C++单例模式

#pragma once

namespace utility
{
    // 需要拼接一下命名空间 utility
#define SINGLETON(x) (friend class utility::##Singleton<x>);

    template <typename T>
    class Singleton
    {
    public:
        static T* instance()
        {
            if (m_instance == nullptr)
            {
                m_instance = new T();
            }
            return m_instance;
        }
    private:
        Singleton(){}
        Singleton(const Singleton<T>&);
        Singleton<T>& operator = (const Singleton<T>&);
        ~Singleton(){}
    private:
        static T* m_instance;
    };

    template <typename T>
    T* Singleton<T>::m_instance = nullptr;
}

#pragma once
#include "Singleton.h"
#include <iostream>
#include <string>

class A
{
    // 声明为友元类Singleton才能创建A的实例
    // friend class utility::Singleton<A>;
    SINGLETON(A);
public:
    void show()
    {
        std::cout << __FUNCTION__ << m_name << std::endl;
    }
private:
    A() : m_name(" name"){}
    A(const A&);
    A& operator = (const A&);
    ~A(){}
private:
    std::string m_name;
};

#include "A.h"
#include "Singleton.h"
using namespace utility;

int main()
{
    //1. 类A构造为私有（无法在模板单例类中创建instance实例）使用友元类解决
    //2. 对于多线程可能创建多个实例的问题（在主线程中先创建规避掉那种情况即可）
    Singleton<A>::instance();

    A* instanceA1 = Singleton<A>::instance();
    A* instanceA2 = Singleton<A>::instance();

    std::cout << "A1: " << instanceA1 << std::endl;
    std::cout << "A2: " << instanceA2 << std::endl;

    instanceA1->show();
    instanceA2->show();

    getchar();
    return 0;
}