C++ named requirements: FunctionObject

 

C++

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Named requirements

Feature test macros (C++20)

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Concepts library (C++20)

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Technical specifications

Symbols index

External libraries

 

C++ named requirements

Basic
DefaultConstructible CopyAssignable CopyConstructible	MoveAssignable (C++11) MoveConstructible (C++11) Destructible
Type properties
ScalarType TriviallyCopyable (C++11) StandardLayoutType (C++11)	PODType TrivialType (C++11) ImplicitLifetimeType (C++20)
Library-Wide
EqualityComparable LessThanComparable Swappable (C++11) ValueSwappable (C++11) NullablePointer (C++11) Hash (C++11)	Allocator FunctionObject Callable Predicate BinaryPredicate Compare
Container
Container SequenceContainer ContiguousContainer (C++17) ReversibleContainer	AssociativeContainer AllocatorAwareContainer UnorderedAssociativeContainer (C++11)
Container Elements
DefaultInsertable (C++11) CopyInsertable (C++11) MoveInsertable (C++11)	EmplaceConstructible (C++11) Erasable (C++11)
Iterator
LegacyIterator LegacyOutputIterator LegacyInputIterator LegacyForwardIterator	LegacyBidirectionalIterator LegacyRandomAccessIterator LegacyContiguousIterator (C++17) ConstexprIterator (C++20)
Stream I/O
FormattedInputFunction UnformattedInputFunction	FormattedOutputFunction UnformattedOutputFunction
Formatting
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)
Other
CharTraits RegexTraits (C++11) BitmaskType NumericType LiteralType (C++11)	UnaryTypeTrait (C++11) BinaryTypeTrait (C++11) TransformationTrait (C++11) Clock (C++11) TrivialClock (C++11)

 

A FunctionObject type is the type of an object that can be used on the left of the function call operator.

Requirements

The type T satisfies FunctionObject if

• The type T satisfies std::is_object, and

Given

• f, a value of type T or const T
• args, suitable argument list, which may be empty

The following expressions must be valid:

Expression	Requirements
f(args)	performs a function call

Notes

Functions and references to functions are not function object types, but can be used where function object types are expected due to function-to-pointer implicit conversion.

Standard library

• All pointers to functions satisfy this requirement.
• All function objects defined in <functional>
• Some return types of functions of <functional>

Example

demonstrates different types of function objects

Run this code

#include <iostream>
 
void foo(int x) { std::cout << "foo(" << x << ")\n"; }
 
int main()
{
    void(*fp)(int) = foo;
    fp(1); // calls foo using the pointer to function
 
    struct S {
        void operator()(int x) const { std::cout << "S::operator(" << x << ")\n"; }
    } s;
    s(2); // calls s.operator()
    s.operator()(3); // the same
 
    auto lam = [](int x) { std::cout << "lambda(" << x << ")\n"; };
    lam(4); // calls the lambda
    lam.operator()(5); // the same
}

Output:

foo(1)
S::operator(2)
S::operator(3)
lambda(4)
lambda(5)

See also

Callable

a type for which the invoke operation is defined (named requirement)