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单例模式，其中对象是由_pInstance指针来保存的，而在使用单例设计模式的过程中，也难免会遇到内存泄漏的问题。那么是否有一个方法，可以让对象自动释放，而不需要程序员自己手动去释放呢？ ——嵌套类

5.1、内存泄漏的检测工具valgrind

安装

sudo apt install valgrind

使用

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5.2、单例模式自动释放的四种方法 & 多线程

1、友元类

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#include <iostream>using std::cout;
using std::endl;class Singleton
{friend class AutoRelease;
public:static Singleton *getInstance(){if(nullptr == _pInstance){_pInstance = new Singleton();}return _pInstance;}static void destroy(){if(_pInstance){delete _pInstance;_pInstance =nullptr;}}private:Singleton(){cout << "Singleton()" << endl;}~Singleton(){cout << "~Singleton()" << endl;}
private:static Singleton *_pInstance;
};
Singleton *Singleton::_pInstance = nullptr;class AutoRelease
{
public:AutoRelease(){cout << "AutoRelease()" << endl;}~AutoRelease(){cout << "~AutoRelease()" << endl;if(Singleton::_pInstance){delete Singleton::_pInstance;Singleton::_pInstance =nullptr;}}
};
int main(int argc, char **argv)
{Singleton *ps1 = Singleton::getInstance();AutoRelease ar;//栈对象/* ps1->destroy(); */return 0;
}

2、内部类 + 静态数据成员

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• 若_ar定义为类成员，则会死锁：
  • new Singleton()，导致_ar在创建的单例堆对象内部，无法使其自动创建&析构（析构自身前，进行delete _pInstance）
  • 应将_ar定义为static
  • [外链图片转存中…(img-8lWGqW65-1728058419414)]

#include <iostream>using std::cout;
using std::endl;// 2、内部类 + 静态数据成员class Singleton
{
public:static Singleton *getInstance(){if (_pInstance == nullptr){_pInstance = new Singleton(); // 构造函数// _ar;}return _pInstance;}static void destroy(){if (_pInstance){delete _pInstance;_pInstance = nullptr;}}private:class AutoRelease{public:AutoRelease(){cout << "AutoRelease()" << endl;}~AutoRelease(){cout << "~AutoRelease()" << endl;if (_pInstance){delete _pInstance;_pInstance = nullptr;}}};private:Singleton(){cout << "Singleton()" << endl;}~Singleton(){cout << "~Singleton()" << endl;}private:static Singleton *_pInstance; // 前向声明；定义为static，位于全局静态区（不属于本类！）static AutoRelease _ar; // 前向声明；对象数据成员, _ar不能存在堆上，否则死锁；定义为static，位于全局静态区（不属于本类！）
};Singleton *Singleton::_pInstance = nullptr; // 静态对象必须在类外进行正式声明！
Singleton::AutoRelease Singleton::_ar; // 静态对象必须在类外进行正式声明！int main(int argc, char **argv)
{Singleton *ps1 = Singleton::getInstance();/* Singleton::AutoRelease ar;//栈对象 *//* ps1->destroy(); */return 0;
}/*
AutoRelease()
Singleton()
~AutoRelease()
~Singleton()*/

采用模板

• Singleton.h

#ifndef __WD_TEMPLATE_SINGLETON_H__
#define __WD_TEMPLATE_SINGLETON_H__#include <iostream>
using std::cout;
using std::endl;
#if 0
class Singleton
{
public:static Point *getInstance(int ix, int iy){if(nullptr == _pInstance) {_pInstance = new Point(ix, iy);_ar;//为了在模板参数推导时创建ar对象}return _pInstance;}
};
#endif
template <class T>
class Singleton
{
public:template <class... Args>static T *getInstance(Args... args){if (nullptr == _pInstance){_pInstance = new T(args...);_ar; // 为了在模板参数推导时创建ar对象}return _pInstance;}private:class AutoRelease{public:AutoRelease(){cout << "AutoRelease()" << endl;}~AutoRelease(){cout << "~AutoRelease()" << endl;if (_pInstance){delete _pInstance;_pInstance = nullptr;}}};private:Singleton(){cout << "Singleton()" << endl;/* _ar; */}~Singleton(){cout << "~Singleton()" << endl;}private:static T *_pInstance;// 前向声明；定义为static，位于全局静态区（不属于本类！）static AutoRelease _ar;// 前向声明；对象数据成员, _ar不能存在堆上，否则死锁；定义为static，位于全局静态区（不属于本类！）
};template <class T>
T *Singleton<T>::_pInstance = nullptr; // 静态对象必须在类外进行正式声明！template <class T>
typename Singleton<T>::AutoRelease Singleton<T>::_ar;  // 静态对象必须在类外进行正式声明！ // typename表名是一个类型#endif

• Test.cpp

  • #include "Singleton.h"#include <iostream>
    using std::cout;
    using std::endl;class Point
    {
    public:Point(int ix = 0, int iy = 0): _ix(ix), _iy(iy){	cout << "Point(int = 0,int = 0)" << endl;	}void print() const{cout << "(" << _ix<< "," << _iy<< ")" << endl;}~Point(){cout << "~Point()" << endl;}private:int _ix;int _iy;
    };int main()
    {Point *pt1 = Singleton<Point>::getInstance(1, 2);Point *pt2 = Singleton<Point>::getInstance(3, 4);pt1->print();pt2->print();cout << "p1 = " << pt1 << endl<< "p2 = " << pt2 << endl;return 0;
    }

3、饿汉模式 + atexit

atexit

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#include <stdlib.h>
#include <iostream>using std::cout;
using std::endl;void func()
{cout << "void func()" << endl;
}void test()
{atexit(func); // atexit: 进程正常结束时候，注册的func会被执行，注册几次就会执行几次atexit(func);atexit(func);atexit(func);atexit(func);
}int main(int argc, char **argv)
{cout << "start test..." << endl;test();cout << "finish test..." << endl;return 0;
}/*
start test...
finish test...
void func()
void func()
void func()
void func()
void func()*/

饿汉模式 + atexit

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#include <stdlib.h>
#include <iostream>using std::cout;
using std::endl;// 3、atexit + 饿汉模式class Singleton
{
public:static Singleton *getInstance(){// 在多线程情况下，是不安全的if (_pInstance == nullptr){_pInstance = new Singleton(); // 构造函数atexit(destroy);              // 使用atexit：注册函数destroy一次, 当进程正常结束后，会调用一次注册的函数destroy} else {cout << "_pInstance != nullptr" << endl;}return _pInstance;}static void destroy(){if (_pInstance){delete _pInstance;_pInstance = nullptr;}}private:Singleton(){cout << "Singleton()" << endl;}~Singleton(){cout << "~Singleton()" << endl;}private:static Singleton *_pInstance;
};/* Singleton *Singleton::_pInstance = nullptr; //饱（懒）汉模式， 问题：多线程下，单例模式失效；*/
Singleton *Singleton::_pInstance = getInstance(); // 饿汉模式，可解决多线程不安全问题void *func1(void *arg)
{Singleton::getInstance();
}void *func2(void *arg)
{Singleton::getInstance();
}void *func3(void *arg)
{Singleton::getInstance();
}
int main(int argc, char **argv)
{Singleton *ps1 = Singleton::getInstance();// 饱（懒）汉模式下，多线程不安全；-- 解决：使用饿汉模式// pthread_t th1, th2, th3;// pthread_create(&th1, nullptr, func1, nullptr);// pthread_create(&th2, nullptr, func2, nullptr);// pthread_create(&th3, nullptr, func3, nullptr);return 0;
}

4、多线程场景：pthread_once + atexit

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#include <pthread.h>
#include <stdlib.h>
#include <iostream>using std::cout;
using std::endl;//4、atexit + pthread_once
//有平台问题，只能在Linux下使用class Singleton
{
public:static Singleton *getInstance(){//当第一个参数是某个固定值的时候，可以保证第一个参数只会被//调用一次  call_oncepthread_once(&_once, init);return _pInstance;}static void init(){_pInstance = new Singleton();//构造函数atexit(destroy);}static void destroy(){if(_pInstance){delete _pInstance;_pInstance = nullptr;}}private:Singleton(){cout << "Singleton()" << endl;}~Singleton(){cout << "~Singleton()" << endl;}
private:static Singleton *_pInstance;static pthread_once_t _once;
};Singleton *Singleton::_pInstance = nullptr; //饱（懒）汉模式
/* Singleton *Singleton::_pInstance = getInstance();//饿汉模式 */
pthread_once_t Singleton::_once = PTHREAD_ONCE_INIT;int main(int argc, char **argv)
{Singleton *ps1 = Singleton::getInstance();return 0;
}