22 General utilities library [utilities]

22.10 Function objects [function.objects]

22.10.17 Polymorphic function wrappers [func.wrap]

22.10.17.1 General [func.wrap.general]

Subclause [func.wrap] describes polymorphic wrapper classes that encapsulate arbitrary callable objects.
Let t be an object of a type that is a specialization of function, copyable_function, or move_only_function, such that the target object x of t has a type that is a specialization of function, copyable_function, or move_only_function.
Each argument of the invocation of x evaluated as part of the invocation of t may alias an argument in the same position in the invocation of t that has the same type, even if the corresponding parameter is not of reference type.
[Example 1: move_only_function<void(T)> f{copyable_function<void(T)>{[](T) {}}}; T t; f(t); // it is unspecified how many copies of T are made — end example]
Recommended practice: Implementations should avoid double wrapping when constructing polymorphic wrappers from one another.

22.10.17.2 Class bad_function_call [func.wrap.badcall]

An exception of type bad_function_call is thrown by function​::​operator() ([func.wrap.func.inv]) when the function wrapper object has no target.
namespace std { class bad_function_call : public exception { public: // see [exception] for the specification of the special member functions const char* what() const noexcept override; }; }
const char* what() const noexcept override;
Returns: An implementation-defined ntbs.

22.10.17.3 Class template function [func.wrap.func]

22.10.17.3.1 General [func.wrap.func.general]

namespace std { template<class R, class... ArgTypes> class function<R(ArgTypes...)> { public: using result_type = R; // [func.wrap.func.con], construct/copy/destroy function() noexcept; function(nullptr_t) noexcept; function(const function&); function(function&&) noexcept; template<class F> function(F&&); function& operator=(const function&); function& operator=(function&&); function& operator=(nullptr_t) noexcept; template<class F> function& operator=(F&&); template<class F> function& operator=(reference_wrapper<F>) noexcept; ~function(); // [func.wrap.func.mod], function modifiers void swap(function&) noexcept; // [func.wrap.func.cap], function capacity explicit operator bool() const noexcept; // [func.wrap.func.inv], function invocation R operator()(ArgTypes...) const; // [func.wrap.func.targ], function target access const type_info& target_type() const noexcept; template<class T> T* target() noexcept; template<class T> const T* target() const noexcept; }; template<class R, class... ArgTypes> function(R(*)(ArgTypes...)) -> function<R(ArgTypes...)>; template<class F> function(F) -> function<see below>; }
The function class template provides polymorphic wrappers that generalize the notion of a function pointer.
Wrappers can store, copy, and call arbitrary callable objects ([func.def]), given a call signature ([func.def]).
A callable type ([func.def]) F is Lvalue-Callable for argument types ArgTypes and return type R if the expression INVOKE<R>(declval<F&>(), declval<ArgTypes>()...), considered as an unevaluated operand ([expr.context]), is well-formed ([func.require]).
The function class template is a call wrapper ([func.def]) whose call signature ([func.def]) is R(ArgTypes...).
[Note 1: 
The types deduced by the deduction guides for function might change in future revisions of C++.
— end note]

22.10.17.3.2 Constructors and destructor [func.wrap.func.con]

function() noexcept;
Postconditions: !*this.
function(nullptr_t) noexcept;
Postconditions: !*this.
function(const function& f);
Postconditions: !*this if !f; otherwise, the target object of *this is a copy of f.target().
Throws: Nothing if f's target is a specialization of reference_wrapper or a function pointer.
Otherwise, may throw bad_alloc or any exception thrown by the copy constructor of the stored callable object.
Recommended practice: Implementations should avoid the use of dynamically allocated memory for small callable objects, for example, where f's target is an object holding only a pointer or reference to an object and a member function pointer.
function(function&& f) noexcept;
Postconditions: If !f, *this has no target; otherwise, the target of *this is equivalent to the target of f before the construction, and f is in a valid state with an unspecified value.
Recommended practice: Implementations should avoid the use of dynamically allocated memory for small callable objects, for example, where f's target is an object holding only a pointer or reference to an object and a member function pointer.
template<class F> function(F&& f);
Let FD be decay_t<F>.
Constraints:
  • is_same_v<remove_cvref_t<F>, function> is false, and
  • FD is Lvalue-Callable ([func.wrap.func]) for argument types ArgTypes... and return type R.
Mandates:
  • is_copy_constructible_v<FD> is true, and
  • is_constructible_v<FD, F> is true.
Preconditions: FD meets the Cpp17CopyConstructible requirements.
Postconditions: !*this is true if any of the following hold:
  • f is a null function pointer value.
  • f is a null member pointer value.
  • remove_cvref_t<F> is a specialization of the function class template, and !f is true.
Otherwise, *this has a target object of type FD direct-non-list-initialized with std​::​forward<F>(f).
Throws: Nothing if FD is a specialization of reference_wrapper or a function pointer type.
Otherwise, may throw bad_alloc or any exception thrown by the initialization of the target object.
Recommended practice: Implementations should avoid the use of dynamically allocated memory for small callable objects, for example, where f refers to an object holding only a pointer or reference to an object and a member function pointer.
template<class F> function(F) -> function<see below>;
Constraints: &F​::​operator() is well-formed when treated as an unevaluated operand and either
  • F​::​operator() is a non-static member function and decltype(&F​::​operator()) is either of the form R(G​::​*)(A...) cv & noexcept or of the form R(*)(G, A...) noexcept for a type G, or
  • F​::​operator() is a static member function and decltype(&F​::​operator()) is of the form R(*)(A...) noexcept.
Remarks: The deduced type is function<R(A...)>.
[Example 1: void f() { int i{5}; function g = [&](double) { return i; }; // deduces function<int(double)> } — end example]
function& operator=(const function& f);
Effects: As if by function(f).swap(*this);
Returns: *this.
function& operator=(function&& f);
Effects: Replaces the target of *this with the target of f.
Returns: *this.
function& operator=(nullptr_t) noexcept;
Effects: If *this != nullptr, destroys the target of this.
Postconditions: !(*this).
Returns: *this.
template<class F> function& operator=(F&& f);
Constraints: decay_t<F> is Lvalue-Callable ([func.wrap.func]) for argument types ArgTypes... and return type R.
Effects: As if by: function(std​::​forward<F>(f)).swap(*this);
Returns: *this.
template<class F> function& operator=(reference_wrapper<F> f) noexcept;
Effects: As if by: function(f).swap(*this);
Returns: *this.
~function();
Effects: If *this != nullptr, destroys the target of this.

22.10.17.3.3 Modifiers [func.wrap.func.mod]

void swap(function& other) noexcept;
Effects: Interchanges the target objects of *this and other.

22.10.17.3.4 Capacity [func.wrap.func.cap]

explicit operator bool() const noexcept;
Returns: true if *this has a target, otherwise false.

22.10.17.3.5 Invocation [func.wrap.func.inv]

R operator()(ArgTypes... args) const;
Returns: INVOKE<R>(f, std​::​forward<ArgTypes>(args)...) ([func.require]), where f is the target object ([func.def]) of *this.
Throws: bad_function_call if !*this; otherwise, any exception thrown by the target object.

22.10.17.3.6 Target access [func.wrap.func.targ]

const type_info& target_type() const noexcept;
Returns: If *this has a target of type T, typeid(T); otherwise, typeid(void).
template<class T> T* target() noexcept; template<class T> const T* target() const noexcept;
Returns: If target_type() == typeid(T) a pointer to the stored function target; otherwise a null pointer.

22.10.17.3.7 Null pointer comparison operator functions [func.wrap.func.nullptr]

template<class R, class... ArgTypes> bool operator==(const function<R(ArgTypes...)>& f, nullptr_t) noexcept;
Returns: !f.

22.10.17.3.8 Specialized algorithms [func.wrap.func.alg]

template<class R, class... ArgTypes> void swap(function<R(ArgTypes...)>& f1, function<R(ArgTypes...)>& f2) noexcept;
Effects: As if by: f1.swap(f2);

22.10.17.4 Move-only wrapper [func.wrap.move]

22.10.17.4.1 General [func.wrap.move.general]

The header provides partial specializations of move_only_function for each combination of the possible replacements of the placeholders cv, ref, and noex where
  • cv is either const or empty,
  • ref is either &, &&, or empty, and
  • noex is either true or false.
For each of the possible combinations of the placeholders mentioned above, there is a placeholder inv-quals defined as follows:
  • If ref is empty, let inv-quals be cv&,
  • otherwise, let inv-quals be cv ref.

22.10.17.4.2 Class template move_only_function [func.wrap.move.class]

namespace std { template<class R, class... ArgTypes> class move_only_function<R(ArgTypes...) cv ref noexcept(noex)> { public: using result_type = R; // [func.wrap.move.ctor], constructors, assignment, and destructor move_only_function() noexcept; move_only_function(nullptr_t) noexcept; move_only_function(move_only_function&&) noexcept; template<class F> move_only_function(F&&); template<class T, class... Args> explicit move_only_function(in_place_type_t<T>, Args&&...); template<class T, class U, class... Args> explicit move_only_function(in_place_type_t<T>, initializer_list<U>, Args&&...); move_only_function& operator=(move_only_function&&); move_only_function& operator=(nullptr_t) noexcept; template<class F> move_only_function& operator=(F&&); ~move_only_function(); // [func.wrap.move.inv], invocation explicit operator bool() const noexcept; R operator()(ArgTypes...) cv ref noexcept(noex); // [func.wrap.move.util], utility void swap(move_only_function&) noexcept; friend void swap(move_only_function&, move_only_function&) noexcept; friend bool operator==(const move_only_function&, nullptr_t) noexcept; private: template<class VT> static constexpr bool is-callable-from = see below; // exposition only }; }
The move_only_function class template provides polymorphic wrappers that generalize the notion of a callable object ([func.def]).
These wrappers can store, move, and call arbitrary callable objects, given a call signature.
Recommended practice: Implementations should avoid the use of dynamically allocated memory for a small contained value.
[Note 1: 
Such small-object optimization can only be applied to a type T for which is_nothrow_move_constructible_v<T> is true.
— end note]

22.10.17.4.3 Constructors, assignment, and destructor [func.wrap.move.ctor]

template<class VT> static constexpr bool is-callable-from = see below;
If noex is true, is-callable-from<VT> is equal to: is_nothrow_invocable_r_v<R, VT cv ref, ArgTypes...> && is_nothrow_invocable_r_v<R, VT inv-quals, ArgTypes...>
Otherwise, is-callable-from<VT> is equal to: is_invocable_r_v<R, VT cv ref, ArgTypes...> && is_invocable_r_v<R, VT inv-quals, ArgTypes...>
move_only_function() noexcept; move_only_function(nullptr_t) noexcept;
Postconditions: *this has no target object.
move_only_function(move_only_function&& f) noexcept;
Postconditions: The target object of *this is the target object f had before construction, and f is in a valid state with an unspecified value.
template<class F> move_only_function(F&& f);
Let VT be decay_t<F>.
Constraints:
  • remove_cvref_t<F> is not the same type as move_only_function, and
  • remove_cvref_t<F> is not a specialization of in_place_type_t, and
  • is-callable-from<VT> is true.
Mandates: is_constructible_v<VT, F> is true.
Preconditions: VT meets the Cpp17Destructible requirements, and if is_move_constructible_v<VT> is true, VT meets the Cpp17MoveConstructible requirements.
Postconditions: *this has no target object if any of the following hold:
  • f is a null function pointer value, or
  • f is a null member pointer value, or
  • remove_cvref_t<F> is a specialization of the move_only_function class template, and f has no target object.
Otherwise, *this has a target object of type VT direct-non-list-initialized with std​::​forward<F>(f).
Throws: Any exception thrown by the initialization of the target object.
May throw bad_alloc unless VT is a function pointer or a specialization of reference_wrapper.
template<class T, class... Args> explicit move_only_function(in_place_type_t<T>, Args&&... args);
Let VT be decay_t<T>.
Constraints:
  • is_constructible_v<VT, Args...> is true, and
  • is-callable-from<VT> is true.
Mandates: VT is the same type as T.
Preconditions: VT meets the Cpp17Destructible requirements, and if is_move_constructible_v<VT> is true, VT meets the Cpp17MoveConstructible requirements.
Postconditions: *this has a target object of type VT direct-non-list-initialized with std​::​forward<Args>(args)....
Throws: Any exception thrown by the initialization of the target object.
May throw bad_alloc unless VT is a function pointer or a specialization of reference_wrapper.
template<class T, class U, class... Args> explicit move_only_function(in_place_type_t<T>, initializer_list<U> ilist, Args&&... args);
Let VT be decay_t<T>.
Constraints:
  • is_constructible_v<VT, initializer_list<U>&, Args...> is true, and
  • is-callable-from<VT> is true.
Mandates: VT is the same type as T.
Preconditions: VT meets the Cpp17Destructible requirements, and if is_move_constructible_v<VT> is true, VT meets the Cpp17MoveConstructible requirements.
Postconditions: *this has a target object of type VT direct-non-list-initialized with ilist, std​::​forward<Args>(args)....
Throws: Any exception thrown by the initialization of the target object.
May throw bad_alloc unless VT is a function pointer or a specialization of reference_wrapper.
move_only_function& operator=(move_only_function&& f);
Effects: Equivalent to: move_only_function(std​::​move(f)).swap(*this);
Returns: *this.
move_only_function& operator=(nullptr_t) noexcept;
Effects: Destroys the target object of *this, if any.
Returns: *this.
template<class F> move_only_function& operator=(F&& f);
Effects: Equivalent to: move_only_function(std​::​forward<F>(f)).swap(*this);
Returns: *this.
~move_only_function();
Effects: Destroys the target object of *this, if any.

22.10.17.4.4 Invocation [func.wrap.move.inv]

explicit operator bool() const noexcept;
Returns: true if *this has a target object, otherwise false.
R operator()(ArgTypes... args) cv ref noexcept(noex);
Preconditions: *this has a target object.
Effects: Equivalent to: return INVOKE<R>(static_cast<F inv-quals>(f), std::forward<ArgTypes>(args)...); where f is an lvalue designating the target object of *this and F is the type of f.

22.10.17.4.5 Utility [func.wrap.move.util]

void swap(move_only_function& other) noexcept;
Effects: Exchanges the target objects of *this and other.
friend void swap(move_only_function& f1, move_only_function& f2) noexcept;
Effects: Equivalent to f1.swap(f2).
friend bool operator==(const move_only_function& f, nullptr_t) noexcept;
Returns: true if f has no target object, otherwise false.

22.10.17.5 Copyable wrapper [func.wrap.copy]

22.10.17.5.1 General [func.wrap.copy.general]

The header provides partial specializations of copyable_function for each combination of the possible replacements of the placeholders cv, ref, and noex where
  • cv is either const or empty,
  • ref is either &, &&, or empty, and
  • noex is either true or false.
For each of the possible combinations of the placeholders mentioned above, there is a placeholder inv-quals defined as follows:
  • If ref is empty, let inv-quals be cv&,
  • otherwise, let inv-quals be cv ref.

22.10.17.5.2 Class template copyable_function [func.wrap.copy.class]

namespace std { template<class R, class... ArgTypes> class copyable_function<R(ArgTypes...) cv ref noexcept(noex)> { public: using result_type = R; // [func.wrap.copy.ctor], constructors, assignments, and destructors copyable_function() noexcept; copyable_function(nullptr_t) noexcept; copyable_function(const copyable_function&); copyable_function(copyable_function&&) noexcept; template<class F> copyable_function(F&&); template<class T, class... Args> explicit copyable_function(in_place_type_t<T>, Args&&...); template<class T, class U, class... Args> explicit copyable_function(in_place_type_t<T>, initializer_list<U>, Args&&...); copyable_function& operator=(const copyable_function&); copyable_function& operator=(copyable_function&&); copyable_function& operator=(nullptr_t) noexcept; template<class F> copyable_function& operator=(F&&); ~copyable_function(); // [func.wrap.copy.inv], invocation explicit operator bool() const noexcept; R operator()(ArgTypes...) cv ref noexcept(noex); // [func.wrap.copy.util], utility void swap(copyable_function&) noexcept; friend void swap(copyable_function&, copyable_function&) noexcept; friend bool operator==(const copyable_function&, nullptr_t) noexcept; private: template<class VT> static constexpr bool is-callable-from = see below; // exposition only }; }
The copyable_function class template provides polymorphic wrappers that generalize the notion of a callable object ([func.def]).
These wrappers can store, copy, move, and call arbitrary callable objects, given a call signature.
Recommended practice: Implementations should avoid the use of dynamically allocated memory for a small contained value.
[Note 1: 
Such small-object optimization can only be applied to a type T for which is_nothrow_move_constructible_v<T> is true.
— end note]

22.10.17.5.3 Constructors, assignments, and destructors [func.wrap.copy.ctor]

template<class VT> static constexpr bool is-callable-from = see below;
If noex is true, is-callable-from<VT> is equal to: is_nothrow_invocable_r_v<R, VT cv ref, ArgTypes...> && is_nothrow_invocable_r_v<R, VT inv-quals, ArgTypes...>
Otherwise, is-callable-from<VT> is equal to: is_invocable_r_v<R, VT cv ref, ArgTypes...> && is_invocable_r_v<R, VT inv-quals, ArgTypes...>
copyable_function() noexcept; copyable_function(nullptr_t) noexcept;
Postconditions: *this has no target object.
copyable_function(const copyable_function& f);
Postconditions: *this has no target object if f had no target object.
Otherwise, the target object of *this is a copy of the target object of f.
Throws: Any exception thrown by the initialization of the target object.
May throw bad_alloc.
copyable_function(copyable_function&& f) noexcept;
Postconditions: The target object of *this is the target object f had before construction, and f is in a valid state with an unspecified value.
template<class F> copyable_function(F&& f);
Let VT be decay_t<F>.
Constraints:
  • remove_cvref_t<F> is not the same type as copyable_function, and
  • remove_cvref_t<F> is not a specialization of in_place_type_t, and
  • is-callable-from<VT> is true.
Mandates:
  • is_constructible_v<VT, F> is true, and
  • is_copy_constructible_v<VT> is true.
Preconditions: VT meets the Cpp17Destructible and Cpp17CopyConstructible requirements.
Postconditions: *this has no target object if any of the following hold:
  • f is a null function pointer value, or
  • f is a null member pointer value, or
  • remove_cvref_t<F> is a specialization of the copyable_function class template, and f has no target object.
Otherwise, *this has a target object of type VT direct-non-list-initialized with std​::​forward<F>(f).
Throws: Any exception thrown by the initialization of the target object.
May throw bad_alloc unless VT is a function pointer or a specialization of reference_wrapper.
template<class T, class... Args> explicit copyable_function(in_place_type_t<T>, Args&&... args);
Let VT be decay_t<T>.
Constraints:
  • is_constructible_v<VT, Args...> is true, and
  • is-callable-from<VT> is true.
Mandates:
  • VT is the same type as T, and
  • is_copy_constructible_v<VT> is true.
Preconditions: VT meets the Cpp17Destructible and Cpp17CopyConstructible requirements.
Postconditions: *this has a target object of type VT direct-non-list-initialized with std​::​forward<Args>(args)....
Throws: Any exception thrown by the initialization of the target object.
May throw bad_alloc unless VT is a pointer or a specialization of reference_wrapper.
template<class T, class U, class... Args> explicit copyable_function(in_place_type_t<T>, initializer_list<U> ilist, Args&&... args);
Let VT be decay_t<T>.
Constraints:
  • is_constructible_v<VT, initializer_list<U>&, Args...> is true, and
  • is-callable-from<VT> is true.
Mandates:
  • VT is the same type as T, and
  • is_copy_constructible_v<VT> is true.
Preconditions: VT meets the Cpp17Destructible and Cpp17CopyConstructible requirements.
Postconditions: *this has a target object of type VT direct-non-list-initialized with ilist, std​::​forward<Args>(args)....
Throws: Any exception thrown by the initialization of the target object.
May throw bad_alloc unless VT is a pointer or a specialization of reference_wrapper.
copyable_function& operator=(const copyable_function& f);
Effects: Equivalent to: copyable_function(f).swap(*this);
Returns: *this.
copyable_function& operator=(copyable_function&& f);
Effects: Equivalent to: copyable_function(std​::​move(f)).swap(*this);
Returns: *this.
copyable_function& operator=(nullptr_t) noexcept;
Effects: Destroys the target object of *this, if any.
Returns: *this.
template<class F> copyable_function& operator=(F&& f);
Effects: Equivalent to: copyable_function(std​::​forward<F>(f)).swap(*this);
Returns: *this.
~copyable_function();
Effects: Destroys the target object of *this, if any.

22.10.17.5.4 Invocation [func.wrap.copy.inv]

explicit operator bool() const noexcept;
Returns: true if *this has a target object, otherwise false.
R operator()(ArgTypes... args) cv ref noexcept(noex);
Preconditions: *this has a target object.
Effects: Equivalent to: return INVOKE<R>(static_cast<F inv-quals>(f), std::forward<ArgTypes>(args)...); where f is an lvalue designating the target object of *this and F is the type of f.

22.10.17.5.5 Utility [func.wrap.copy.util]

void swap(copyable_function& other) noexcept;
Effects: Exchanges the target objects of *this and other.
friend void swap(copyable_function& f1, copyable_function& f2) noexcept;
Effects: Equivalent to f1.swap(f2).
friend bool operator==(const copyable_function& f, nullptr_t) noexcept;
Returns: true if f has no target object, otherwise false.

22.10.17.6 Non-owning wrapper [func.wrap.ref]

22.10.17.6.1 General [func.wrap.ref.general]

The header provides partial specializations of function_ref for each combination of the possible replacements of the placeholders cv and noex where:
  • cv is either const or empty, and
  • noex is either true or false.

22.10.17.6.2 Class template function_ref [func.wrap.ref.class]

namespace std { template<class R, class... ArgTypes> class function_ref<R(ArgTypes...) cv noexcept(noex)> { public: // [func.wrap.ref.ctor], constructors and assignment operators template<class F> function_ref(F*) noexcept; template<class F> constexpr function_ref(F&&) noexcept; template<auto f> constexpr function_ref(nontype_t<f>) noexcept; template<auto f, class U> constexpr function_ref(nontype_t<f>, U&&) noexcept; template<auto f, class T> constexpr function_ref(nontype_t<f>, cv T*) noexcept; constexpr function_ref(const function_ref&) noexcept = default; constexpr function_ref& operator=(const function_ref&) noexcept = default; template<class T> function_ref& operator=(T) = delete; // [func.wrap.ref.inv], invocation R operator()(ArgTypes...) const noexcept(noex); private: template<class... T> static constexpr bool is-invocable-using = see below; // exposition only R (*thunk-ptr)(BoundEntityType, Args&&...) noexcept(noex); // exposition only BoundEntityType bound-entity; // exposition only }; // [func.wrap.ref.deduct], deduction guides template<class F> function_ref(F*) -> function_ref<F>; template<auto f> function_ref(nontype_t<f>) -> function_ref<see below>; template<auto f, class T> function_ref(nontype_t<f>, T&&) -> function_ref<see below>; }
An object of class function_ref<R(Args...) cv noexcept(noex)> stores a pointer to function thunk-ptr and an object bound-entity.
bound-entity has an unspecified trivially copyable type BoundEntityType, that models copyable and is capable of storing a pointer to object value or a pointer to function value.
The type of thunk-ptr is R(*)(BoundEntityType, Args&&...) noexcept(noex).
Each specialization of function_ref is a trivially copyable type ([basic.types.general]) that models copyable.
Within [func.wrap.ref], call-args is an argument pack with elements such that decltype((call-args))... denote Args&&... respectively.

22.10.17.6.3 Constructors and assignment operators [func.wrap.ref.ctor]

template<class... T> static constexpr bool is-invocable-using = see below;
If noex is true, is-invocable-using<T...> is equal to: is_nothrow_invocable_r_v<R, T..., ArgTypes...>
Otherwise, is-invocable-using<T...> is equal to: is_invocable_r_v<R, T..., ArgTypes...>
template<class F> function_ref(F* f) noexcept;
Constraints:
  • is_function_v<F> is true, and
  • is-invocable-using<F> is true.
Preconditions: f is not a null pointer.
Effects: Initializes bound-entity with f, and thunk-ptr with the address of a function thunk such that thunk(bound-entity, call-args...) is expression-equivalent ([defns.expression.equivalent]) to invoke_r<R>(f, call-args...).
template<class F> constexpr function_ref(F&& f) noexcept;
Let T be remove_reference_t<F>.
Constraints:
  • remove_cvref_t<F> is not the same type as function_ref,
  • is_member_pointer_v<T> is false, and
  • is-invocable-using<cv T&> is true.
Effects: Initializes bound-entity with addressof(f), and thunk-ptr with the address of a function thunk such that thunk(bound-entity, call-args...) is expression-equivalent ([defns.expression.equivalent]) to invoke_r<R>(static_cast<cv T&>(f), call-args...).
template<auto f> constexpr function_ref(nontype_t<f>) noexcept;
Let F be decltype(f).
Constraints: is-invocable-using<F> is true.
Mandates: If is_pointer_v<F> || is_member_pointer_v<F> is true, then f != nullptr is true.
Effects: Initializes bound-entity with a pointer to an unspecified object or null pointer value, and thunk-ptr with the address of a function thunk such that thunk(bound-entity, call-args...) is expression-equivalent ([defns.expression.equivalent]) to invoke_r<R>(f, call-args...).
template<auto f, class U> constexpr function_ref(nontype_t<f>, U&& obj) noexcept;
Let T be remove_reference_t<U> and F be decltype(f).
Constraints:
  • is_rvalue_reference_v<U&&> is false, and
  • is-invocable-using<F, cv T&> is true.
Mandates: If is_pointer_v<F> || is_member_pointer_v<F> is true, then f != nullptr is true.
Effects: Initializes bound-entity with addressof(obj), and thunk-ptr with the address of a function thunk such that thunk(bound-entity, call-args...) is expression-equivalent ([defns.expression.equivalent]) to invoke_r<R>(f, static_cast<cv T&>(obj), call-args...).
template<auto f, class T> constexpr function_ref(nontype_t<f>, cv T* obj) noexcept;
Let F be decltype(f).
Constraints: is-invocable-using<F, cv T*> is true.
Mandates: If is_pointer_v<F> || is_member_pointer_v<F> is true, then f != nullptr is true.
Preconditions: If is_member_pointer_v<F> is true, obj is not a null pointer.
Effects: Initializes bound-entity with obj, and thunk-ptr with the address of a function thunk such that thunk(bound-entity, call-args...) is expression-equivalent ([defns.expression.equivalent]) to invoke_r<R>(f, obj, call-args...).
template<class T> function_ref& operator=(T) = delete;
Constraints:
  • T is not the same type as function_ref,
  • is_pointer_v<T> is false, and
  • T is not a specialization of nontype_t.

22.10.17.6.4 Invocation [func.wrap.ref.inv]

R operator()(ArgTypes... args) const noexcept(noex);
Effects: Equivalent to: return thunk-ptr(bound-entity, std​::​forward<ArgTypes>(args)...);

22.10.17.6.5 Deduction guides [func.wrap.ref.deduct]

template<class F> function_ref(F*) -> function_ref<F>;
Constraints: is_function_v<F> is true.
template<auto f> function_ref(nontype_t<f>) -> function_ref<see below>;
Let F be remove_pointer_t<decltype(f)>.
Constraints: is_function_v<F> is true.
Remarks: The deduced type is function_ref<F>.
template<auto f, class T> function_ref(nontype_t<f>, T&&) -> function_ref<see below>;
Let F be decltype(f).
Constraints:
  • F is of the form R(G​::​*)(A...) cv & noexcept(E) for a type G, or
  • F is of the form M G​::​* for a type G and an object type M, in which case let R be invoke_result_t<F, T&>, A... be an empty pack, and E be false, or
  • F is of the form R(*)(G, A...) noexcept(E) for a type G.
Remarks: The deduced type is function_ref<R(A...) noexcept(E)>.