/*
* 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.
*/
/**
* Discriminated pointer: Type-safe pointer to one of several types.
*
* Similar to std::variant, but has no space overhead over a raw pointer, as
* it relies on the fact that (on x86_64) there are 16 unused bits in a
* pointer.
*/
#pragma once
#include <limits>
#include <stdexcept>
#include <glog/logging.h>
#include <folly/Likely.h>
#include <folly/Portability.h>
#include <folly/detail/DiscriminatedPtrDetail.h>
#if !FOLLY_X64 && !FOLLY_AARCH64 && !FOLLY_PPC64 && !FOLLY_RISCV64
#error "DiscriminatedPtr is x64, arm64, ppc64 and riscv64 specific code."
#endif
namespace folly {
/**
* Discriminated pointer.
*
* Given a list of types, a DiscriminatedPtr<Types...> may point to an object
* of one of the given types, or may be empty. DiscriminatedPtr is type-safe:
* you may only get a pointer to the type that you put in, otherwise get
* throws an exception (and get_nothrow returns nullptr)
*
* This pointer does not do any kind of lifetime management -- it's not a
* "smart" pointer. You are responsible for deallocating any memory used
* to hold pointees, if necessary.
*/
template <typename... Types>
class DiscriminatedPtr {
// <, not <=, as our indexes are 1-based (0 means "empty")
static_assert(
sizeof...(Types) < std::numeric_limits<uint16_t>::max(),
"too many types");
public:
/**
* Create an empty DiscriminatedPtr.
*/
DiscriminatedPtr() : data_(0) {}
/**
* Create a DiscriminatedPtr that points to an object of type T.
* Fails at compile time if T is not a valid type (listed in Types)
*/
template <typename T>
explicit DiscriminatedPtr(T* ptr) {
set(ptr, typeIndex<T>());
}
/**
* Set this DiscriminatedPtr to point to an object of type T.
* Fails at compile time if T is not a valid type (listed in Types)
*/
template <typename T>
void set(T* ptr) {
set(ptr, typeIndex<T>());
}
/**
* Get a pointer to the object that this DiscriminatedPtr points to, if it is
* of type T. Fails at compile time if T is not a valid type (listed in
* Types), and returns nullptr if this DiscriminatedPtr is empty or points to
* an object of a different type.
*/
template <typename T>
T* get_nothrow() noexcept {
void* p = FOLLY_LIKELY(hasType<T>()) ? ptr() : nullptr;
return static_cast<T*>(p);
}
template <typename T>
const T* get_nothrow() const noexcept {
const void* p = FOLLY_LIKELY(hasType<T>()) ? ptr() : nullptr;
return static_cast<const T*>(p);
}
/**
* Get a pointer to the object that this DiscriminatedPtr points to, if it is
* of type T. Fails at compile time if T is not a valid type (listed in
* Types), and throws std::invalid_argument if this DiscriminatedPtr is empty
* or points to an object of a different type.
*/
template <typename T>
T* get() {
if (FOLLY_UNLIKELY(!hasType<T>())) {
throw std::invalid_argument("Invalid type");
}
return static_cast<T*>(ptr());
}
template <typename T>
const T* get() const {
if (FOLLY_UNLIKELY(!hasType<T>())) {
throw std::invalid_argument("Invalid type");
}
return static_cast<const T*>(ptr());
}
/**
* Return true iff this DiscriminatedPtr is empty.
*/
bool empty() const { return index() == 0; }
/**
* Return true iff the object pointed by this DiscriminatedPtr has type T,
* false otherwise. Fails at compile time if T is not a valid type (listed
* in Types...)
*/
template <typename T>
bool hasType() const {
return index() == typeIndex<T>();
}
/**
* Clear this DiscriminatedPtr, making it empty.
*/
void clear() { data_ = 0; }
/**
* Assignment operator from a pointer of type T.
*/
template <typename T>
DiscriminatedPtr& operator=(T* ptr) {
set(ptr);
return *this;
}
/**
* Apply a visitor to this object, calling the appropriate overload for
* the type currently stored in DiscriminatedPtr. Throws invalid_argument
* if the DiscriminatedPtr is empty.
*
* The visitor must meet the following requirements:
*
* - The visitor must allow invocation as a function by overloading
* operator(), unambiguously accepting all values of type T* (or const T*)
* for all T in Types...
* - All operations of the function object on T* (or const T*) must
* return the same type (or a static_assert will fire).
*/
template <typename V>
typename dptr_detail::VisitorResult<V, Types...>::type apply(V&& visitor) {
size_t n = index();
if (n == 0) {
throw std::invalid_argument("Empty DiscriminatedPtr");
}
return dptr_detail::ApplyVisitor<V, Types...>()(
n, std::forward<V>(visitor), ptr());
}
template <typename V>
typename dptr_detail::ConstVisitorResult<V, Types...>::type apply(
V&& visitor) const {
size_t n = index();
if (n == 0) {
throw std::invalid_argument("Empty DiscriminatedPtr");
}
return dptr_detail::ApplyConstVisitor<V, Types...>()(
n, std::forward<V>(visitor), ptr());
}
private:
/**
* Get the 1-based type index of T in Types.
*/
template <typename T>
uint16_t typeIndex() const {
return uint16_t(dptr_detail::GetTypeIndex<T, Types...>::value);
}
uint16_t index() const { return data_ >> 48; }
void* ptr() const {
return reinterpret_cast<void*>(data_ & ((1ULL << 48) - 1));
}
void set(void* p, uint16_t v) {
uintptr_t ip = reinterpret_cast<uintptr_t>(p);
CHECK(!(ip >> 48));
ip |= static_cast<uintptr_t>(v) << 48;
data_ = ip;
}
/**
* We store a pointer in the least significant 48 bits of data_, and a type
* index (0 = empty, or 1-based index in Types) in the most significant 16
* bits. We rely on the fact that pointers have their most significant 16
* bits clear on x86_64.
*/
uintptr_t data_;
};
template <typename Visitor, typename... Args>
decltype(auto) apply_visitor(
Visitor&& visitor, const DiscriminatedPtr<Args...>& variant) {
return variant.apply(std::forward<Visitor>(visitor));
}
template <typename Visitor, typename... Args>
decltype(auto) apply_visitor(
Visitor&& visitor, DiscriminatedPtr<Args...>& variant) {
return variant.apply(std::forward<Visitor>(visitor));
}
template <typename Visitor, typename... Args>
decltype(auto) apply_visitor(
Visitor&& visitor, DiscriminatedPtr<Args...>&& variant) {
return variant.apply(std::forward<Visitor>(visitor));
}
} // namespace folly
Codebase Browser
folly