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/**
* Copyright (C) 2018-present MongoDB, Inc.
*
* This program is free software: you can redistribute it and/or modify
* it under the terms of the Server Side Public License, version 1,
* as published by MongoDB, Inc.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* Server Side Public License for more details.
*
* You should have received a copy of the Server Side Public License
* along with this program. If not, see
* <http://www.mongodb.com/licensing/server-side-public-license>.
*
* As a special exception, the copyright holders give permission to link the
* code of portions of this program with the OpenSSL library under certain
* conditions as described in each individual source file and distribute
* linked combinations including the program with the OpenSSL library. You
* must comply with the Server Side Public License in all respects for
* all of the code used other than as permitted herein. If you modify file(s)
* with this exception, you may extend this exception to your version of the
* file(s), but you are not obligated to do so. If you do not wish to do so,
* delete this exception statement from your version. If you delete this
* exception statement from all source files in the program, then also delete
* it in the license file.
*/
#pragma once
#include <boost/intrusive_ptr.hpp>
#include <stdlib.h>
#include "mongo/base/disallow_copying.h"
#include "mongo/base/string_data.h"
#include "mongo/platform/atomic_word.h"
#include "mongo/util/allocator.h"
namespace mongo {
/// This is an alternative base class to the above ones (will replace them eventually)
class RefCountable {
MONGO_DISALLOW_COPYING(RefCountable);
public:
/// If false you have exclusive access to this object. This is useful for implementing COW.
bool isShared() const {
// This needs to be at least acquire to ensure that it is sequenced-after any
// intrusive_ptr_release calls. Otherwise there is a subtle race where the releaser's memory
// accesses could see writes done by a thread that thinks it has exclusive access to this
// object. Note that acquire reads are free on x86 and cheap on most other platforms.
return _count.load(std::memory_order_acquire) > 1;
}
/**
* Sets the refcount to count, assuming it is currently one less. This should only be used
* during logical initialization before another thread could possibly have access to this
* object.
*/
void threadUnsafeIncRefCountTo(uint32_t count) const {
dassert(_count.load(std::memory_order_relaxed) == (count - 1));
_count.store(count, std::memory_order_relaxed);
}
friend void intrusive_ptr_add_ref(const RefCountable* ptr) {
// See this for a description of why relaxed is OK here. It is also used in libc++.
// http://www.boost.org/doc/libs/1_66_0/doc/html/atomic/usage_examples.html#boost_atomic.usage_examples.example_reference_counters.discussion
ptr->_count.fetch_add(1, std::memory_order_relaxed);
};
friend void intrusive_ptr_release(const RefCountable* ptr) {
if (ptr->_count.fetch_sub(1, std::memory_order_acq_rel) == 1) {
delete ptr;
}
};
protected:
RefCountable() {}
virtual ~RefCountable() {}
private:
mutable std::atomic<uint32_t> _count{0}; // NOLINT
};
template <typename T,
typename... Args,
typename = std::enable_if_t<std::is_base_of<RefCountable, T>::value>>
boost::intrusive_ptr<T> make_intrusive(Args&&... args) {
auto ptr = new T(std::forward<Args>(args)...);
ptr->threadUnsafeIncRefCountTo(1);
return boost::intrusive_ptr<T>(ptr, /*add ref*/ false);
}
/// This is an immutable reference-counted string
class RCString : public RefCountable {
public:
const char* c_str() const {
return reinterpret_cast<const char*>(this) + sizeof(RCString);
}
int size() const {
return _size;
}
StringData stringData() const {
return StringData(c_str(), _size);
}
static boost::intrusive_ptr<const RCString> create(StringData s);
// MSVC: C4291: 'declaration' : no matching operator delete found; memory will not be freed if
// initialization throws an exception
// We simply rely on the default global placement delete since a local placement delete would be
// ambiguous for some compilers
#pragma warning(push)
#pragma warning(disable : 4291)
void operator delete(void* ptr) {
free(ptr);
}
#pragma warning(pop)
private:
// these can only be created by calling create()
RCString(){};
void* operator new(size_t objSize, size_t realSize) {
return mongoMalloc(realSize);
}
int _size; // does NOT include trailing NUL byte.
// char[_size+1] array allocated past end of class
};
} // namespace mongo
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