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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

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：

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 
   
    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 
   
    #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 
   
    #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

main()的大括号

对象间赋值

对象重新初始化

对象也能够使用列表初始化 只要顺序对

关于对象函数的const

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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