<Linux多线程服务端编程>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
", this, name_.c_str());
    }

    ~Stock()
    {
        printf("~Stock[%p] %s
", 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
");

    return 0;
}

 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
", this, name_.c_str());
    }

    ~Stock()
    {
        printf("~Stock[%p] %s
", 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
");

    return 0;
}

 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
", this, name_.c_str());
    }

    ~Stock()
    {
        printf("~Stock[%p] %s
", 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]
", 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
");

    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 0;
}

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