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/*
* Copyright (c) 2016-present, Facebook, Inc.
* All rights reserved.
*
* This source code is licensed under both the BSD-style license (found in the
* LICENSE file in the root directory of this source tree) and the GPLv2 (found
* in the COPYING file in the root directory of this source tree).
*/
#pragma once
#include <cassert>
#include <functional>
#include <memory>
#include <mutex>
#include <vector>
namespace pzstd {
/**
* An unbounded pool of resources.
* A `ResourcePool<T>` requires a factory function that takes allocates `T*` and
* a free function that frees a `T*`.
* Calling `ResourcePool::get()` will give you a new `ResourcePool::UniquePtr`
* to a `T`, and when it goes out of scope the resource will be returned to the
* pool.
* The `ResourcePool<T>` *must* survive longer than any resources it hands out.
* Remember that `ResourcePool<T>` hands out mutable `T`s, so make sure to clean
* up the resource before or after every use.
*/
template <typename T>
class ResourcePool {
public:
class Deleter;
using Factory = std::function<T*()>;
using Free = std::function<void(T*)>;
using UniquePtr = std::unique_ptr<T, Deleter>;
private:
std::mutex mutex_;
Factory factory_;
Free free_;
std::vector<T*> resources_;
unsigned inUse_;
public:
/**
* Creates a `ResourcePool`.
*
* @param factory The function to use to create new resources.
* @param free The function to use to free resources created by `factory`.
*/
ResourcePool(Factory factory, Free free)
: factory_(std::move(factory)), free_(std::move(free)), inUse_(0) {}
/**
* @returns A unique pointer to a resource. The resource is null iff
* there are no avaiable resources and `factory()` returns null.
*/
UniquePtr get() {
std::lock_guard<std::mutex> lock(mutex_);
if (!resources_.empty()) {
UniquePtr resource{resources_.back(), Deleter{*this}};
resources_.pop_back();
++inUse_;
return resource;
}
UniquePtr resource{factory_(), Deleter{*this}};
++inUse_;
return resource;
}
~ResourcePool() noexcept {
assert(inUse_ == 0);
for (const auto resource : resources_) {
free_(resource);
}
}
class Deleter {
ResourcePool *pool_;
public:
explicit Deleter(ResourcePool &pool) : pool_(&pool) {}
void operator() (T *resource) {
std::lock_guard<std::mutex> lock(pool_->mutex_);
// Make sure we don't put null resources into the pool
if (resource) {
pool_->resources_.push_back(resource);
}
assert(pool_->inUse_ > 0);
--pool_->inUse_;
}
};
};
}
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