[C++]C++学习笔记（二）

（一）基本知识

//stock00.h
#ifndef STOCK00_H_
#define STOCK00_H_

#include <string>

class Stock  // class declaration
{
private:
    std::string company;
    long shares;
    double share_val;
    double total_val;
    void set_tot() { total_val = shares * share_val; }
public:
    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();
};    // note semicolon at the end

#endif



#include <iostream>
#include "stock00.h"

void Stock::acquire(const std::string & co, long n, double pr)
{
    company = co;
    if (n < 0)
    {
        std::cout << "Number of shares can't be negative; "
                  << company << " shares set to 0.
";
        shares = 0;
    }
    else
        shares = n;
    share_val = pr;
    set_tot();
}

void Stock::buy(long num, double price)
{
     if (num < 0)
    {
        std::cout << "Number of shares purchased can't be negative. "
             << "Transaction is aborted.
";
    }
    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.
";
    }
    else if (num > shares)
    {
        cout << "You can't sell more than you have! "
             << "Transaction is aborted.
";
    }
    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 << '
'
              << "  Share Price: $" << share_val
              << "  Total Worth: $" << total_val << '
';
}

1,类的私有成员变量只能通过类的成员函数去访问，类的成员函数可以通过类对象程序访问
2，声明类对象 
Stack Jack;
类的每个对象都有自己的存储空间，每个对象都有自己的成员变量，但是多个成员变量共享一份成员函数

（二）类的构造函数和析构函数

Stock::Stock(const string &co,long n,double pr )
{
    company = co;
    if (n < 0)
    {
        std::cerr<<"can not be negative";
        shares = 0;
    }
    else
        shares - n;
    share_val = pr;
    set_tot()
}

构造函数在创建类对象的时候调用，构造函数被用来创建对象，而不能被对象调用
示例：
Stock flood = Stock("hello",4,6.9);
或者 Stock flood("hello",4,6.9);
或者 Stock *pstock = new Stock("hello",4,6.9);
析构函数用来做清理工作

Stock::~Stock()
{

}

(三)类对象可以互相赋值

Stock Stock1 =Stock("hello",4,6.9);
Stock Stock2("welcome",4,6.0);
Stock2 = Stock1;

(四)const成员函数 为了保证成员函数不会修改调用的类对象，C++是将const关键字放在函数括号的后面 void show()const;

(五)this指针 const Stock &topval(const Stock &s)const; 括号内的const是指该函数不会修改被显式的访问的对象 最后面的const是指该函数不会修改被隐式的访问的对象 例：top = Stock1.topval(Stock2);  //Stock2被显式访问，Stock1被隐式访问 this指针指向用来调用成员函数的对象，作为隐藏参数传递给函数 示例程序

const Stock &Stock::topval(const Stock &s)const
{
    if(s.total_val > total_val)
    {
        return s;
    }
    else
    {
        return *s
    }
}

(六)类作用域 （1）定义作用域为类的常量

class Bakery{
    const int months = 12;  //出错，因为在声明类的时候类还没有存储空间
    ....
    };

正确的如下：

class Bakery{
    enum {months = 12};  //出错，因为在声明类的时候类还没有存储空间
    ....
    };
class Bakery{
    static const int months = 12;  //出错，因为在声明类的时候类还没有存储空间
    ....
    };