/
Function.h
1136 lines (1012 loc) · 39.1 KB
/
Function.h
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/*
* Copyright (c) Meta Platforms, Inc. and affiliates.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
//
// Docs: https://fburl.com/fbcref_function
//
/*
* @author Eric Niebler (eniebler@fb.com), Sven Over (over@fb.com)
* Acknowledgements: Giuseppe Ottaviano (ott@fb.com)
*/
/**
* @class folly::Function
* @refcode folly/docs/examples/folly/Function.cpp
*
* A polymorphic function wrapper that is not copyable and does not
* require the wrapped function to be copy constructible.
*
* `folly::Function` is a polymorphic function wrapper, similar to
* `std::function`. The template parameters of the `folly::Function` define
* the parameter signature of the wrapped callable, but not the specific
* type of the embedded callable. E.g. a `folly::Function<int(int)>`
* can wrap callables that return an `int` when passed an `int`. This can be a
* function pointer or any class object implementing one or both of
*
* int operator(int);
* int operator(int) const;
*
* If both are defined, the non-const one takes precedence.
*
* Unlike `std::function`, a `folly::Function` can wrap objects that are not
* copy constructible. As a consequence of this, `folly::Function` itself
* is not copyable, either.
*
* Another difference is that, unlike `std::function`, `folly::Function` treats
* const-ness of methods correctly. While a `std::function` allows to wrap
* an object that only implements a non-const `operator()` and invoke
* a const-reference of the `std::function`, `folly::Function` requires you to
* declare a function type as const in order to be able to execute it on a
* const-reference.
*
* For example:
*
* class Foo {
* public:
* void operator()() {
* // mutates the Foo object
* }
* };
*
* class Bar {
* std::function<void(void)> foo_; // wraps a Foo object
* public:
* void mutateFoo() const
* {
* foo_();
* }
* };
*
* Even though `mutateFoo` is a const-method, so it can only reference `foo_`
* as const, it is able to call the non-const `operator()` of the Foo
* object that is embedded in the foo_ function.
*
* `folly::Function` will not allow you to do that. You will have to decide
* whether you need to invoke your wrapped callable from a const reference
* (like in the example above), in which case it will only wrap a
* `operator() const`. If your functor does not implement that,
* compilation will fail. If you do not require to be able to invoke the
* wrapped function in a const context, you can wrap any functor that
* implements either or both of const and non-const `operator()`.
*
* The template parameter of `folly::Function`, the `FunctionType`, can be
* const-qualified. Be aware that the const is part of the function signature.
* It does not mean that the function type is a const type.
*
* using FunctionType = R(Args...);
* using ConstFunctionType = R(Args...) const;
*
* In this example, `FunctionType` and `ConstFunctionType` are different
* types. `ConstFunctionType` is not the same as `const FunctionType`.
* As a matter of fact, trying to use the latter should emit a compiler
* warning or error, because it has no defined meaning.
*
* // This will not compile:
* folly::Function<void(void) const> func = Foo();
* // because Foo does not have a member function of the form:
* // void operator()() const;
*
* // This will compile just fine:
* folly::Function<void(void)> func = Foo();
* // and it will wrap the existing member function:
* // void operator()();
*
* When should a const function type be used? As a matter of fact, you will
* probably not need to use const function types very often. See the following
* example:
*
* class Bar {
* folly::Function<void()> func_;
* folly::Function<void() const> constFunc_;
*
* void someMethod() {
* // Can call func_.
* func_();
* // Can call constFunc_.
* constFunc_();
* }
*
* void someConstMethod() const {
* // Can call constFunc_.
* constFunc_();
* // However, cannot call func_ because a non-const method cannot
* // be called from a const one.
* }
* };
*
* As you can see, whether the `folly::Function`'s function type should
* be declared const or not is identical to whether a corresponding method
* would be declared const or not.
*
* You only require a `folly::Function` to hold a const function type, if you
* intend to invoke it from within a const context. This is to ensure that
* you cannot mutate its inner state when calling in a const context.
*
* This is how the const/non-const choice relates to lambda functions:
*
* // Non-mutable lambdas: can be stored in a non-const...
* folly::Function<void(int)> print_number =
* [] (int number) { std::cout << number << std::endl; };
*
* // ...as well as in a const folly::Function
* folly::Function<void(int) const> print_number_const =
* [] (int number) { std::cout << number << std::endl; };
*
* // Mutable lambda: can only be stored in a non-const folly::Function:
* int number = 0;
* folly::Function<void()> print_number =
* [number] () mutable { std::cout << ++number << std::endl; };
* // Trying to store the above mutable lambda in a
* // `folly::Function<void() const>` would lead to a compiler error:
* // error: no viable conversion from '(lambda at ...)' to
* // 'folly::Function<void () const>'
*
* Casting between const and non-const `folly::Function`s:
* conversion from const to non-const signatures happens implicitly. Any
* function that takes a `folly::Function<R(Args...)>` can be passed
* a `folly::Function<R(Args...) const>` without explicit conversion.
* This is safe, because casting from const to non-const only entails giving
* up the ability to invoke the function from a const context.
* Casting from a non-const to a const signature is potentially dangerous,
* as it means that a function that may change its inner state when invoked
* is made possible to call from a const context. Therefore this cast does
* not happen implicitly. The function `folly::constCastFunction` can
* be used to perform the cast.
*
* // Mutable lambda: can only be stored in a non-const folly::Function:
* int number = 0;
* folly::Function<void()> print_number =
* [number] () mutable { std::cout << ++number << std::endl; };
*
* // const-cast to a const folly::Function:
* folly::Function<void() const> print_number_const =
* constCastFunction(std::move(print_number));
*
* When to use const function types?
* Generally, only when you need them. When you use a `folly::Function` as a
* member of a struct or class, only use a const function signature when you
* need to invoke the function from const context.
* When passing a `folly::Function` to a function, the function should accept
* a non-const `folly::Function` whenever possible, i.e. when it does not
* need to pass on or store a const `folly::Function`. This is the least
* possible constraint: you can always pass a const `folly::Function` when
* the function accepts a non-const one.
*
* How does the const behaviour compare to `std::function`?
* `std::function` can wrap object with non-const invocation behaviour but
* exposes them as const. The equivalent behaviour can be achieved with
* `folly::Function` like so:
*
* std::function<void(void)> stdfunc = someCallable;
*
* folly::Function<void(void) const> uniqfunc = constCastFunction(
* folly::Function<void(void)>(someCallable)
* );
*
* You need to wrap the callable first in a non-const `folly::Function` to
* select a non-const invoke operator (or the const one if no non-const one is
* present), and then move it into a const `folly::Function` using
* `constCastFunction`.
*/
#pragma once
#include <cstring>
#include <functional>
#include <memory>
#include <new>
#include <type_traits>
#include <utility>
#include <folly/CppAttributes.h>
#include <folly/Portability.h>
#include <folly/Traits.h>
#include <folly/functional/Invoke.h>
#include <folly/lang/Align.h>
#include <folly/lang/Exception.h>
namespace folly {
template <typename FunctionType>
class Function;
template <typename ReturnType, typename... Args>
Function<ReturnType(Args...) const> constCastFunction(
Function<ReturnType(Args...)>&&) noexcept;
template <typename ReturnType, typename... Args>
Function<ReturnType(Args...) const noexcept> constCastFunction(
Function<ReturnType(Args...) noexcept>&&) noexcept;
namespace detail {
namespace function {
enum class Op { MOVE, NUKE, HEAP };
union Data {
void* big;
std::aligned_storage<6 * sizeof(void*)>::type tiny;
};
struct CoerceTag {};
template <typename T>
using FunctionNullptrTest =
decltype(static_cast<bool>(static_cast<T const&>(T(nullptr)) == nullptr));
template <typename T>
constexpr bool IsNullptrCompatible = is_detected_v<FunctionNullptrTest, T>;
template <typename T, std::enable_if_t<!IsNullptrCompatible<T>, int> = 0>
constexpr bool isEmptyFunction(T const&) {
return false;
}
template <typename T, std::enable_if_t<IsNullptrCompatible<T>, int> = 0>
constexpr bool isEmptyFunction(T const& t) {
return static_cast<bool>(t == nullptr);
}
template <typename F, typename... Args>
using CallableResult = decltype(FOLLY_DECLVAL(F &&)(FOLLY_DECLVAL(Args &&)...));
template <typename F, typename... Args>
constexpr bool CallableNoexcept =
noexcept(FOLLY_DECLVAL(F&&)(FOLLY_DECLVAL(Args&&)...));
template <
typename From,
typename To,
typename = typename std::enable_if<
!std::is_reference<To>::value || std::is_reference<From>::value>::type>
using IfSafeResultImpl = decltype(void(static_cast<To>(FOLLY_DECLVAL(From))));
#if defined(_MSC_VER)
// Need a workaround for MSVC to avoid the inscrutable error:
//
// folly\function.h(...) : fatal error C1001: An internal error has
// occurred in the compiler.
// (compiler file 'f:\dd\vctools\compiler\utc\src\p2\main.c', line 258)
// To work around this problem, try simplifying or changing the program
// near the locations listed above.
template <typename T>
using CallArg = T&&;
#else
template <typename T>
using CallArg = conditional_t<is_register_pass_v<T>, T, T&&>;
#endif
template <typename F, bool Nx, typename R, typename... A>
class FunctionTraitsSharedProxy {
std::shared_ptr<Function<F>> sp_;
public:
explicit FunctionTraitsSharedProxy(std::nullptr_t) noexcept {}
explicit FunctionTraitsSharedProxy(Function<F>&& func)
: sp_(func ? std::make_shared<Function<F>>(std::move(func))
: std::shared_ptr<Function<F>>()) {}
R operator()(A... args) const noexcept(Nx) {
if (!sp_) {
throw_exception<std::bad_function_call>();
}
return (*sp_)(static_cast<A&&>(args)...);
}
explicit operator bool() const noexcept { return sp_ != nullptr; }
friend bool operator==(
FunctionTraitsSharedProxy const& proxy, std::nullptr_t) noexcept {
return proxy.sp_ == nullptr;
}
friend bool operator!=(
FunctionTraitsSharedProxy const& proxy, std::nullptr_t) noexcept {
return proxy.sp_ != nullptr;
}
friend bool operator==(
std::nullptr_t, FunctionTraitsSharedProxy const& proxy) noexcept {
return proxy.sp_ == nullptr;
}
friend bool operator!=(
std::nullptr_t, FunctionTraitsSharedProxy const& proxy) noexcept {
return proxy.sp_ != nullptr;
}
};
template <typename FunctionType>
struct FunctionTraits;
template <typename ReturnType, typename... Args>
struct FunctionTraits<ReturnType(Args...)> {
using Call = ReturnType (*)(CallArg<Args>..., Data&);
using ConstSignature = ReturnType(Args...) const;
using NonConstSignature = ReturnType(Args...);
using OtherSignature = ConstSignature;
template <typename F, typename R = CallableResult<F&, Args...>>
using IfSafeResult = IfSafeResultImpl<R, ReturnType>;
template <typename Fun>
static ReturnType callSmall(CallArg<Args>... args, Data& p) {
auto& fn = *static_cast<Fun*>(static_cast<void*>(&p.tiny));
if constexpr (std::is_void<ReturnType>::value) {
fn(static_cast<Args&&>(args)...);
} else {
return fn(static_cast<Args&&>(args)...);
}
}
template <typename Fun>
static ReturnType callBig(CallArg<Args>... args, Data& p) {
auto& fn = *static_cast<Fun*>(p.big);
if constexpr (std::is_void<ReturnType>::value) {
fn(static_cast<Args&&>(args)...);
} else {
return fn(static_cast<Args&&>(args)...);
}
}
static ReturnType uninitCall(CallArg<Args>..., Data&) {
throw_exception<std::bad_function_call>();
}
ReturnType operator()(Args... args) {
auto& fn = *static_cast<Function<NonConstSignature>*>(this);
return fn.call_(static_cast<Args&&>(args)..., fn.data_);
}
using SharedProxy =
FunctionTraitsSharedProxy<NonConstSignature, false, ReturnType, Args...>;
};
template <typename ReturnType, typename... Args>
struct FunctionTraits<ReturnType(Args...) const> {
using Call = ReturnType (*)(CallArg<Args>..., Data&);
using ConstSignature = ReturnType(Args...) const;
using NonConstSignature = ReturnType(Args...);
using OtherSignature = NonConstSignature;
template <typename F, typename R = CallableResult<const F&, Args...>>
using IfSafeResult = IfSafeResultImpl<R, ReturnType>;
template <typename Fun>
static ReturnType callSmall(CallArg<Args>... args, Data& p) {
auto& fn = *static_cast<const Fun*>(static_cast<void*>(&p.tiny));
if constexpr (std::is_void<ReturnType>::value) {
fn(static_cast<Args&&>(args)...);
} else {
return fn(static_cast<Args&&>(args)...);
}
}
template <typename Fun>
static ReturnType callBig(CallArg<Args>... args, Data& p) {
auto& fn = *static_cast<const Fun*>(p.big);
if constexpr (std::is_void<ReturnType>::value) {
fn(static_cast<Args&&>(args)...);
} else {
return fn(static_cast<Args&&>(args)...);
}
}
static ReturnType uninitCall(CallArg<Args>..., Data&) {
throw_exception<std::bad_function_call>();
}
ReturnType operator()(Args... args) const {
auto& fn = *static_cast<const Function<ConstSignature>*>(this);
return fn.call_(static_cast<Args&&>(args)..., fn.data_);
}
using SharedProxy =
FunctionTraitsSharedProxy<ConstSignature, false, ReturnType, Args...>;
};
template <typename ReturnType, typename... Args>
struct FunctionTraits<ReturnType(Args...) noexcept> {
using Call = ReturnType (*)(CallArg<Args>..., Data&) noexcept;
using ConstSignature = ReturnType(Args...) const noexcept;
using NonConstSignature = ReturnType(Args...) noexcept;
using OtherSignature = ConstSignature;
template <
typename F,
typename R = CallableResult<F&, Args...>,
std::enable_if_t<CallableNoexcept<F&, Args...>, int> = 0>
using IfSafeResult = IfSafeResultImpl<R, ReturnType>;
template <typename Fun>
static ReturnType callSmall(CallArg<Args>... args, Data& p) noexcept {
auto& fn = *static_cast<Fun*>(static_cast<void*>(&p.tiny));
if constexpr (std::is_void<ReturnType>::value) {
fn(static_cast<Args&&>(args)...);
} else {
return fn(static_cast<Args&&>(args)...);
}
}
template <typename Fun>
static ReturnType callBig(CallArg<Args>... args, Data& p) noexcept {
auto& fn = *static_cast<Fun*>(p.big);
if constexpr (std::is_void<ReturnType>::value) {
fn(static_cast<Args&&>(args)...);
} else {
return fn(static_cast<Args&&>(args)...);
}
}
static ReturnType uninitCall(CallArg<Args>..., Data&) noexcept {
terminate_with<std::bad_function_call>();
}
ReturnType operator()(Args... args) noexcept {
auto& fn = *static_cast<Function<NonConstSignature>*>(this);
return fn.call_(static_cast<Args&&>(args)..., fn.data_);
}
using SharedProxy =
FunctionTraitsSharedProxy<NonConstSignature, true, ReturnType, Args...>;
};
template <typename ReturnType, typename... Args>
struct FunctionTraits<ReturnType(Args...) const noexcept> {
using Call = ReturnType (*)(CallArg<Args>..., Data&) noexcept;
using ConstSignature = ReturnType(Args...) const noexcept;
using NonConstSignature = ReturnType(Args...) noexcept;
using OtherSignature = NonConstSignature;
template <
typename F,
typename R = CallableResult<const F&, Args...>,
std::enable_if_t<CallableNoexcept<const F&, Args...>, int> = 0>
using IfSafeResult = IfSafeResultImpl<R, ReturnType>;
template <typename Fun>
static ReturnType callSmall(CallArg<Args>... args, Data& p) noexcept {
auto& fn = *static_cast<const Fun*>(static_cast<void*>(&p.tiny));
if constexpr (std::is_void<ReturnType>::value) {
fn(static_cast<Args&&>(args)...);
} else {
return fn(static_cast<Args&&>(args)...);
}
}
template <typename Fun>
static ReturnType callBig(CallArg<Args>... args, Data& p) noexcept {
auto& fn = *static_cast<const Fun*>(p.big);
if constexpr (std::is_void<ReturnType>::value) {
fn(static_cast<Args&&>(args)...);
} else {
return fn(static_cast<Args&&>(args)...);
}
}
static ReturnType uninitCall(CallArg<Args>..., Data&) noexcept {
terminate_with<std::bad_function_call>();
}
ReturnType operator()(Args... args) const noexcept {
auto& fn = *static_cast<const Function<ConstSignature>*>(this);
return fn.call_(static_cast<Args&&>(args)..., fn.data_);
}
using SharedProxy =
FunctionTraitsSharedProxy<ConstSignature, true, ReturnType, Args...>;
};
// These are control functions. They type-erase the operations of move-
// construction, destruction, and conversion to bool.
//
// The interface operations are noexcept, so the implementations are as well.
// Having the implementations be noexcept in the type permits callers to omit
// exception-handling machinery.
//
// This is intentionally instantiated per size rather than per function in order
// to minimize the number of instantiations. It would be safe to minimize
// instantiations even more by simply having a single non-template function that
// copies sizeof(Data) bytes rather than only copying sizeof(Fun) bytes, but
// then for small function types it would be likely to cross cache lines without
// need. But it is only necessary to handle those sizes which are multiples of
// the alignof(Data), and to round up other sizes.
struct DispatchSmallTrivial {
template <typename Fun, typename Base>
static constexpr auto call = Base::template callSmall<Fun>;
template <std::size_t Size>
static std::size_t exec_(Op o, Data* src, Data* dst) noexcept {
switch (o) {
case Op::MOVE:
std::memcpy(static_cast<void*>(dst), static_cast<void*>(src), Size);
break;
case Op::NUKE:
break;
case Op::HEAP:
break;
}
return 0U;
}
template <std::size_t size, std::size_t adjust = alignof(Data) - 1>
static constexpr std::size_t size_ = (size + adjust) & ~adjust;
template <typename Fun>
static constexpr auto exec = exec_<size_<sizeof(Fun)>>;
};
struct DispatchSmall {
template <typename Fun, typename Base>
static constexpr auto call = Base::template callSmall<Fun>;
template <typename Fun>
static std::size_t exec(Op o, Data* src, Data* dst) noexcept {
switch (o) {
case Op::MOVE:
::new (static_cast<void*>(&dst->tiny)) Fun(static_cast<Fun&&>(
*static_cast<Fun*>(static_cast<void*>(&src->tiny))));
[[fallthrough]];
case Op::NUKE:
static_cast<Fun*>(static_cast<void*>(&src->tiny))->~Fun();
break;
case Op::HEAP:
break;
}
return 0U;
}
};
struct DispatchBig {
template <typename Fun, typename Base>
static constexpr auto call = Base::template callBig<Fun>;
template <typename Fun>
static std::size_t exec(Op o, Data* src, Data* dst) noexcept {
switch (o) {
case Op::MOVE:
dst->big = src->big;
src->big = nullptr;
break;
case Op::NUKE:
delete static_cast<Fun*>(src->big);
break;
case Op::HEAP:
break;
}
return sizeof(Fun);
}
};
} // namespace function
} // namespace detail
template <typename FunctionType>
class Function final : private detail::function::FunctionTraits<FunctionType> {
// These utility types are defined outside of the template to reduce
// the number of instantiations, and then imported in the class
// namespace for convenience.
using Data = detail::function::Data;
using Op = detail::function::Op;
using CoerceTag = detail::function::CoerceTag;
using Traits = detail::function::FunctionTraits<FunctionType>;
using Call = typename Traits::Call;
using Exec = std::size_t (*)(Op, Data*, Data*) noexcept;
// The `data_` member is mutable to allow `constCastFunction` to work without
// invoking undefined behavior. Const-correctness is only violated when
// `FunctionType` is a const function type (e.g., `int() const`) and `*this`
// is the result of calling `constCastFunction`.
mutable Data data_{unsafe_default_initialized};
Call call_{&Traits::uninitCall};
Exec exec_{nullptr};
std::size_t exec(Op o, Data* src, Data* dst) const {
if (!exec_) {
return 0U;
}
return exec_(o, src, dst);
}
friend Traits;
friend Function<typename Traits::ConstSignature> folly::constCastFunction<>(
Function<typename Traits::NonConstSignature>&&) noexcept;
friend class Function<typename Traits::OtherSignature>;
template <typename Signature>
Function(Function<Signature>&& fun, CoerceTag) {
using Fun = Function<Signature>;
if (fun) {
data_.big = new Fun(static_cast<Fun&&>(fun));
call_ = Traits::template callBig<Fun>;
exec_ = Exec(detail::function::DispatchBig::exec<Fun>);
}
}
Function(Function<typename Traits::OtherSignature>&& that, CoerceTag) noexcept
: call_(that.call_), exec_(that.exec_) {
that.call_ = &Traits::uninitCall;
that.exec_ = nullptr;
exec(Op::MOVE, &that.data_, &data_);
}
public:
/**
* Default constructor. Constructs an empty Function.
*/
constexpr Function() = default;
// not copyable
Function(const Function&) = delete;
#ifdef __OBJC__
// Make sure Objective C blocks are copied
template <class ReturnType, class... Args>
/*implicit*/ Function(ReturnType (^objCBlock)(Args... args))
: Function([blockCopy = (ReturnType(^)(Args...))[objCBlock copy]](
Args... args) { return blockCopy(args...); }){};
#endif
/**
* Move constructor
*/
Function(Function&& that) noexcept : call_(that.call_), exec_(that.exec_) {
// that must be uninitialized before exec() call in the case of self move
that.call_ = &Traits::uninitCall;
that.exec_ = nullptr;
exec(Op::MOVE, &that.data_, &data_);
}
/**
* Constructs an empty `Function`.
*/
/* implicit */ constexpr Function(std::nullptr_t) noexcept {}
/**
* Constructs a new `Function` from any callable object that is _not_ a
* `folly::Function`.
*
* \note `typename Traits::template IfSafeResult<Fun>` prevents this overload
* from being selected by overload resolution when `fun` is not a compatible
* function.
*
* \note The noexcept requires some explanation. `IsSmall` is true when the
* decayed type fits within the internal buffer and is noexcept-movable. But
* this ctor might copy, not move. What we need here, if this ctor does a
* copy, is that this ctor be noexcept when the copy is noexcept. That is not
* checked in `IsSmall`, and shouldn't be, because once the `Function` is
* constructed, the contained object is never copied. This check is for this
* ctor only, in the case that this ctor does a copy.
*
* @param fun function pointers, pointers to static
* member functions, `std::reference_wrapper` objects, `std::function`
* objects, and arbitrary objects that implement `operator()` if the
* parameter signature matches (i.e. it returns an object convertible to `R`
* when called with `Args...`).
*/
template <
typename Fun,
typename =
std::enable_if_t<!detail::is_similar_instantiation_v<Function, Fun>>,
typename = typename Traits::template IfSafeResult<Fun>,
bool IsSmall = ( //
sizeof(Fun) <= sizeof(Data) && //
alignof(Fun) <= alignof(Data) && //
noexcept(Fun(FOLLY_DECLVAL(Fun))))>
/* implicit */ constexpr Function(Fun fun) noexcept(IsSmall) {
using Dispatch = conditional_t<
IsSmall && is_trivially_copyable_v<Fun>,
detail::function::DispatchSmallTrivial,
conditional_t<
IsSmall,
detail::function::DispatchSmall,
detail::function::DispatchBig>>;
if constexpr (detail::function::IsNullptrCompatible<Fun>) {
if (detail::function::isEmptyFunction(fun)) {
return;
}
}
if constexpr (IsSmall) {
if constexpr (
!std::is_empty<Fun>::value || !is_trivially_copyable_v<Fun>) {
::new (&data_.tiny) Fun(static_cast<Fun&&>(fun));
}
} else {
data_.big = new Fun(static_cast<Fun&&>(fun));
}
call_ = Dispatch::template call<Fun, Traits>;
exec_ = Exec(Dispatch::template exec<Fun>);
}
/**
* For move-constructing from a `folly::Function<X(Ys...) [const?]>`.
*
* For a `Function` with a `const` function type, the object must be
* callable from a `const`-reference, i.e. implement `operator() const`.
* For a `Function` with a non-`const` function type, the object will
* be called from a non-const reference, which means that it will execute
* a non-const `operator()` if it is defined, and falls back to
* `operator() const` otherwise.
*/
template <
typename Signature,
typename Fun = Function<Signature>,
// prevent gcc from making this a better match than move-ctor
typename = std::enable_if_t<!std::is_same<Function, Fun>::value>,
typename = typename Traits::template IfSafeResult<Fun>>
Function(Function<Signature>&& that) noexcept(
noexcept(Function(std::move(that), CoerceTag{})))
: Function(std::move(that), CoerceTag{}) {}
/**
* If `ptr` is null, constructs an empty `Function`. Otherwise,
* this constructor is equivalent to `Function(std::mem_fn(ptr))`.
*/
template <
typename Member,
typename Class,
// Prevent this overload from being selected when `ptr` is not a
// compatible member function pointer.
typename = decltype(Function(std::mem_fn((Member Class::*)0)))>
/* implicit */ Function(Member Class::*ptr) noexcept {
if (ptr) {
*this = std::mem_fn(ptr);
}
}
~Function() { exec(Op::NUKE, &data_, nullptr); }
Function& operator=(const Function&) = delete;
#ifdef __OBJC__
// Make sure Objective C blocks are copied
template <class ReturnType, class... Args>
/* implicit */ Function& operator=(ReturnType (^objCBlock)(Args... args)) {
(*this) = [blockCopy = (ReturnType(^)(Args...))[objCBlock copy]](
Args... args) { return blockCopy(args...); };
return *this;
}
#endif
/**
* Move assignment operator
*
* \note Leaves `that` in a valid but unspecified state. If `&that == this`
* then `*this` is left in a valid but unspecified state.
*/
Function& operator=(Function&& that) noexcept {
exec(Op::NUKE, &data_, nullptr);
if (FOLLY_LIKELY(this != &that)) {
that.exec(Op::MOVE, &that.data_, &data_);
exec_ = that.exec_;
call_ = that.call_;
}
that.exec_ = nullptr;
that.call_ = &Traits::uninitCall;
return *this;
}
/**
* Assigns a callable object to this `Function`. If the operation fails,
* `*this` is left unmodified.
*
* \note `typename = decltype(Function(FOLLY_DECLVAL(Fun&&)))` prevents this
* overload from being selected by overload resolution when `fun` is not a
* compatible function.
*/
template <
typename Fun,
typename...,
bool Nx = noexcept(Function(FOLLY_DECLVAL(Fun&&)))>
Function& operator=(Fun fun) noexcept(Nx) {
// Doing this in place is more efficient when we can do so safely.
if (Nx) {
// Q: Why is is safe to destroy and reconstruct this object in place?
// A: See the explanation in the move assignment operator.
this->~Function();
::new (this) Function(static_cast<Fun&&>(fun));
} else {
// Construct a temporary and (nothrow) swap.
Function(static_cast<Fun&&>(fun)).swap(*this);
}
return *this;
}
/**
* For assigning from a `Function<X(Ys..) [const?]>`.
*/
template <
typename Signature,
typename...,
typename = typename Traits::template IfSafeResult<Function<Signature>>>
Function& operator=(Function<Signature>&& that) noexcept(
noexcept(Function(std::move(that)))) {
return (*this = Function(std::move(that)));
}
/**
* Clears this `Function`.
*/
Function& operator=(std::nullptr_t) noexcept { return (*this = Function()); }
/**
* If `ptr` is null, clears this `Function`. Otherwise, this assignment
* operator is equivalent to `*this = std::mem_fn(ptr)`.
*/
template <typename Member, typename Class>
auto operator=(Member Class::*ptr) noexcept
// Prevent this overload from being selected when `ptr` is not a
// compatible member function pointer.
-> decltype(operator=(std::mem_fn(ptr))) {
return ptr ? (*this = std::mem_fn(ptr)) : (*this = Function());
}
/**
* Call the wrapped callable object with the specified arguments.
*/
using Traits::operator();
/**
* Exchanges the callable objects of `*this` and `that`.
*
* @param that a folly::Function ref
*/
void swap(Function& that) noexcept { std::swap(*this, that); }
/**
* Returns `true` if this `Function` contains a callable, i.e. is
* non-empty.
*/
explicit operator bool() const noexcept { return exec_ != nullptr; }
/**
* Returns the size of the allocation made to store the callable on the
* heap. If `0` is returned, there has been no additional memory
* allocation because the callable is stored within the `Function` object.
*/
std::size_t heapAllocatedMemory() const noexcept {
return exec(Op::HEAP, nullptr, nullptr);
}
using typename Traits::SharedProxy;
/**
* Move this `Function` into a copyable callable object, of which all copies
* share the state.
*/
SharedProxy asSharedProxy() && { return SharedProxy{std::move(*this)}; }
/**
* Construct a `std::function` by moving in the contents of this `Function`.
* Note that the returned `std::function` will share its state (i.e. captured
* data) across all copies you make of it, so be very careful when copying.
*/
std::function<typename Traits::NonConstSignature> asStdFunction() && {
return std::move(*this).asSharedProxy();
}
};
template <typename FunctionType>
void swap(Function<FunctionType>& lhs, Function<FunctionType>& rhs) noexcept {
lhs.swap(rhs);
}
template <typename FunctionType>
bool operator==(const Function<FunctionType>& fn, std::nullptr_t) {
return !fn;
}
template <typename FunctionType>
bool operator==(std::nullptr_t, const Function<FunctionType>& fn) {
return !fn;
}
template <typename FunctionType>
bool operator!=(const Function<FunctionType>& fn, std::nullptr_t) {
return !(fn == nullptr);
}
template <typename FunctionType>
bool operator!=(std::nullptr_t, const Function<FunctionType>& fn) {
return !(nullptr == fn);
}
/**
* Casts a `folly::Function` from non-const to a const signature.
*
* NOTE: The name of `constCastFunction` should warn you that something
* potentially dangerous is happening. As a matter of fact, using
* `std::function` always involves this potentially dangerous aspect, which
* is why it is not considered fully const-safe or even const-correct.
* However, in most of the cases you will not need the dangerous aspect at all.
* Either you do not require invocation of the function from a const context,
* in which case you do not need to use `constCastFunction` and just
* use a non-const `folly::Function`. Or, you may need invocation from const,
* but the callable you are wrapping does not mutate its state (e.g. it is a
* class object and implements `operator() const`, or it is a normal,
* non-mutable lambda), in which case you can wrap the callable in a const
* `folly::Function` directly, without using `constCastFunction`.
* Only if you require invocation from a const context of a callable that
* may mutate itself when invoked you have to go through the above procedure.
* However, in that case what you do is potentially dangerous and requires
* the equivalent of a `const_cast`, hence you need to call
* `constCastFunction`.
*
* @param that a non-const folly::Function.
*/
template <typename ReturnType, typename... Args>
Function<ReturnType(Args...) const> constCastFunction(
Function<ReturnType(Args...)>&& that) noexcept {
return Function<ReturnType(Args...) const>{
std::move(that), detail::function::CoerceTag{}};
}
template <typename ReturnType, typename... Args>
Function<ReturnType(Args...) const> constCastFunction(
Function<ReturnType(Args...) const>&& that) noexcept {
return std::move(that);
}
template <typename ReturnType, typename... Args>
Function<ReturnType(Args...) const noexcept> constCastFunction(
Function<ReturnType(Args...) noexcept>&& that) noexcept {
return Function<ReturnType(Args...) const noexcept>{
std::move(that), detail::function::CoerceTag{}};
}
template <typename ReturnType, typename... Args>
Function<ReturnType(Args...) const noexcept> constCastFunction(
Function<ReturnType(Args...) const noexcept>&& that) noexcept {
return std::move(that);
}
namespace detail {
template <typename Void, typename>
struct function_ctor_deduce_;
template <typename P>
struct function_ctor_deduce_<
std::enable_if_t<std::is_function<std::remove_pointer_t<P>>::value>,
P> {
using type = std::remove_pointer_t<P>;
};
template <typename F>
struct function_ctor_deduce_<void_t<decltype(&F::operator())>, F> {
using type =
typename member_pointer_traits<decltype(&F::operator())>::member_type;
};
template <typename F>
using function_ctor_deduce_t = typename function_ctor_deduce_<void, F>::type;
} // namespace detail
template <typename F>
Function(F) -> Function<detail::function_ctor_deduce_t<F>>;
/**
* @class folly::FunctionRef
*