C++单例模式

#ifndef FOO_H
#define FOO_H

class Foo
{
public:
    static Foo* getInstance();
    ~Foo();

private:
    Foo();
    static Foo* instance;

};
#endif

#include "Foo.h"
#include <mutex>

std::mutex fooMutex;
Foo* Foo::instance = NULL;
Foo::Foo()
{
}

Foo* Foo::getInstance()
{
    fooMutex.lock();
    if (NULL == instance) {
        instance = new Foo();
    }
    fooMutex.unlock();
    return instance;
}

Foo::~Foo()
{
    if (NULL != instance) {
        delete instance;
        instance = NULL;
    }
}