C++ Primer Plus 学习笔记第十章类和对象构造函数和析构函数类函数的const

叁歲伎倆 2022-01-29 07:43 217阅读 0赞

![20190521133409283.png][]

类中的各个变量和函数 默认是private 所以 一般可以不用写 如果是public 或者protected的话要显式的声明

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如果定义了非默认的构造函数，那么必须要在定义一个默认构造函数

定义默认构造函数的方法有两种，一种是参数全部给默认值，一种不传参（）

两种方法只能选其一不能同时用 不然会有二义性

举例：

Stock(const string& co = "Error", int n =0);

Stock();

析构函数

~Stock();

成员名和形参：

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3NvdWx3eWI_size_16_color_FFFFFF_t_70 1][]

![20190521143032262.png][]

关于析构函数：

创建对象的两个方法 一种是直接创建，另一种是穿件临时兑现然后复制到指定的对象变量所指的内存中

如果是第二种的话在释放临时对象时调用析构函数

这个释放时间根据编译器而有所不同。

上示例代码

stock10.h

#ifndef STOCK00_H_
    #define STOCK00_H_
    
    #include <string>
    
    class Stock
    {
      private:
        std::string company;
        long shares;
        double share_val;
        double total_val;
        void set_tot() { total_val = shares * share_val;}
      public:
        Stock();
        Stock(const std::string & co, long n = 0, double pr = 0.0);
        ~Stock();
        void acquire(const std::string & co, long n, double pr);
        void buy(long num, double price);
        void sell(long num, double price);
        void update(double price);
        void show();
    };
    
    #endif

stock10.cpp

#include <iostream>
    #include "stock10.h"
    
    Stock::Stock()
    {
      std::cout << "Default constructor called\n";
      company = "no name";
      shares = 0;
      share_val = 0.0;
      total_val = 0.0;
    }
    
    Stock::Stock(const std::string & co, long n, double pr)
    {
      std::cout << "Constructor using " << co << " called\n";
      company = co;
    
      if (n < 0)
      {
        std::cout << "Number of shares can't be negative; "
                  << company << " shares set to 0.\n";
        shares = 0;
      }
      else
        shares = 1;
      share_val = pr;
      set_tot();
    }
    
    Stock::~Stock()
    {
      std::cout << "Bye, " << company << "!\n";
    }
    
    void Stock::buy(long num, double price)
    {
      if (num < 0)
      {
        std::cout << "Number of shares purchased can't be negative. "
                  << "Transaction is aborted.\n";
      }
      else
      {
        shares += num;
        share_val = price;
        set_tot();
      }
    }
    void Stock::sell(long num, double price)
    {
      using std::cout;
      if (num < 0)
      {
        cout << "Number of shares sold can't be negative. "
             << "Transaction is aborted.\n";
      }
      else if (num > shares)
      {
        cout << "You can't sell more than you have! "
             << "Transaction is aborted.\n";
      }
      else
      {
        shares -= num;
        share_val = price;
        set_tot();
      }
    }
    void Stock::update(double price)
    {
      share_val = price;
      set_tot();
    }
    
    void Stock::show()
    {
      // std::cout << "Company: " << company
      //           << " Shares: " << shares << '\n'
      //           << " shares Price: $" << share_val
      //           << " Total Worth: $" << total_val << '\n';
      using std::cout;
      using std::ios_base;
      ios_base::fmtflags orig = 
        cout.setf(ios_base::fixed, ios_base::floatfield);
      std::streamsize prec = cout.precision(3);
    
      cout << "Company: " << company
           << " Shares: " << shares << '\n'
           << " shares Price: $" << share_val;
      cout.precision(2);
      cout << " Total Worth: $" << total_val << '\n';
      cout .setf(orig, ios_base::floatfield);
      cout.precision(prec);
    }

usestock2.cpp

#include <iostream>
    #include "stock10.h"
    int main()
    {
      {
        using std::cout;
        cout << "Using constructors to create new objects\n";
        Stock stock1("NanoSmart", 12, 20.0);
        stock1.show();
        Stock stock2 = Stock("Boffo Objects", 2, 2.0);
        stock2.show();
    
        cout << "Assigning stock1 to stock2:\n";
        stock2 = stock1;
        cout << "Listing stock1 and stock2:\n";
        stock1.show();
        stock2.show();
    
        cout << "Using a constructor to reset an object\n";
        stock1 = Stock("Nifty Foods", 10, 50.0);
        cout << "Revised stock1:\n";
        stock1.show();
        cout << "Done\n";
      }
      return 0;
    }

运行结果

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3NvdWx3eWI_size_16_color_FFFFFF_t_70 2][]

main()的大括号

![20190521171643843.png][]

对象间赋值

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3NvdWx3eWI_size_16_color_FFFFFF_t_70 3][]

对象重新初始化

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3NvdWx3eWI_size_16_color_FFFFFF_t_70 4][]

对象也能够使用列表初始化 只要顺序对

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3NvdWx3eWI_size_16_color_FFFFFF_t_70 5][]

关于对象函数的const

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3NvdWx3eWI_size_16_color_FFFFFF_t_70 6][]

最后说个 对象初始化可以这样（针对构造函数只有1个形参）：

Bozo tubby = 32;

但是可能会出问题 所以 在后面会介绍如何关闭这种特性

完结 明天继续

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