c++设计模式二：原型模式

        使用场景：当需要构建多个相同的类对象时，而且该类对象结构较为复杂，如果每个都重新组织构建会很麻烦。

        其实，就是写一个拷贝构造函数，或者写一个拷贝每个成员变量的clone()方法。

        举例说明：比如一个相亲网站需要定义一个“男人”类，可想而知描述清楚一个男人肯定需要很多属性吧，比如姓名、身高、体重、职业、财产大致情况、联系电话、微信号、QQ号等。如果一个男人同时有多个相亲对象时，每次都重新构建填写属性显然太麻烦，所以类中需要定义一个拷贝的方法。

先写一个简单的类：

#pragma once
#include <QString>
class singleMan
{
public:singleMan(QString name,double height,double weight,QString career,QString phoneNum);~singleMan();QString m_name;//姓名double m_height;//身高double m_weight;//体重QString m_career;//职业QString m_phoneNum;//电话};

#include "singleMan.h"
singleMan::singleMan(QString name, double height, double weight, QString career, QString phoneNum):m_name(name),m_height(height),m_weight(weight),m_career(career),m_phoneNum(phoneNum)
{
}singleMan::~singleMan()
{
}

直接赋值，打印出的内容也完全一样，还需要写什么拷贝构造吗？

singleMan sman("LaoWang", 178, 153, "programmer", "19969961024");singleMan sman1 = sman;cout << "sman:" << sman.m_name.toStdString() << ";" << sman.m_phoneNum.toStdString() << endl;cout << "sman1:" << sman1.m_name.toStdString() << ";" << sman1.m_phoneNum.toStdString() << endl;

        这里c++会自动创建一个默认的拷贝构造函数，将成员变量中的值复制一次。但是，当类中有指针类型成员变量时，默认的构造函数仅仅把地址复制一份，两个对象指向的是同一个内存地址，对象销毁时会造成同一个内存被释放两次造成double free，例子如下，添加一个指针类型成员变量-近半年工资流水：

singleman类中添加double* m_monthSalary指针成员变量：

double* m_monthSalary = nullptr;//近半年工资流水

类的析构中释放指针空间：

singleMan::~singleMan()
{if (m_monthSalary!=nullptr){delete m_monthSalary;m_monthSalary = nullptr;}
}

    double salary[6] = { 30000,30000,40000,50000,60000 };sman.m_monthSalary = salary;singleMan sman2 = sman;cout << "sman:" << sman.m_monthSalary[0] << sman.m_monthSalary[1] <<                     sman.m_monthSalary[2] << sman.m_monthSalary[3] << endl;cout << "sman2:" << sman2.m_monthSalary[0] << sman2.m_monthSalary[1] << sman2.m_monthSalary[2] << sman2.m_monthSalary[3] << endl;

调用打印没问题，但是程序结束后会报错：

         所以，这里需要自己写一个拷贝构造函数，实现中需要重新分配新的内存地址（即所谓的深拷贝）：

//拷贝构造singleMan(const singleMan& man);//这里要用引用，引用做参数时可减少一次对象拷贝，同时避免了循环拷贝构造问题；

singleMan::singleMan(const singleMan& man)
{m_career = man.m_career;m_height = man.m_height;m_name = man.m_name;m_phoneNum = man.m_phoneNum;m_weight = man.m_weight;m_monthSalary = new double[6];memcpy(m_monthSalary, man.m_monthSalary, 6 * sizeof(double*));
}

    double* salary = new double[6];salary[0] = 30000;salary[1] = 30000;salary[2] = 40000;salary[3] = 50000;salary[4] = 60000;salary[5] = 65000;sman.m_monthSalary = salary;singleMan sman2 = sman;cout << "sman  salary: " << sman.m_monthSalary << endl;cout << "sman2 salary: " << sman2.m_monthSalary << endl;cout << "sman: " << sman.m_monthSalary[0]<<"," << sman.m_monthSalary[1] << "," << sman.m_monthSalary[2] << "," << sman.m_monthSalary[3] << endl;cout << "sman2:" << sman2.m_monthSalary[0] << "," << sman2.m_monthSalary[1] << "," << sman2.m_monthSalary[2] << "," << sman2.m_monthSalary[3] << endl;

        此时，上述问题貌似已经解决了，但是在实际使用过程中会有这种用法：将一个类对象赋值给另一个已经构造好的对象：

    singleMan sman3("LaoLi", 183, 160, "programmer", "19969960512");sman3 = sman;cout << "sman: " << sman.m_name.toStdString() << ";" << sman.m_phoneNum.toStdString() << endl;cout << "sman3:" << sman3.m_name.toStdString() << ";" << sman3.m_phoneNum.toStdString() << endl;cout << "sman  salary: " << sman.m_monthSalary << endl;cout << "sman3 salary: " << sman3.m_monthSalary << endl;cout << "sman: " << sman.m_monthSalary[0] << "," << sman.m_monthSalary[1] << "," << sman.m_monthSalary[2] << "," << sman.m_monthSalary[3] << endl;cout << "sman3:" << sman3.m_monthSalary[0] << "," << sman3.m_monthSalary[1] << "," << sman3.m_monthSalary[2] << "," << sman3.m_monthSalary[3] << endl; 

        这时，就不进入拷贝构造函数了，而是进入了默认的赋值操作符函数中，会有同样的问题，这时需要写一个赋值操作符重载函数：

    //赋值运算符重载（用新的对象的值全部替换原有项）singleMan& operator=(const singleMan& man);

singleMan& singleMan::operator=(const singleMan& man)
{if (m_monthSalary != nullptr){delete m_monthSalary;m_monthSalary = nullptr;}m_career = man.m_career;m_height = man.m_height;m_name = man.m_name;m_phoneNum = man.m_phoneNum;m_weight = man.m_weight;m_monthSalary = new double[6];memcpy(m_monthSalary, man.m_monthSalary, 6 * sizeof(double*));return *this;
}

再次调用问题得到解决：

最后，由于拷贝构造函数中代码和赋值运算符代码几乎一致，可以在拷贝构造函数中直接调研赋值运算符重载函数：

singleMan::singleMan(const singleMan& man)
{/*m_career = man.m_career;m_height = man.m_height;m_name = man.m_name;m_phoneNum = man.m_phoneNum;m_weight = man.m_weight;m_monthSalary = new double[6];memcpy(m_monthSalary, man.m_monthSalary, 6 * sizeof(double*));*/*this = man;
}

        如有问题，欢迎指正！ 

参考文献：

【精选】C++创建型模式-原型模式_c++ 原型模式-CSDN博客

4. 原型模式（Prototype） (yuque.com)