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 operation (used by, e.g., std::function, std::bind, and std::thread::thread) is applicable. This operation may be performed explicitly using the library function std::invoke.(since C++17)

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

where INVOKE<R>(f, t1, t2, ..., tN) is defined as static_cast<void>(INVOKE(f, t1, t2, ..., tN)) if R is possibly cv-qualified void, otherwise, INVOKE(f, t1, t2, ..., tN) is implicitly converted to R

where INVOKE(f, t1, t2, ..., tN) is defined as follows:

• if f is a pointer to member function of class T:

• If std::is_base_of<T, std::remove_reference_t<decltype(t1)>>::value is true, then INVOKE(f, t1, t2, ..., tN) is equivalent to (t1.*f)(t2, ..., tN)
• otherwise, if std::remove_cvref_t<decltype(t1)> is a specialization of std::reference_wrapper, then INVOKE(f, t1, t2, ..., tN) is equivalent to (t1.get().*f)(t2, ..., tN)(since C++17)
• otherwise, if t1 does not satisfy the previous items, then INVOKE(f, t1, t2, ..., tN) is equivalent to ((*t1).*f)(t2, ..., tN).

• otherwise, if N == 1 and f is a pointer to data member of class T:

• If std::is_base_of<T, std::remove_reference_t<decltype(t1)>>::value is true, then INVOKE(f, t1) is equivalent to t1.*f
• otherwise, if std::remove_cvref_t<decltype(t1)> is a specialization of std::reference_wrapper, then INVOKE(f, t1) is equivalent to t1.get().*f(since C++17)
• otherwise, if t1 does not satisfy the previous items, then INVOKE(f, t1) is equivalent to (*t1).*f

• otherwise, INVOKE(f, t1, t2, ..., tN) is equivalent to f(t1, t2, ..., tN) (that is, f is a FunctionObject)

Notes

For pointers to member functions and pointers to data members, t1 may be a regular pointer or an object of class type that overloads operator*, such as std::unique_ptr or std::shared_ptr.

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)
bind (C++11)	binds one or more arguments 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)
(constructor)	constructs new thread object (public member function of std::thread)
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)

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 2420	C++11	impossible implicit conversion from the result to void was required when R is void	the result is explicitly converted to void when R is cv void

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)