)

 

A Callable type is a type for which the INVOKE and INVOKE<R> operations (used by, e.g., std::function, std::bind, and std::thread::thread) are applicable.

INVOKE can be performed explicitly using the library function std::invoke.	(since C++17)
INVOKE<R> can be performed explicitly using the library function std::invoke_r.	(since C++23)

Requirements

The type T satisfies Callable if

Given

• f, an object of type T,
• ArgTypes, suitable list of argument types,
• R, suitable return type.

The following expressions must be valid:

Expression	Requirements
INVOKE<R>(f, std::declval<ArgTypes>()...)	The expression is well-formed in unevaluated context.

Notes

Pointers to data members are Callable, even though no function calls take place.

Standard library

In addition, the following standard library facilities accept any Callable type (not just FunctionObject):

function (C++11)	copyable wrapper of any copy constructible callable object (class template)
move_only_function (C++23)	move-only wrapper of any callable object that supports qualifiers in a given call signature (class template)
copyable_function (C++26)	copyable wrapper of any copy constructible callable object that supports qualifiers in a given call signature (class template)
function_ref (C++26)	non-owning wrapper of any callable object (class template)
bind (C++11)	binds one or more arguments to a function object (function template)
bind_frontbind_back (C++20)(C++23)	bind a variable number of arguments, in order, to a function object (function template)
reference_wrapper (C++11)	CopyConstructible and CopyAssignable reference wrapper (class template)
result_ofinvoke_result (C++11)(removed in C++20)(C++17)	deduces the result type of invoking a callable object with a set of arguments (class template)
thread (C++11)	manages a separate thread (class)
jthread (C++20)	std::thread with support for auto-joining and cancellation (class)
call_once (C++11)	invokes a function only once even if called from multiple threads (function template)
async (C++11)	runs a function asynchronously (potentially in a new thread) and returns a std::future that will hold the result (function template)
packaged_task (C++11)	packages a function to store its return value for asynchronous retrieval (class template)

See also

is_invocableis_invocable_ris_nothrow_invocableis_nothrow_invocable_r (C++17)	checks if a type can be invoked (as if by std::invoke) with the given argument types (class template)
invocableregular_invocable (C++20)	specifies that a callable type can be invoked with a given set of argument types (concept)
invokeinvoke_r (C++17)(C++23)	invokes any Callable object with given arguments and possibility to specify return type(since C++23) (function template)