/*
* 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.
*/
// TODO: [x] "cast" from Poly<C&> to Poly<C&&>
// TODO: [ ] copy/move from Poly<C&>/Poly<C&&> to Poly<C>
// TODO: [ ] copy-on-write?
// TODO: [ ] down- and cross-casting? (Possible?)
// TODO: [ ] shared ownership? (Dubious.)
// TODO: [ ] can games be played with making the VTable a member of a struct
// with strange alignment such that the address of the VTable can
// be used to tell whether the object is stored in-situ or not?
#pragma once
#include <cassert>
#include <new>
#include <type_traits>
#include <typeinfo>
#include <utility>
#include <folly/CPortability.h>
#include <folly/CppAttributes.h>
#include <folly/Traits.h>
#include <folly/detail/TypeList.h>
#include <folly/lang/Assume.h>
#include <folly/PolyException.h>
#include <folly/detail/PolyDetail.h>
namespace folly {
template <class I>
struct Poly;
// MSVC workaround
template <class Node, class Tfx, class Access>
struct PolySelf_ {
using type = decltype(Access::template self_<Node, Tfx>());
};
/**
* Within the definition of interface `I`, `PolySelf<Base>` is an alias for
* the instance of `Poly` that is currently being instantiated. It is
* one of: `Poly<J>`, `Poly<J&&>`, `Poly<J&>`, or `Poly<J const&>`; where
* `J` is either `I` or some interface that extends `I`.
*
* It can be used within interface definitions to declare members that accept
* other `Poly` objects of the same type as `*this`.
*
* The first parameter may optionally be cv- and/or reference-qualified, in
* which case, the qualification is applies to the type of the interface in the
* resulting `Poly<>` instance. The second template parameter controls whether
* or not the interface is decayed before the cv-ref qualifiers of the first
* argument are applied. For example, given the following:
*
* struct Foo {
* template <class Base>
* struct Interface : Base {
* using A = PolySelf<Base>;
* using B = PolySelf<Base &>;
* using C = PolySelf<Base const &>;
* using X = PolySelf<Base, PolyDecay>;
* using Y = PolySelf<Base &, PolyDecay>;
* using Z = PolySelf<Base const &, PolyDecay>;
* };
* // ...
* };
* struct Bar : PolyExtends<Foo> {
* // ...
* };
*
* Then for `Poly<Bar>`, the typedefs are aliases for the following types:
* - `A` is `Poly<Bar>`
* - `B` is `Poly<Bar &>`
* - `C` is `Poly<Bar const &>`
* - `X` is `Poly<Bar>`
* - `Y` is `Poly<Bar &>`
* - `Z` is `Poly<Bar const &>`
*
* And for `Poly<Bar &>`, the typedefs are aliases for the following types:
* - `A` is `Poly<Bar &>`
* - `B` is `Poly<Bar &>`
* - `C` is `Poly<Bar &>`
* - `X` is `Poly<Bar>`
* - `Y` is `Poly<Bar &>`
* - `Z` is `Poly<Bar const &>`
*/
template <
class Node,
class Tfx = detail::MetaIdentity,
class Access = detail::PolyAccess>
using PolySelf = _t<PolySelf_<Node, Tfx, Access>>;
/**
* When used in conjunction with `PolySelf`, controls how to construct `Poly`
* types related to the one currently being instantiated.
*
* \sa PolySelf
*/
using PolyDecay = detail::MetaQuote<std::decay_t>;
#define FOLLY_POLY_MEMBER(SIG, MEM) ::folly::sig<SIG>(MEM)
#define FOLLY_POLY_MEMBERS(...) ::folly::PolyMembers<__VA_ARGS__>
template <auto... Ps>
struct PolyMembers {};
/**
* Used in the definition of a `Poly` interface to say that the current
* interface is an extension of a set of zero or more interfaces.
*
* Example:
*
* struct IFoo {
* template <class Base> struct Interface : Base {
* void foo() { folly::poly_call<0>(*this); }
* };
* template <class T> using Members = FOLLY_POLY_MEMBERS(&T::foo);
* }
* struct IBar : PolyExtends<IFoo> {
* template <class Base> struct Interface : Base {
* void bar(int i) { folly::poly_call<0>(*this, i); }
* };
* template <class T> using Members = FOLLY_POLY_MEMBERS(&T::bar);
* }
*/
template <class... I>
struct PolyExtends : virtual I... {
using Subsumptions = detail::TypeList<I...>;
template <class Base>
struct Interface : Base {
Interface() = default;
using Base::Base;
};
template <class...>
using Members = PolyMembers<>;
};
////////////////////////////////////////////////////////////////////////////////
/**
* Call the N-th member of the currently-being-defined interface. When the
* first parameter is an object of type `PolySelf<Base>` (as opposed to `*this`)
* you must explicitly specify which interface through which to dispatch.
* For instance:
*
* struct IAddable {
* template <class Base>
* struct Interface : Base {
* friend folly::PolySelf<Base, folly::PolyDecay>
* operator+(
* folly::PolySelf<Base> const& a,
* folly::PolySelf<Base> const& b) {
* return folly::poly_call<0, IAddable>(a, b);
* }
* };
* template <class T>
* static auto plus_(T const& a, T const& b) -> decltype(a + b) {
* return a + b;
* }
* template <class T>
* using Members = FOLLY_POLY_MEMBERS(&plus_<std::decay_t<T>>);
* };
*
* \sa PolySelf
*/
template <std::size_t N, typename This, typename... As>
auto poly_call(This&& _this, As&&... as)
-> decltype(detail::PolyAccess::call<N>(
static_cast<This&&>(_this), static_cast<As&&>(as)...)) {
return detail::PolyAccess::call<N>(
static_cast<This&&>(_this), static_cast<As&&>(as)...);
}
/// \overload
template <std::size_t N, class I, class Tail, typename... As>
decltype(auto) poly_call(detail::PolyNode<I, Tail>&& _this, As&&... as) {
using This = detail::InterfaceOf<I, detail::PolyNode<I, Tail>>;
return detail::PolyAccess::call<N>(
static_cast<This&&>(_this), static_cast<As&&>(as)...);
}
/// \overload
template <std::size_t N, class I, class Tail, typename... As>
decltype(auto) poly_call(detail::PolyNode<I, Tail>& _this, As&&... as) {
using This = detail::InterfaceOf<I, detail::PolyNode<I, Tail>>;
return detail::PolyAccess::call<N>(
static_cast<This&>(_this), static_cast<As&&>(as)...);
}
/// \overload
template <std::size_t N, class I, class Tail, typename... As>
decltype(auto) poly_call(detail::PolyNode<I, Tail> const& _this, As&&... as) {
using This = detail::InterfaceOf<I, detail::PolyNode<I, Tail>>;
return detail::PolyAccess::call<N>(
static_cast<This const&>(_this), static_cast<As&&>(as)...);
}
/// \overload
template <
std::size_t N,
class I,
class Poly,
typename... As,
std::enable_if_t<detail::IsPoly<Poly>::value, int> = 0>
auto poly_call(Poly&& _this, As&&... as) -> decltype(poly_call<N, I>(
static_cast<Poly&&>(_this).get(),
static_cast<As&&>(as)...)) {
return poly_call<N, I>(
static_cast<Poly&&>(_this).get(), static_cast<As&&>(as)...);
}
/// \cond
/// \overload
template <std::size_t N, class I, typename... As>
[[noreturn]] detail::Bottom poly_call(detail::ArchetypeBase const&, As&&...) {
assume_unreachable();
}
/// \endcond
////////////////////////////////////////////////////////////////////////////////
/**
* Try to cast the `Poly` object to the requested type. If the `Poly` stores an
* object of that type, return a reference to the object; otherwise, throw an
* exception.
* \tparam T The (unqualified) type to which to cast the `Poly` object.
* \tparam Poly The type of the `Poly` object.
* \param that The `Poly` object to be cast.
* \return A reference to the `T` object stored in or referred to by `that`.
* \throw BadPolyAccess if `that` is empty.
* \throw BadPolyCast if `that` does not store or refer to an object of type
* `T`.
*/
template <class T, class I>
detail::AddCvrefOf<T, I>&& poly_cast(detail::PolyRoot<I>&& that) {
return detail::PolyAccess::cast<T>(std::move(that));
}
/// \overload
template <class T, class I>
detail::AddCvrefOf<T, I>& poly_cast(detail::PolyRoot<I>& that) {
return detail::PolyAccess::cast<T>(that);
}
/// \overload
template <class T, class I>
detail::AddCvrefOf<T, I> const& poly_cast(detail::PolyRoot<I> const& that) {
return detail::PolyAccess::cast<T>(that);
}
/// \cond
/// \overload
template <class T, class I>
[[noreturn]] detail::AddCvrefOf<T, I>&& poly_cast(detail::ArchetypeRoot<I>&&) {
assume_unreachable();
}
/// \overload
template <class T, class I>
[[noreturn]] detail::AddCvrefOf<T, I>& poly_cast(detail::ArchetypeRoot<I>&) {
assume_unreachable();
}
/// \overload
template <class T, class I>
[[noreturn]] detail::AddCvrefOf<T, I> const& poly_cast(
detail::ArchetypeRoot<I> const&) {
assume_unreachable();
}
/// \endcond
/// \overload
template <
class T,
class Poly,
std::enable_if_t<detail::IsPoly<Poly>::value, int> = 0>
constexpr auto poly_cast(Poly&& that)
-> decltype(poly_cast<T>(std::declval<Poly>().get())) {
return poly_cast<T>(static_cast<Poly&&>(that).get());
}
////////////////////////////////////////////////////////////////////////////////
/**
* Returns a reference to the `std::type_info` object corresponding to the
* object currently stored in `that`. If `that` is empty, returns
* `typeid(void)`.
*/
template <class I>
std::type_info const& poly_type(detail::PolyRoot<I> const& that) noexcept {
return detail::PolyAccess::type(that);
}
/// \cond
/// \overload
[[noreturn]] inline std::type_info const& poly_type(
detail::ArchetypeBase const&) noexcept {
assume_unreachable();
}
/// \endcond
/// \overload
template <class Poly, std::enable_if_t<detail::IsPoly<Poly>::value, int> = 0>
constexpr auto poly_type(Poly const& that) noexcept
-> decltype(poly_type(that.get())) {
return poly_type(that.get());
}
////////////////////////////////////////////////////////////////////////////////
/**
* Returns `true` if `that` is not currently storing an object; `false`,
* otherwise.
*/
template <class I>
bool poly_empty(detail::PolyRoot<I> const& that) noexcept {
return detail::State::eEmpty == detail::PolyAccess::vtable(that)->state_;
}
/// \overload
template <class I>
constexpr bool poly_empty(detail::PolyRoot<I&&> const&) noexcept {
return false;
}
/// \overload
template <class I>
constexpr bool poly_empty(detail::PolyRoot<I&> const&) noexcept {
return false;
}
/// \overload
template <class I>
constexpr bool poly_empty(Poly<I&&> const&) noexcept {
return false;
}
/// \overload
template <class I>
constexpr bool poly_empty(Poly<I&> const&) noexcept {
return false;
}
/// \cond
[[noreturn]] inline bool poly_empty(detail::ArchetypeBase const&) noexcept {
assume_unreachable();
}
/// \endcond
////////////////////////////////////////////////////////////////////////////////
/**
* Given a `Poly<I&>`, return a `Poly<I&&>`. Otherwise, when `I` is not a
* reference type, returns a `Poly<I>&&` when given a `Poly<I>&`, like
* `std::move`.
*/
template <
class I,
std::enable_if_t<Negation<std::is_reference<I>>::value, int> = 0>
constexpr Poly<I>&& poly_move(detail::PolyRoot<I>& that) noexcept {
return static_cast<Poly<I>&&>(static_cast<Poly<I>&>(that));
}
/// \overload
template <class I, std::enable_if_t<Negation<std::is_const<I>>::value, int> = 0>
Poly<I&&> poly_move(detail::PolyRoot<I&> const& that) noexcept {
return detail::PolyAccess::move(that);
}
/// \overload
template <class I>
Poly<I const&> poly_move(detail::PolyRoot<I const&> const& that) noexcept {
return detail::PolyAccess::move(that);
}
/// \cond
/// \overload
[[noreturn]] inline detail::ArchetypeBase poly_move(
detail::ArchetypeBase const&) noexcept {
assume_unreachable();
}
/// \endcond
/// \overload
template <class Poly, std::enable_if_t<detail::IsPoly<Poly>::value, int> = 0>
constexpr auto poly_move(Poly& that) noexcept
-> decltype(poly_move(that.get())) {
return poly_move(that.get());
}
/// \cond
namespace detail {
/**
* The implementation for `Poly` for when the interface type is not
* reference-like qualified, as in `Poly<SemiRegular>`.
*/
template <class I>
struct PolyVal : PolyImpl<I> {
private:
friend PolyAccess;
struct NoneSuch {};
using Copyable = std::is_copy_constructible<PolyImpl<I>>;
using PolyOrNonesuch = If<Copyable::value, PolyVal, NoneSuch>;
using PolyRoot<I>::vptr_;
PolyRoot<I>& _polyRoot_() noexcept { return *this; }
PolyRoot<I> const& _polyRoot_() const noexcept { return *this; }
Data* _data_() noexcept { return PolyAccess::data(*this); }
Data const* _data_() const noexcept { return PolyAccess::data(*this); }
public:
/**
* Default constructor.
* \post `poly_empty(*this) == true`
*/
PolyVal() = default;
/**
* Move constructor.
* \post `poly_empty(that) == true`
*/
PolyVal(PolyVal&& that) noexcept;
/**
* A copy constructor if `I` is copyable; otherwise, a useless constructor
* from a private, incomplete type.
*/
/* implicit */ PolyVal(PolyOrNonesuch const& that);
~PolyVal();
/**
* Inherit any constructors defined by any of the interfaces.
*/
using PolyImpl<I>::PolyImpl;
/**
* Copy assignment, destroys the object currently held (if any) and makes
* `*this` equal to `that` by stealing its guts.
*/
Poly<I>& operator=(PolyVal that) noexcept;
/**
* Construct a Poly<I> from a concrete type that satisfies the I concept
*/
template <class T, std::enable_if_t<ModelsInterface<T, I>::value, int> = 0>
/* implicit */ PolyVal(T&& t);
/**
* Construct a `Poly` from a compatible `Poly`. "Compatible" here means: the
* other interface extends this one either directly or indirectly.
*/
template <class I2, std::enable_if_t<ValueCompatible<I, I2>::value, int> = 0>
/* implicit */ PolyVal(Poly<I2> that);
/**
* Assign to this `Poly<I>` from a concrete type that satisfies the `I`
* concept.
*/
template <class T, std::enable_if_t<ModelsInterface<T, I>::value, int> = 0>
Poly<I>& operator=(T&& t);
/**
* Assign a compatible `Poly` to `*this`. "Compatible" here means: the
* other interface extends this one either directly or indirectly.
*/
template <class I2, std::enable_if_t<ValueCompatible<I, I2>::value, int> = 0>
Poly<I>& operator=(Poly<I2> that);
/**
* Swaps the values of two `Poly` objects.
*/
void swap(Poly<I>& that) noexcept;
};
////////////////////////////////////////////////////////////////////////////////
/**
* The implementation of `Poly` for when the interface type is
* reference-qualified, like `Poly<SemiRegular &>`.
*/
template <class I>
struct PolyRef : private PolyImpl<I> {
private:
friend PolyAccess;
AddCvrefOf<PolyRoot<I>, I>& _polyRoot_() const noexcept;
Data* _data_() noexcept { return PolyAccess::data(*this); }
Data const* _data_() const noexcept { return PolyAccess::data(*this); }
static constexpr RefType refType() noexcept;
protected:
template <class That, class I2>
PolyRef(That&& that, Type<I2>);
public:
/**
* Copy constructor
* \post `&poly_cast<T>(*this) == &poly_cast<T>(that)`, where `T` is the
* type of the object held by `that`.
*/
PolyRef(PolyRef const& that) noexcept;
/**
* Copy assignment
* \post `&poly_cast<T>(*this) == &poly_cast<T>(that)`, where `T` is the
* type of the object held by `that`.
*/
Poly<I>& operator=(PolyRef const& that) noexcept;
/**
* Construct a `Poly<I>` from a concrete type that satisfies concept `I`.
* \post `!poly_empty(*this)`
*/
template <class T, std::enable_if_t<ModelsInterface<T, I>::value, int> = 0>
/* implicit */ PolyRef(T&& t) noexcept;
/**
* Construct a `Poly<I>` from a compatible `Poly<I2>`.
*/
template <
class I2,
std::enable_if_t<ReferenceCompatible<I, I2, I2&&>::value, int> = 0>
/* implicit */ PolyRef(Poly<I2>&& that) noexcept(
std::is_reference<I2>::value);
template <
class I2,
std::enable_if_t<ReferenceCompatible<I, I2, I2&>::value, int> = 0>
/* implicit */ PolyRef(Poly<I2>& that) noexcept(std::is_reference<I2>::value)
: PolyRef{that, Type<I2>{}} {}
template <
class I2,
std::enable_if_t<ReferenceCompatible<I, I2, I2 const&>::value, int> = 0>
/* implicit */ PolyRef(Poly<I2> const& that) noexcept(
std::is_reference<I2>::value)
: PolyRef{that, Type<I2>{}} {}
/**
* Assign to a `Poly<I>` from a concrete type that satisfies concept `I`.
* \post `!poly_empty(*this)`
*/
template <class T, std::enable_if_t<ModelsInterface<T, I>::value, int> = 0>
Poly<I>& operator=(T&& t) noexcept;
/**
* Assign to `*this` from another compatible `Poly`.
*/
template <
class I2,
std::enable_if_t<ReferenceCompatible<I, I2, I2&&>::value, int> = 0>
Poly<I>& operator=(Poly<I2>&& that) noexcept(std::is_reference<I2>::value);
/**
* \overload
*/
template <
class I2,
std::enable_if_t<ReferenceCompatible<I, I2, I2&>::value, int> = 0>
Poly<I>& operator=(Poly<I2>& that) noexcept(std::is_reference<I2>::value);
/**
* \overload
*/
template <
class I2,
std::enable_if_t<ReferenceCompatible<I, I2, I2 const&>::value, int> = 0>
Poly<I>& operator=(Poly<I2> const& that) noexcept(
std::is_reference<I2>::value);
/**
* Swap which object this `Poly` references ("shallow" swap).
*/
void swap(Poly<I>& that) noexcept;
/**
* Get a reference to the interface, with correct `const`-ness applied.
*/
AddCvrefOf<PolyImpl<I>, I>& get() const noexcept;
/**
* Get a reference to the interface, with correct `const`-ness applied.
*/
AddCvrefOf<PolyImpl<I>, I>& operator*() const noexcept { return get(); }
/**
* Get a pointer to the interface, with correct `const`-ness applied.
*/
auto operator->() const noexcept { return &get(); }
};
template <class I>
using PolyValOrRef = If<std::is_reference<I>::value, PolyRef<I>, PolyVal<I>>;
} // namespace detail
/// \endcond
/**
* `Poly` is a class template that makes it relatively easy to define a
* type-erasing polymorphic object wrapper.
*
* \par Type-erasure
*
* \par
* `std::function` is one example of a type-erasing polymorphic object wrapper;
* `folly::exception_wrapper` is another. Type-erasure is often used as an
* alternative to dynamic polymorphism via inheritance-based virtual dispatch.
* The distinguishing characteristic of type-erasing wrappers are:
* \li **Duck typing:** Types do not need to inherit from an abstract base
* class in order to be assignable to a type-erasing wrapper; they merely
* need to satisfy a particular interface.
* \li **Value semantics:** Type-erasing wrappers are objects that can be
* passed around _by value_. This is in contrast to abstract base classes
* which must be passed by reference or by pointer or else suffer from
* _slicing_, which causes them to lose their polymorphic behaviors.
* Reference semantics make it difficult to reason locally about code.
* \li **Automatic memory management:** When dealing with inheritance-based
* dynamic polymorphism, it is often necessary to allocate and manage
* objects on the heap. This leads to a proliferation of `shared_ptr`s and
* `unique_ptr`s in APIs, complicating their point-of-use. APIs that take
* type-erasing wrappers, on the other hand, can often store small objects
* in-situ, with no dynamic allocation. The memory management, if any, is
* handled for you, and leads to cleaner APIs: consumers of your API don't
* need to pass `shared_ptr<AbstractBase>`; they can simply pass any object
* that satisfies the interface you require. (`std::function` is a
* particularly compelling example of this benefit. Far worse would be an
* inheritance-based callable solution like
* `shared_ptr<ICallable<void(int)>>`. )
*
* \par Example: Defining a type-erasing function wrapper with `folly::Poly`
*
* \par
* Defining a polymorphic wrapper with `Poly` is a matter of defining two
* things:
* \li An *interface*, consisting of public member functions, and
* \li A *mapping* from a concrete type to a set of member function bindings.
*
* Below is a (heavily commented) example of a simple implementation of a
* `std::function`-like polymorphic wrapper. Its interface has only a single
* member function: `operator()`
*
* // An interface for a callable object of a particular signature, Fun
* // (most interfaces don't need to be templates, FWIW).
* template <class Fun>
* struct IFunction;
*
* template <class R, class... As>
* struct IFunction<R(As...)> {
* // An interface is defined as a nested class template called
* // Interface that takes a single template parameter, Base, from
* // which it inherits.
* template <class Base>
* struct Interface : Base {
* // The Interface has public member functions. These become the
* // public interface of the resulting Poly instantiation.
* // (Implementation note: Poly<IFunction<Sig>> will publicly
* // inherit from this struct, which is what gives it the right
* // member functions.)
* R operator()(As... as) const {
* // The definition of each member function in your interface will
* // always consist of a single line dispatching to
* // folly::poly_call<N>. The "N" corresponds to the N-th member
* // function in the list of member function bindings, Members,
* // defined below. The first argument will always be *this, and the
* // rest of the arguments should simply forward (if necessary) the
* // member function's arguments.
* return static_cast<R>(
* folly::poly_call<0>(*this, std::forward<As>(as)...));
* }
* };
*
* // The "Members" alias template is a comma-separated list of bound
* // member functions for a given concrete type "T". The
* // "FOLLY_POLY_MEMBERS" macro accepts a comma-separated list, and the
* // (optional) "FOLLY_POLY_MEMBER" macro lets you disambiguate overloads
* // by explicitly specifying the function signature the target member
* // function should have. In this case, we require "T" to have a
* // function call operator with the signature `R(As...) const`.
* //
* // If you are using a C++17-compatible compiler, you can do away with
* // the macros and write this as:
* //
* // template <class T>
* // using Members = folly::PolyMembers<
* // folly::sig<R(As...) const>(&T::operator())>;
* //
* // And since `folly::sig` is only needed for disambiguation in case of
* // overloads, if you are not concerned about objects with overloaded
* // function call operators, it could be further simplified to:
* //
* // template <class T>
* // using Members = folly::PolyMembers<&T::operator()>;
* //
* template <class T>
* using Members = FOLLY_POLY_MEMBERS(
* FOLLY_POLY_MEMBER(R(As...) const, &T::operator()));
* };
*
* // Now that we have defined the interface, we can pass it to Poly to
* // create our type-erasing wrapper:
* template <class Fun>
* using Function = Poly<IFunction<Fun>>;
*
* \par
* Given the above definition of `Function`, users can now initialize instances
* of (say) `Function<int(int, int)>` with function objects like
* `std::plus<int>` and `std::multiplies<int>`, as below:
*
* Function<int(int, int)> fun = std::plus<int>{};
* assert(5 == fun(2, 3));
* fun = std::multiplies<int>{};
* assert(6 = fun(2, 3));
*
* \par Defining an interface with C++17
*
* \par
* With C++17, defining an interface to be used with `Poly` is fairly
* straightforward. As in the `Function` example above, there is a struct with
* a nested `Interface` class template and a nested `Members` alias template.
* No macros are needed with C++17.
* \par
* Imagine we were defining something like a Java-style iterator. If we are
* using a C++17 compiler, our interface would look something like this:
*
* template <class Value>
* struct IJavaIterator {
* template <class Base>
* struct Interface : Base {
* bool Done() const { return folly::poly_call<0>(*this); }
* Value Current() const { return folly::poly_call<1>(*this); }
* void Next() { folly::poly_call<2>(*this); }
* };
* // NOTE: This works in C++17 only:
* template <class T>
* using Members = folly::PolyMembers<&T::Done, &T::Current, &T::Next>;
* };
*
* template <class Value>
* using JavaIterator = Poly<IJavaIterator>;
*
* \par
* Given the above definition, `JavaIterator<int>` can be used to hold instances
* of any type that has `Done`, `Current`, and `Next` member functions with the
* correct (or compatible) signatures.
*
* \par
* The presence of overloaded member functions complicates this picture. Often,
* property members are faked in C++ with `const` and non-`const` member
* function overloads, like in the interface specified below:
*
* struct IIntProperty {
* template <class Base>
* struct Interface : Base {
* int Value() const { return folly::poly_call<0>(*this); }
* void Value(int i) { folly::poly_call<1>(*this, i); }
* };
* // NOTE: This works in C++17 only:
* template <class T>
* using Members = folly::PolyMembers<
* folly::sig<int() const>(&T::Value),
* folly::sig<void(int)>(&T::Value)>;
* };
*
* using IntProperty = Poly<IIntProperty>;
*
* \par
* Now, any object that has `Value` members of compatible signatures can be
* assigned to instances of `IntProperty` object. Note how `folly::sig` is used
* to disambiguate the overloads of `&T::Value`.
*
* \par Defining an interface with C++14
*
* \par
* In C++14, the nice syntax above doesn't work, so we have to resort to macros.
* The two examples above would look like this:
*
* template <class Value>
* struct IJavaIterator {
* template <class Base>
* struct Interface : Base {
* bool Done() const { return folly::poly_call<0>(*this); }
* Value Current() const { return folly::poly_call<1>(*this); }
* void Next() { folly::poly_call<2>(*this); }
* };
* // NOTE: This works in C++14 and C++17:
* template <class T>
* using Members = FOLLY_POLY_MEMBERS(&T::Done, &T::Current, &T::Next);
* };
*
* template <class Value>
* using JavaIterator = Poly<IJavaIterator>;
*
* \par
* and
*
* struct IIntProperty {
* template <class Base>
* struct Interface : Base {
* int Value() const { return folly::poly_call<0>(*this); }
* void Value(int i) { return folly::poly_call<1>(*this, i); }
* };
* // NOTE: This works in C++14 and C++17:
* template <class T>
* using Members = FOLLY_POLY_MEMBERS(
* FOLLY_POLY_MEMBER(int() const, &T::Value),
* FOLLY_POLY_MEMBER(void(int), &T::Value));
* };
*
* using IntProperty = Poly<IIntProperty>;
*
* \par Extending interfaces
*
* \par
* One typical advantage of inheritance-based solutions to runtime polymorphism
* is that one polymorphic interface could extend another through inheritance.
* The same can be accomplished with type-erasing polymorphic wrappers. In
* the `Poly` library, you can use `folly::PolyExtends` to say that one
* interface extends another.
*
* struct IFoo {
* template <class Base>
* struct Interface : Base {
* void Foo() const { return folly::poly_call<0>(*this); }
* };
* template <class T>
* using Members = FOLLY_POLY_MEMBERS(&T::Foo);
* };
*
* // The IFooBar interface extends the IFoo interface
* struct IFooBar : PolyExtends<IFoo> {
* template <class Base>
* struct Interface : Base {
* void Bar() const { return folly::poly_call<0>(*this); }
* };
* template <class T>
* using Members = FOLLY_POLY_MEMBERS(&T::Bar);
* };
*
* using FooBar = Poly<IFooBar>;
*
* \par
* Given the above defintion, instances of type `FooBar` have both `Foo()` and
* `Bar()` member functions.
*
* \par
* The sensible conversions exist between a wrapped derived type and a wrapped
* base type. For instance, assuming `IDerived` extends `IBase` with
* `PolyExtends`:
*
* Poly<IDerived> derived = ...;
* Poly<IBase> base = derived; // This conversion is OK.
*
* \par
* As you would expect, there is no conversion in the other direction, and at
* present there is no `Poly` equivalent to `dynamic_cast`.
*
* \par Type-erasing polymorphic reference wrappers
*
* \par
* Sometimes you don't need to own a copy of an object; a reference will do. For
* that you can use `Poly` to capture a _reference_ to an object satisfying an
* interface rather than the whole object itself. The syntax is intuitive.
*
* int i = 42;
* // Capture a mutable reference to an object of any IRegular type:
* Poly<IRegular &> intRef = i;
* assert(42 == folly::poly_cast<int>(intRef));
* // Assert that we captured the address of "i":
* assert(&i == &folly::poly_cast<int>(intRef));
*
* \par
* A reference-like `Poly` has a different interface than a value-like `Poly`.
* Rather than calling member functions with the `obj.fun()` syntax, you would
* use the `obj->fun()` syntax. This is for the sake of `const`-correctness.
* For example, consider the code below:
*
* struct IFoo {
* template <class Base>
* struct Interface {
* void Foo() { folly::poly_call<0>(*this); }
* };
* template <class T>
* using Members = folly::PolyMembers<&T::Foo>;
* };
*
* struct SomeFoo {
* void Foo() { std::printf("SomeFoo::Foo\n"); }
* };
*
* SomeFoo foo;
* Poly<IFoo &> const anyFoo = foo;
* anyFoo->Foo(); // prints "SomeFoo::Foo"
*
* \par
* Notice in the above code that the `Foo` member function is non-`const`.
* Notice also that the `anyFoo` object is `const`. However, since it has
* captured a non-`const` reference to the `foo` object, it should still be
* possible to dispatch to the non-`const` `Foo` member function. When
* instantiated with a reference type, `Poly` has an overloaded `operator->`
* member that returns a pointer to the `IFoo` interface with the correct
* `const`-ness, which makes this work.
*
* \par
* The same mechanism also prevents users from calling non-`const` member
* functions on `Poly` objects that have captured `const` references, which
* would violate `const`-correctness.
*
* \par
* Sensible conversions exist between non-reference and reference `Poly`s. For
* instance:
*
* Poly<IRegular> value = 42;
* Poly<IRegular &> mutable_ref = value;
* Poly<IRegular const &> const_ref = mutable_ref;
*
* assert(&poly_cast<int>(value) == &poly_cast<int>(mutable_ref));
* assert(&poly_cast<int>(value) == &poly_cast<int>(const_ref));
*
* \par Non-member functions (C++17)
*
* \par
* If you wanted to write the interface `ILogicallyNegatable`, which captures
* all types that can be negated with unary `operator!`, you could do it
* as we've shown above, by binding `&T::operator!` in the nested `Members`
* alias template, but that has the problem that it won't work for types that
* have defined unary `operator!` as a free function. To handle this case,
* the `Poly` library lets you use a free function instead of a member function
* when creating a binding.
*
* \par
* With C++17 you may use a lambda to create a binding, as shown in the example
* below:
*
* struct ILogicallyNegatable {
* template <class Base>
* struct Interface : Base {
* bool operator!() const { return folly::poly_call<0>(*this); }
* };
* template <class T>
* using Members = folly::PolyMembers<
* +[](T const& t) -> decltype(!t) { return !t; }>;
* };
*
* \par
* This requires some explanation. The unary `operator+` in front of the lambda
* is necessary! It causes the lambda to decay to a C-style function pointer,
* which is one of the types that `folly::PolyMembers` accepts. The `decltype`
* in the lambda return type is also necessary. Through the magic of SFINAE, it
* will cause `Poly<ILogicallyNegatable>` to reject any types that don't support
* unary `operator!`.
*
* \par
* If you are using a free function to create a binding, the first parameter is
* implicitly the `this` parameter. It will receive the type-erased object.
*
* \par Non-member functions (C++14)
*
* \par
* If you are using a C++14 compiler, the defintion of `ILogicallyNegatable`
* above will fail because lambdas are not `constexpr`. We can get the same
* effect by writing the lambda as a named free function, as show below:
*
* struct ILogicallyNegatable {
* template <class Base>
* struct Interface : Base {
* bool operator!() const { return folly::poly_call<0>(*this); }
* };
*
* template <class T>
* static auto negate(T const& t) -> decltype(!t) { return !t; }
*
* template <class T>
* using Members = FOLLY_POLY_MEMBERS(&negate<T>);
* };
*
* \par
* As with the example that uses the lambda in the preceding section, the first
* parameter is implicitly the `this` parameter. It will receive the type-erased
* object.
*
* \par Multi-dispatch
*
* \par
* What if you want to create an `IAddable` interface for things that can be
* added? Adding requires _two_ objects, both of which are type-erased. This
* interface requires dispatching on both objects, doing the addition only
* if the types are the same. For this we make use of the `PolySelf` template
* alias to define an interface that takes more than one object of the
* erased type.
*
* struct IAddable {
* template <class Base>
* struct Interface : Base {
* friend PolySelf<Base, Decay>
* operator+(PolySelf<Base> const& a, PolySelf<Base> const& b) {
* return folly::poly_call<0, IAddable>(a, b);
* }
* };
*
* template <class T>
* using Members = folly::PolyMembers<
* +[](T const& a, T const& b) -> decltype(a + b) { return a + b; }>;
* };
*
* \par
* Given the above defintion of `IAddable` we would be able to do the following:
*
* Poly<IAddable> a = 2, b = 3;
* Poly<IAddable> c = a + b;
* assert(poly_cast<int>(c) == 5);
*
* \par
* If `a` and `b` stored objects of different types, a `BadPolyCast` exception
* would be thrown.
*
* \par Move-only types
*
* \par
* If you want to store move-only types, then your interface should extend the
* `IMoveOnly` interface.
*
* \par Implementation notes
* \par
* `Poly` will store "small" objects in an internal buffer, avoiding the cost of
* of dynamic allocations. At present, this size is not configurable; it is
* pegged at the size of two `double`s.
*
* \par
* `Poly` objects are always nothrow movable. If you store an object in one that
* has a potentially throwing move contructor, the object will be stored on the
* heap, even if it could fit in the internal storage of the `Poly` object.
* (So be sure to give your objects nothrow move constructors!)
*
* \par
* `Poly` implements type-erasure in a manner very similar to how the compiler
* accomplishes virtual dispatch. Every `Poly` object contains a pointer to a
* table of function pointers. Member function calls involve a double-
* indirection: once through the v-pointer, and other indirect function call
* through the function pointer.
*/
template <class I>
struct Poly final : detail::PolyValOrRef<I> {
friend detail::PolyAccess;
Poly() = default;
using detail::PolyValOrRef<I>::PolyValOrRef;
using detail::PolyValOrRef<I>::operator=;
};
/**
* Swap two `Poly<I>` instances.
*/
template <class I>
void swap(Poly<I>& left, Poly<I>& right) noexcept {
left.swap(right);
}
/**
* Pseudo-function template handy for disambiguating function overloads.
*
* For example, given:
* struct S {
* int property() const;
* void property(int);
* };
*
* You can get a member function pointer to the first overload with:
* folly::sig<int()const>(&S::property);
*
* This is arguably a nicer syntax that using the built-in `static_cast`:
* static_cast<int (S::*)() const>(&S::property);
*
* `sig` is also more permissive than `static_cast` about `const`. For instance,
* the following also works:
* folly::sig<int()>(&S::property);
*
* The above is permitted
*/
template <class Sig>
inline constexpr detail::Sig<Sig> const sig = {};
} // namespace folly
#include <folly/Poly-inl.h>
Codebase Browser
folly