C++ concepts: Callable
A Callable
type is a type, for which the INVOKE operation, as defined by std::function, std::bind, or std::thread::thread is applicable.
[edit] Requirements
The type T
satisfies Callable
if
Given
-
f
, an object of typeT
orconst T
-
ArgTypes
, suitable argument list -
R
, suitable return type
The following expressions must be valid:
Expression | Requirements |
---|---|
INVOKE(f, std::declval<ArgTypes>..., R) | the expression is well-formed in unevaluated context |
where INVOKE(f, t1, t2, ..., tn, R) 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
andt1
is an object of classT
or reference to an object of classT
or derived fromT
, then INVOKE(f, t1, t2, ..., tn) is equivalent to (t1.*f)(t2, ..., tn) - otherwise, if
f
is a pointer to member function andt1
is not one of the types described above, 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 classT
andt1
is an object of classT
or reference to an object of classT
or derived fromT
, then INVOKE(f, t1) is equivalent to t1.*f - otherwise, if N == 1 and
f
is a pointer to data member of classT
andt1
is not one of the types described above, 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 aFunctionObject
)
[edit] 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.
[edit] 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 |