Callable
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 typeT
ArgTypes
, suitable list of argument typesR
, 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 (since C++17) INVOKE(f, t1, t2, ..., tN), implicitly converted to R
.
where INVOKE(f, t1, t2, ..., tN) is defined as follows:
- if
f
is a pointer to member function of classT
:- If
std::is_base_of<T,
std::decay_t<decltype(t1)>>::value
istrue
, thenINVOKE(f, t1, t2, ..., tN)
is equivalent to(t1.*f)(t2, ..., tN)
- otherwise, if
std::decay_t<decltype(t1)>
is a specialization ofstd::reference_wrapper
, thenINVOKE(f, t1, t2, ..., tN)
is equivalent to(t1.get().*f)(t2, ..., tN)
(since C++17) - otherwise, if
t1
does not satisfy the previous items, thenINVOKE(f, t1, t2, ..., tN)
is equivalent to((*t1).*f)(t2, ..., tN)
.
- If
- otherwise, if N == 1 and
f
is a pointer to data member of classT
:- If
std::is_base_of<T,
std::decay_t<decltype(t1)>>::value
istrue
, thenINVOKE(f, t1)
is equivalent tot1.*f
- otherwise, if
std::decay_t<decltype(t1)>
is a specialization ofstd::reference_wrapper
, thenINVOKE(f, t1)
is equivalent tot1.get().*f
(since C++17) - otherwise, if
t1
does not satisfy the previous items, thenINVOKE(f, t1)
is equivalent to(*t1).*f
- If
- otherwise,
INVOKE(f, t1, t2, ..., tN)
is equivalent tof(t1, t2, ..., tN)
(that is,f
is aFunctionObject
)
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
).
| std::function |
|:----|
| std::bind |
| std::result_of |
| std::thread::thread |
| std::call_once |
| std::async |
| std::packaged_task |
| std::reference_wrapper |
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) |
---|
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