c++primer plus 第12章 编程题第1题

　　一开始的程序里 类不能连续赋值，内存会报错。debug了一整晚发现是new里面的数组n++出错。经过测试++n和n+1都能使程序正确运行。

　　n++会报错是因为先使用n，再对n加1。而我们的期望是开辟一个n+1长度的数组而不是n，所以会报错。

　　在单语句中，自增运算和普通的加法没有区别，但是在这种复合语句中千万小心，尽量不使用n++和++n

　　

#pragma once
#ifndef COW_H_
#define COW_H_

class Cow
{
private:    //class default private,struct default public
    char name[20];
    char * hobby;
    double weight;
public:
    Cow();
    Cow(const char * nm, const char * ho, double wt);
    Cow(const Cow & c);
    ~Cow();
    Cow & operator=( const Cow & c);
    void ShowCow() const;    //display all cow data
};



#endif // !COW_H_

cow.h

#include"Cow.h"
#include<cstring>
#include<iostream>
using std::strcpy;
Cow::Cow()
{
    name[0] = '\0';
    hobby = new char[1];//为了统一构造函数的型式，兼容析构函数
    hobby[0] = '\0';
    weight = 0;
}

Cow::Cow(const char * nm, const char * ho, double wt)
{
    weight = wt;
    strcpy(name, nm);
    int n;
    n = strlen(ho);
    hobby = new char[n+1];//记得加1
    strcpy(hobby, ho);
}

Cow::Cow(const Cow & c)
{
    weight = c.weight;
    strcpy(name, c.name);
    int n;
    n = strlen(c.hobby);
    hobby = new char[n+1];
    strcpy(hobby, c.hobby);
}


Cow::~Cow()
{
    delete[]hobby;
    hobby = nullptr;
}

void Cow::ShowCow() const
{
    using std::cout;
    using std::endl;
    cout << name << endl;
    cout << hobby << endl;
    cout << weight << endl;
}

Cow & Cow::operator=(const Cow & c)//记得要return
{
    //先删除后创建,节约内存
    if (this == &c)//判断万一是自身
    {
        return *this;
    }
    
    delete[] hobby;
    strcpy(name, c.name);
    int n;
    n = strlen(c.hobby);
    hobby = new char[n+1];
    strcpy(hobby, c.hobby);
    weight = c.weight;
    return *this;
}
//连续赋值出错原因竟然是++n！！
//千万不要在new里面n++，尽量少用自增
//++n和n+1都可以,但是n++不行，因为new是先new了n，n再加1，当n++自增运算不是但语句时候，不要用，容易出错，就像这种在括号里的型式

cow.cpp

#include <iostream>
#include <cstring>
#include"Cow.h"

using std::strcpy;
using std::cin;
using std::cout;
using std::endl;
int main()
{
    Cow milk("a", "eat", 56.9);//Cow::Cow ( const char *nm, const char *ho, double wt )

    Cow junk("t", "c", 200.66);//Cow::Cow ( const char *nm, const char *ho, double wt )

    milk.ShowCow();
    cout << endl;
    Cow m1;//Cow::Cow()
    m1 = milk;//Cow & Cow::operator= ( const Cow & c )
    m1.ShowCow();
    cout << endl;
    Cow m2 = junk;//Cow::Cow ( const Cow & c )，等价于m2(junk)
    m2.ShowCow();
    cout << endl;
    Cow m3;//Cow::Cow()
    m3.ShowCow();
    cout << endl;
    m3 = m1 = m2;//Cow & Cow::operator= ( const Cow & c )
    //m3 = m1;
    //m3.ShowCow();
    //m3 = m2;
    m3.ShowCow();
    //为什么两次赋值会出错？
    cout << "ok" << endl;
    return 0;
}

test.cpp