C ++对象池，它提供作为智能指针的项，在删除后返回到池 [英] C++ object-pool that provides items as smart-pointers that are returned to pool upon deletion
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问题描述
我想要一个 LIFO 管理资源池的类。
当请求资源时（通过 acquire（）），它返回对象作为 unique_ptr
单元测试将是：
//创建池（为简单起见，int对象）
SharedPool< int>池;
TS_ASSERT（pool.empty（））;
//添加一个对象到池，现在不再为空
pool.add（std :: unique_ptr< int>（new int（42）））;
TS_ASSERT（！pool.empty（））;
//在自己的范围内调用此对象，使池为空
{
auto v = pool.acquire（）;
TS_ASSERT_EQUALS（* v，42）;
TS_ASSERT (pool.empty（））;
}
//对象应该已经返回到池
TS_ASSERT（！pool.empty（））
基本实现，通过测试，但最重要的测试除外：
模板< class T>
类SharedPool
{
public：
SharedPool（）{}
virtual〜SharedPool（）{}
void add（std： ：unique_ptr< T> t）{
pool_.push（std :: move（t）
}
std :: unique_ptr< T> acquire（）{
assert（！pool_.empty（））;
std :: unique_ptr< T> tmp（std :: move（pool_.top（）））;
pool_.pop（）;
return std :: move（tmp）;
}
bool empty（）const {
return pool_.empty（）;
}
private：
std :: stack< std :: unique_ptr< T> > pool_;
};
问题：如何去使得 acquire（）返回类型的 unique_ptr ，使得删除者知道 this ，并调用 this-> add（...），将资源返回到池。 >
解决方案
原生实现
实现使用 unique_ptr 使用自定义删除器将对象返回到池。 获取和发布都是 O（1）。此外，自定义删除器的 unique_ptr 可以隐式转换为 shared_ptr 。
模板< class T>
class SharedPool
{
public：
using ptr_type = std :: unique_ptr< T，std :: function< void（T *）& > ;;
SharedPool（）{}
virtual〜SharedPool（）{}
void add（std :: unique_ptr< T> t）{
pool .push（std :: move（t））;
}
ptr_type acquire（）{
assert（！pool_.empty（））;
ptr_type tmp（pool_.top（）。release（），
[this]（T * ptr）{
this-> add（std :: unique_ptr& ）;
}）;
pool_.pop（）;
return std :: move（tmp）;
}
bool empty（）const {
return pool_.empty（）;
}
size_t size（）const {
return pool_.size（）;
}
private：
std :: stack< std :: unique_ptr< T> > pool_;
};
用法示例：
SharedPool< int>池;
pool.add（std :: unique_ptr< int>（new int（42）））;
pool.add（std :: unique_ptr< int>（new int（84）））;
pool.add（std :: unique_ptr< int>（new int（1024）））;
pool.add（std :: unique_ptr< int>（new int（1337））））;
//三种表达unique_ptr对象的方法
auto v1 = pool.acquire（）;
SharedPool< int> :: ptr_type v2 = pool.acquire（）;
std :: unique_ptr< int，std :: function< void（int *）> > v3 = pool.acquire（）;
//使用正确的删除程序隐式转换shared_ptr
std :: shared_ptr< int> v4 = pool.acquire（）;
//注意，添加一个获取的对象是（正确地）不允许的：
// pool.add（v1）; //编译器错误
这个实现的问题。以下用法是不可想象的：
std :: unique_ptr< SharedPool< Widget> > pool（new SharedPool< Widget>）;
pool-> add（std :: unique_ptr< Widget>（new Widget（42）））;
pool-> add（std :: unique_ptr< Widget>（new Widget（84）））;
// [Widget，42] acquire（），并从池中释放
auto v1 = pool-> acquire
// [Widget，84]与pool
pool.reset（nullptr）一起被正确销毁;
// [Widget，42]不会被销毁，池不再存在。
v1.reset（nullptr）;
//内存泄漏
- 我应该将对象返回到池吗？
- 我应该删除实际对象吗？
SharedPool 中的 weak_ptr 到 shared_ptr 成员。
正确的实现：
template< class T>
class SharedPool
{
private：
struct External_Deleter {
explicit External_Deleter（std :: weak_ptr< SharedPool< T> * pool）
： pool_（pool）{}
void operator（）（T * ptr）{
if（auto pool_ptr = pool_.lock（））{
try {
（* pool_ptr.get（）） - > add（std :: unique_ptr< T> {ptr}）;
return;
} catch（...）{}
}
std :: default_delete< T> {}（ptr）;
}
private：
std :: weak_ptr< SharedPool< T> *> pool_;
};
public：
using ptr_type = std :: unique_ptr< T，External_Deleter> ;;
SharedPool（）：this_ptr_（new SharedPool< T> *（this））{}
virtual〜SharedPool（）{}
void add（std： ：unique_ptr< T> t）{
pool_.push（std :: move（t））;
}
ptr_type acquire（）{
assert（！pool_.empty（））;
ptr_type tmp（pool_.top（）。release（），
External_Deleter {std :: weak_ptr< SharedPool< T> * {this_ptr_}}）;
pool_.pop（）;
return std :: move（tmp）;
}
bool empty（）const {
return pool_.empty（）;
}
size_t size（）const {
return pool_.size（）;
}
private：
std :: shared_ptr< SharedPool< T> *> this_ptr_;
std :: stack< std :: unique_ptr< T> > pool_;
};
I'm having fun with c++-ideas, and got a little stuck with this problem.
I would like a LIFO class that manages a pool of resources.
When a resource is requested (through acquire()), it returns the object as a unique_ptr that, upon deletion, causes the resource to be returned to the pool.
The unit tests would be:
// Create the pool, that holds (for simplicity, int objects)
SharedPool<int> pool;
TS_ASSERT(pool.empty());
// Add an object to the pool, which is now, no longer empty
pool.add(std::unique_ptr<int>(new int(42)));
TS_ASSERT(!pool.empty());
// Pop this object within its own scope, causing the pool to be empty
{
auto v = pool.acquire();
TS_ASSERT_EQUALS(*v, 42);
TS_ASSERT(pool.empty());
}
// Object should now have returned to the pool
TS_ASSERT(!pool.empty())
Basic implementation, which would pass the tests, except for the important final test:
template <class T>
class SharedPool
{
public:
SharedPool(){}
virtual ~SharedPool(){}
void add(std::unique_ptr<T> t) {
pool_.push(std::move(t));
}
std::unique_ptr<T> acquire() {
assert(!pool_.empty());
std::unique_ptr<T> tmp(std::move(pool_.top()));
pool_.pop();
return std::move(tmp);
}
bool empty() const {
return pool_.empty();
}
private:
std::stack<std::unique_ptr<T> > pool_;
};
The question: How to go about so that acquire() returns a unique_ptr of a type such that the deleter has knowledge of this, and calls this->add(...), returning the resource back to the pool.
解决方案
Naive implementation
The implementation uses unique_ptr with a custom deleter that returns objects to the pool. Both acquire and release are O(1). Additionally, unique_ptr with custom deleters can be implicitly converted to shared_ptr.
template <class T>
class SharedPool
{
public:
using ptr_type = std::unique_ptr<T, std::function<void(T*)> >;
SharedPool() {}
virtual ~SharedPool(){}
void add(std::unique_ptr<T> t) {
pool_.push(std::move(t));
}
ptr_type acquire() {
assert(!pool_.empty());
ptr_type tmp(pool_.top().release(),
[this](T* ptr) {
this->add(std::unique_ptr<T>(ptr));
});
pool_.pop();
return std::move(tmp);
}
bool empty() const {
return pool_.empty();
}
size_t size() const {
return pool_.size();
}
private:
std::stack<std::unique_ptr<T> > pool_;
};
Example usage:
SharedPool<int> pool;
pool.add(std::unique_ptr<int>(new int(42)));
pool.add(std::unique_ptr<int>(new int(84)));
pool.add(std::unique_ptr<int>(new int(1024)));
pool.add(std::unique_ptr<int>(new int(1337)));
// Three ways to express the unique_ptr object
auto v1 = pool.acquire();
SharedPool<int>::ptr_type v2 = pool.acquire();
std::unique_ptr<int, std::function<void(int*)> > v3 = pool.acquire();
// Implicitly converted shared_ptr with correct deleter
std::shared_ptr<int> v4 = pool.acquire();
// Note that adding an acquired object is (correctly) disallowed:
// pool.add(v1); // compiler error
You might have caught a severe problem with this implementation. The following usage isn't unthinkable:
std::unique_ptr< SharedPool<Widget> > pool( new SharedPool<Widget> );
pool->add(std::unique_ptr<Widget>(new Widget(42)));
pool->add(std::unique_ptr<Widget>(new Widget(84)));
// [Widget,42] acquired(), and released from pool
auto v1 = pool->acquire();
// [Widget,84] is destroyed properly, together with pool
pool.reset(nullptr);
// [Widget,42] is not destroyed, pool no longer exists.
v1.reset(nullptr);
// Memory leak
We need a way to keep alive information necessary for the deleter to make the distinction
- Should I return object to pool?
- Should I delete the actual object?
One way of doing this (suggested by T.C.), is having each deleter keep a weak_ptr to shared_ptr member in SharedPool. This lets the deleter know if the pool has been destroyed.
Correct implementation:
template <class T>
class SharedPool
{
private:
struct External_Deleter {
explicit External_Deleter(std::weak_ptr<SharedPool<T>* > pool)
: pool_(pool) {}
void operator()(T* ptr) {
if (auto pool_ptr = pool_.lock()) {
try {
(*pool_ptr.get())->add(std::unique_ptr<T>{ptr});
return;
} catch(...) {}
}
std::default_delete<T>{}(ptr);
}
private:
std::weak_ptr<SharedPool<T>* > pool_;
};
public:
using ptr_type = std::unique_ptr<T, External_Deleter >;
SharedPool() : this_ptr_(new SharedPool<T>*(this)) {}
virtual ~SharedPool(){}
void add(std::unique_ptr<T> t) {
pool_.push(std::move(t));
}
ptr_type acquire() {
assert(!pool_.empty());
ptr_type tmp(pool_.top().release(),
External_Deleter{std::weak_ptr<SharedPool<T>*>{this_ptr_}});
pool_.pop();
return std::move(tmp);
}
bool empty() const {
return pool_.empty();
}
size_t size() const {
return pool_.size();
}
private:
std::shared_ptr<SharedPool<T>* > this_ptr_;
std::stack<std::unique_ptr<T> > pool_;
};
这篇关于C ++对象池，它提供作为智能指针的项，在删除后返回到池的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！
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