对象池的设计及其实现

对象池概述：

对象池模型创建并拥有固定数量的对象，当程序需要一个新的对象时，如果对象池中有空闲对象，则立即返回，否则才创建新的该类对象。当一个对象不再被使用时，其应该应该将其放回对象池，以便后来的程序使用。由于系统资源有限，一个对象池模型应该指定其可容纳的最大对象数量。当达到该数量时，如果仍然有对象创建请求，则抛出异常或者阻塞当前调用线程，直到一个对象被放回对象池中。

对象池模型适用的场景：

(1)需要使用大量对象

(2)这些对象的实例化开销比较大且生存期比较短

对象池优势：

一个对象池可以在可容忍时间内创建成功并投入使用。但是创建对象时并不总是这样，尤其是当这些对象的创建过程比较耗时，而且创建和销毁频率又比较大时更是如此。比如数据库连接、网络套接字连接、线程对象、诸如字体或位图等图像对象等。

实现：

假设有如下类定义：

class Object
{
public:
Object(const string& name) : name_(name)
{
printf("Construct Object[%p] %s.\n", this, name_.c_str());
}

~Object()
{
printf("~Destruct Object[%p] %s.\n", this, name_.c_str());
}

const string& key() const { return name_; }

private:
string name_;
};

如下对象池类的设计，用来提供Object类对象，分为2个版本介绍：（注意，如下仅考虑了对象池本身所涉及的特性，没有涉及同步控制机制）

版本1：

class ObjectPool
{
public:
boost::shared_ptr<Object> get(const string& key)
{
boost::shared_ptr<Object> pObject;
boost::weak_ptr<Object>& k_object = objects_[key];
pObject = k_object.lock();
if (!pObject)
{
pObject.reset(new Object(key),
boost::bind(&ObjectPool::releaseObject, this, _1));
k_object = pObject;
}
return pObject;
}

private:
void releaseObject(Object* object)
{
printf("releaseObject[%p].\n", object);
if (object)
{
objects_.erase(object->key());
}
delete object;
}

std::map<string, boost::weak_ptr<Object> > objects_;
};

ObjectPool的get函数返回map中key对应的Object对象。如果该对象不存在，则新建一个Object，将其放入map中，然后返回这个新建的Object。同时，重置shared_ptr（新增Object对象）时指定析构器releaseObject，使得对象析构时执行releaseObject(object);

但是上述实现存在一个问题：将this传入bind函数中，如果ObjectPool对象先于Object对象析构了，那么在析构Object对象时，如何调用releaseObject函数呢？（因为releaseObject函数属于ObjectPool类）

版本2：

class ObjectPool : public boost::enable_shared_from_this<ObjectPool>
{
public:
boost::shared_ptr<Object> get(const string& key)
{
boost::shared_ptr<Object> pObject;
boost::weak_ptr<Object>& k_object = objects_[key];
pObject = k_object.lock();
if (!pObject)
{
pObject.reset(new Object(key),
boost::bind(&ObjectPool::releaseObject, shared_from_this(), _1));
k_object = pObject;
}
return pObject;
}

private:
void releaseObject(Object* object)
{
printf("releaseObject[%p].\n", object);
if (object)
{
objects_.erase(object->key());
}
delete object;
}

std::map<string, boost::weak_ptr<Object> > objects_;
};

要解决版本1中的问题，只需增加ObjectPool的寿命就可以了。可以利用boost::enable_shared_from_this模板类中的shared_from_this()，如此可以将this转换为shared_ptr<ObjectPool>。如此，由于bind是值传递语义，因此其必然保存一份shared_ptr<ObjectPool>的副本，可以保证shared_ptr的引用计数不为0。

测试用例：

// object_pool.cc
#include <map>

#include <boost/bind.hpp>
#include <boost/enable_shared_from_this.hpp>
#include <boost/shared_ptr.hpp>
#include <boost/weak_ptr.hpp>

#include <stdio.h>

using std::string;
const int MAXNUM = 5;  // the largest amounts of objects
int nums = 0;          // the current amounts of objects

class Object
{
public:
Object(const string& name) : name_(name)
{
printf("Construct Object[%p] %s.\n", this, name_.c_str());
}

~Object()
{
printf("~Destruct Object[%p] %s.\n", this, name_.c_str());
}

const string& key() const { return name_; }

private:
string name_;
};

namespace version1
{

class ObjectPool
{
public:
boost::shared_ptr<Object> get(const string& key)
{
boost::shared_ptr<Object> pObject;
boost::weak_ptr<Object>& k_object = objects_[key];
pObject = k_object.lock();
if (!pObject)
{
++nums;
BOOST_ASSERT(nums <= MAXNUM);
pObject.reset(new Object(key),
boost::bind(&ObjectPool::releaseObject, this, _1));
k_object = pObject;
}
return pObject;
}

private:
void releaseObject(Object* object)
{
printf("releaseObject[%p].\n", object);
if (object)
{
--nums;
objects_.erase(object->key());
}
delete object;
}

std::map<string, boost::weak_ptr<Object> > objects_;
};

}

namespace version2
{

class ObjectPool : public boost::enable_shared_from_this<ObjectPool>
{
public:
boost::shared_ptr<Object> get(const string& key)
{
boost::shared_ptr<Object> pObject;
boost::weak_ptr<Object>& k_object = objects_[key];
pObject = k_object.lock();
if (!pObject)
{
++nums;
BOOST_ASSERT(nums <= MAXNUM);
pObject.reset(new Object(key),
boost::bind(&ObjectPool::releaseObject, shared_from_this(), _1));
k_object = pObject;
}
return pObject;
}

private:
void releaseObject(Object* object)
{
printf("releaseObject[%p].\n", object);
if (object)
{
--nums;
objects_.erase(object->key());
}
delete object;
}

std::map<string, boost::weak_ptr<Object> > objects_;
};

}

int main()
{
boost::shared_ptr<version1::ObjectPool> op1(new version1::ObjectPool);

boost::shared_ptr<Object> object1 = op1->get("object1");
boost::shared_ptr<Object> object2 = op1->get("object2");
boost::shared_ptr<Object> object3 = op1->get("object3");
boost::shared_ptr<Object> object4 = op1->get("object4");
boost::shared_ptr<Object> object5 = op1->get("object5");
boost::shared_ptr<Object> object6 = op1->get("object5");

//boost::shared_ptr<version2::ObjectPool> op2(new version2::ObjectPool);
//boost::shared_ptr<Object> object7 = op2->get("object2");
//boost::shared_ptr<Object> object8 = op2->get("object2");

return 0;
}

// output
Construct Object[003e1060] object1.
Construct Object[003e10e0] object2.
Construct Object[003e1160] object3.
Construct Object[003e11e0] object4.
Construct Object[003e1260] object5.
releaseObject[003e1260].
~Destruct Object[003e1260] object5.
releaseObject[003e11e0].
~Destruct Object[003e11e0] object4.
releaseObject[003e1160].
~Destruct Object[003e1160] object3.
releaseObject[003e10e0].
~Destruct Object[003e10e0] object2.
releaseObject[003e1060].
~Destruct Object[003e1060] object1.

View Code

References
https://en.wikipedia.org/wiki/Object_pool_pattern