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// Copyright Yahoo. Licensed under the terms of the Apache 2.0 license. See LICENSE in the project root.
#pragma once
#include <atomic>
#include <concepts>
#include <memory>
#include <utility>
// This file contains code that implements intrusive reference
// counting with smart handles (ref_counted<T> points to T, T inherits
// enable_ref_counted).
// Functions with names starting with 'internal' are not intended for
// direct use, but are available for completeness (enables incremental
// re-write of code from bald to smart pointers).
namespace vespalib {
// This is the actual reference count. It cannot be moved or copied,
// but it can be changed even when const. Classes that want to be
// counted need to inherit from this. The destructor is virtual to
// make sure the right one is called. This also enables the subref
// function to directly delete the shared object.
class enable_ref_counted
{
static constexpr uint32_t MAGIC = 0xcc56a933;
private:
uint32_t _guard;
mutable std::atomic<int32_t> _refs;
protected:
enable_ref_counted() noexcept : _guard(MAGIC), _refs(1) {}
public:
virtual ~enable_ref_counted() noexcept;
enable_ref_counted(enable_ref_counted &&) = delete;
enable_ref_counted(const enable_ref_counted &) = delete;
enable_ref_counted &operator=(enable_ref_counted &&) = delete;
enable_ref_counted &operator=(const enable_ref_counted &) = delete;
void internal_addref() const noexcept;
void internal_subref() const noexcept;
int32_t count_refs() const noexcept;
};
// This is the handle to a shared object. The handle itself is not
// thread safe.
template <typename T>
class ref_counted
{
// carefully placed here to give the best error messages
static_assert(std::derived_from<T,enable_ref_counted>);
template <typename X> friend class ref_counted;
private:
T *_ptr;
ref_counted(T *ptr) noexcept : _ptr(ptr) {}
void maybe_subref() noexcept {
if (_ptr) [[likely]] {
_ptr->internal_subref();
}
}
static T *maybe_addref(T *ptr) noexcept {
if (ptr) [[likely]] {
ptr->internal_addref();
}
return ptr;
}
void replace_with(T *ptr) noexcept {
maybe_subref();
_ptr = ptr;
}
public:
ref_counted() noexcept : _ptr(nullptr) {}
ref_counted(ref_counted &&rhs) noexcept : _ptr(std::exchange(rhs._ptr, nullptr)) {}
ref_counted(const ref_counted &rhs) noexcept : _ptr(maybe_addref(rhs._ptr)) {}
ref_counted &operator=(ref_counted &&rhs) noexcept {
replace_with(std::exchange(rhs._ptr, nullptr));
return *this;
}
ref_counted &operator=(const ref_counted &rhs) noexcept {
replace_with(maybe_addref(rhs._ptr));
return *this;
}
template <typename X> ref_counted(ref_counted<X> &&rhs) noexcept : _ptr(std::exchange(rhs._ptr, nullptr)) {}
template <typename X> ref_counted(const ref_counted<X> &rhs) noexcept : _ptr(maybe_addref(rhs._ptr)) {}
template <typename X> ref_counted &operator=(ref_counted<X> &&rhs) noexcept {
replace_with(std::exchange(rhs._ptr, nullptr));
return *this;
}
template <typename X> ref_counted &operator=(const ref_counted<X> &rhs) noexcept {
replace_with(maybe_addref(rhs._ptr));
return *this;
}
T *operator->() const noexcept { return _ptr; }
T &operator*() const noexcept { return *_ptr; }
operator bool() const noexcept { return (_ptr != nullptr); }
~ref_counted() noexcept { maybe_subref(); }
// NB: will not call subref
T *internal_detach() noexcept { return std::exchange(_ptr, nullptr); }
// NB: will not call addref
static ref_counted internal_attach(T *ptr) noexcept { return ref_counted(ptr); }
};
// similar to make_shared;
// create a reference counted object and return a handle to it
template <typename T, typename ... Args>
ref_counted<T> make_ref_counted(Args && ... args) {
return ref_counted<T>::internal_attach(new T(std::forward<Args>(args)...));
}
// similar to shared_from_this;
// create a new handle to a reference counted object
// (NB: object must still be valid)
template <typename T>
ref_counted<T> ref_counted_from(T &t) noexcept {
t.internal_addref();
return ref_counted<T>::internal_attach(std::addressof(t));
}
}
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