学习记录

使用智能指针解决多线程下 类的解析冲突问题

有这样一个场景

使用StockFactory记录Stock的信息  容器是map<string,smart_ptr>;

代码如下:

 #include <functional>
 #include <memory>
 #include <string>
 #include <map>
 #include <assert.h>
 #include <mutex>

 using std::string;

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

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

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

 private:
     string name_;
 };

 // questionable code
 class StockFactory
 {
 public:

     std::shared_ptr<Stock> get(const string& key)
     {
         std::lock_guard<std::mutex> lock(mutex_);
         std::shared_ptr<Stock>& pStock = stocks_[key];
         if (!pStock)
         {
             pStock.reset(new Stock(key));
         }
         return pStock;
     }

 private:
     std::mutex mutex_;
     std::map<string, std::shared_ptr<Stock> > stocks_;
 };

 int main()
 {
     StockFactory sf1;
     {
         std::shared_ptr<Stock> s1 = sf1.get("stock1");
     }
     printf("s1 should destruct\n");

     return ;
 }

cpp1

运行显示如下

Stock[007E8818] stock1
s1 should destruct
~Stock[007E8818] stock1
没有按照预想的进行析构

说明我们的map容器应该使用弱指针weak_ptr，否则就会出现STOCKFACTOR不析构，STOCK也不会析构的强引用

代码如下

 #include <functional>
 #include <memory>
 #include <string>
 #include <map>
 #include <assert.h>
 #include <mutex>

 using std::string;

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

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

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

 private:
     string name_;
 };

 // questionable code
 class StockFactory
 {
 public:
     std::shared_ptr<Stock> get(const string& key)
     {
         std::shared_ptr<Stock> pStock;
         std::lock_guard<std::mutex> lock(mutex_);
         std::weak_ptr<Stock>& wkStock = stocks_[key];
         pStock = wkStock.lock();
         if (!pStock)
         {
             pStock.reset(new Stock(key));
             wkStock = pStock;
         }
         return pStock;
     }

 private:
     std::mutex mutex_;
     std::map<string, std::weak_ptr<Stock> > stocks_;
 };

 int main()
 {
     StockFactory sf2;
     {
         std::shared_ptr<Stock> s1 = sf2.get("stock2");
     }
     printf("s1 should destruct\n");

     return ;
 }

cpp2

运行显示如下

Stock[00468818] stock2
~Stock[00468818] stock2
s1 should destruct

运行情况与我们预想的情况一致 下面的问题就是我们使用的map容器中，当STOCK解析了，map却没有将其智能指针移除

所以在使用智能指针的时候我们要订制析构函数

 // 11111.cpp : 定义控制台应用程序的入口点。
 //

 #include "stdafx.h"
 #include <functional>
 #include <memory>
 #include <string>
 #include <map>
 #include <assert.h>
 #include <mutex>

 using std::string;

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

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

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

 private:
     string name_;
 };

 class StockFactory
 {
 public:

     std::shared_ptr<Stock> get(const string& key)
     {
         std::shared_ptr<Stock> pStock;
         std::lock_guard<std::mutex> lock(mutex_);
         std::weak_ptr<Stock>& wkStock = stocks_[key];
         pStock = wkStock.lock();
         if (!pStock)
         {
             pStock.reset(new Stock(key),
                 std::bind(&StockFactory::deleteStock, this, std::placeholders::_1));
             wkStock = pStock;
         }
         return pStock;
     }

 private:

     void deleteStock(Stock* stock)
     {
         printf("deleteStock[%p]\n", stock);
         if (stock)
         {
             std::lock_guard<std::mutex> lock(mutex_);
             stocks_.erase(stock->key());
         }
         delete stock;  // sorry, I lied
     }
     std::mutex mutex_;
     std::map<string, std::weak_ptr<Stock> > stocks_;
 };

 void testLongLifeFactory()
 {
     std::shared_ptr<StockFactory> factory(new StockFactory);
     {
         std::shared_ptr<Stock> stock = factory->get("NYSE:IBM");
         std::shared_ptr<Stock> stock2 = factory->get("NYSE:IBM");
         assert(stock == stock2);
         // stock destructs here
     }
     // factory destructs here
 }

 void testShortLifeFactory()
 {
     std::shared_ptr<Stock> stock;
     {
         std::shared_ptr<StockFactory> factory(new StockFactory);
         stock = factory->get("NYSE:IBM");
         std::shared_ptr<Stock> stock2 = factory->get("NYSE:IBM");
         assert(stock == stock2);
         // factory destructs here
     }
     // stock destructs here
 }

 int main()
 {
     StockFactory sf3;
     {
         std::shared_ptr<Stock> s3 = sf3.get("stock3");
     }
     printf("s1 should destruct\n");

     testLongLifeFactory();
     testShortLifeFactory();//此处出错 因为我们BIND函数的时候使用的是this指针 而STOCK析构的时候 FACTORY早已不在

     return ;
 }

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