面试题2：实现Singleton模式

书上给的是C#实现，自己写几个C++实现：

1. 基本版，适用于单线程

class Singleton
{
public:
    static Singleton * Instance();
    static void Destroy();
protected:
    Singleton(){}
private:
    static Singleton * m_pInstance;
};

Singleton * Singleton::m_pInstance = NULL;

Singleton * Singleton::Instance()
{
    if (m_pInstance == NULL)
        m_pInstance = new Singleton;
    return m_pInstance;
}

void Singleton::Destroy()
{
    delete m_pInstance;
}

2. 多线程，使用加锁机制

利用winAPI中的加锁

#include <Windows.h>

class Locker
{
protected:
    friend class Singleton;
    CRITICAL_SECTION cs;

public:
    Locker(){
        ::InitializeCriticalSection(&cs);
    }
    void lock(){
        ::EnterCriticalSection(&cs);
    }
    void unlock(){
        ::LeaveCriticalSection(&cs);
    }
    ~Locker(){
        ::DeleteCriticalSection(&cs);
    }
};

class Singleton
{
public:
    static Singleton * Instance();
    static void Destroy();
protected:
    Singleton(){}
private:
    static Singleton * m_pInstance;
};

Singleton * Singleton::m_pInstance = NULL;

Singleton * Singleton::Instance()
{
    Locker *lock = new Locker;
    lock->lock();
    if (m_pInstance == NULL)
        m_pInstance = new Singleton;
    lock->unlock();
    return m_pInstance;
}

void Singleton::Destroy()
{
    delete m_pInstance;
}

3. 模版实现

class Singleton
{
public:
    static T * Instance();
    static void Destroy();
protected:
    Singleton(){}
private:
    static T * m_pInstance;
};

template <typename T>
T * Singleton<T>::m_pInstance = NULL;

template <typename T>
T * Singleton<T>::Instance()
{
    if (m_pInstance == NULL)
        m_pInstance = new T;
    return m_pInstance;
}

template <typename T>
void Singleton<T>::Destroy()
{
    delete m_pInstance;
}

参考资料：

http://blog.csdn.net/a342374071/article/details/18270643/

http://www.cnblogs.com/liyuan989/p/4264889.html