C++（10）：类基础

2019-11-10 21:50:06

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面向对象编程是一种特殊的设计程序的概念性方法，C++通过一些特性改进了C语言，使得应用这种方法更容易。下面是最重要的OPP特性：抽象封装和数据隐藏多态继承代码的可重用性

为了实现这些特性并将它们组合在一起，C++最重要的改进是提供了类。

1，类的设计和使用

1.1，类的声明，头文件(.h)

在类声明中一般包括两部分，一般情况下，数据（PRivite）,成员函数（public）。成员函数可以调用私有数据，是接口。

// stock00.h -- Stock class interface（用户接口）// version 00#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();}; // 冒号#endif

1.2,实现类成员函数，源文件（.cpp）

对于类的成员函数在源文件中实现，如果成员函数很短，可以在类声明里实现，这样的话就会变成内联函数，如void set_tot()。

// stock00.cpp -- implementing the Stock class（实现类成员函数）// version 00#include <iostream>#include "stock00.h"//使用类作用域符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./n"; 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./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' << " Share Price: $" << share_val << " Total Worth: $" << total_val << '/n';}

1.3，创建一个程序，使用创建的类

// usestok0.cpp -- the client program// compile with stock.cpp#include <iostream>#include "stock00.h"int main(){ Stock fluffy_the_cat; fluffy_the_cat.acquire("NanoSmart", 20, 12.50); fluffy_the_cat.show(); fluffy_the_cat.buy(15, 18.125); fluffy_the_cat.show(); fluffy_the_cat.sell(400, 20.00); fluffy_the_cat.show(); fluffy_the_cat.buy(300000,40.125); fluffy_the_cat.show(); fluffy_the_cat.sell(300000,0.125); fluffy_the_cat.show(); // std::cin.get(); return 0;}

三个文件编译后结果

2，类的构造函数和析构函数

2.1构造函数

使用C++类的目标之一是让使用类对象就像使用标准类型一样。然而以上并不能像初始化int类型那样初始化Stock对象。不能初始化的原因在于，数据的状态是私有的，意味着程序不能直接访问数据成员。因此，C++提供了也特殊的成员函数，类构造函数。它专门用于构造新对象，将值赋给它们的数据成员。

2.2析构函数

用构造函数创建对象后，程序负责跟踪该对象，直到过期为止，此时程序自动调用析构函数完成清理工作，因此很有必要。如果构造函数用new来分配内存，则析构函数将用delete来释放这些内存。Stock的构造函数没有使用new,则析构函数也没什么任务。

析构函数的名称：在类名前加~,没有返回值和声明类型。增加构造函数析构函数后的三个文件如下：

2.3 构造函数与析构函数例

// stock10.h ?Stock class declaration with constructors, destructor added#ifndef STOCK1_H_#define STOCK1_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(); // default constructor Stock(const std::string & co, long n = 0, double pr = 0.0); ~Stock(); // noisy destructor void buy(long num, double price); void sell(long num, double price); void update(double price); void show();};#endif

// stock1.cpp ?Stock class implementation with constructors, destructor added#include <iostream>#include "stock10.h"// constructors (verbose versions)Stock::Stock() // default constructor{ 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 = n; share_val = pr; set_tot();}// class destructorStock::~Stock() // verbose class destructor{ std::cout << "Bye, " << company << "!/n";}// other methodsvoid 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(){ using std::cout; using std::ios_base; // set format to #.### ios_base::fmtflags orig = cout.setf(ios_base::fixed, ios_base::floatfield); std::streamsize prec = cout.precision(3); cout << "Company: " << company << " Shares: " << shares << '/n'; cout << " Share Price: $" << share_val; // set format to #.## cout.precision(2); cout << " Total Worth: $" << total_val << '/n'; // restore original format cout.setf(orig, ios_base::floatfield); cout.precision(prec);}

// usestok1.cpp -- using the Stock class// compile with stock10.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); // syntax 1 stock1.show(); Stock stock2 = Stock ("Boffo Objects", 2, 2.0); // syntax 2 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); // temp object cout << "Revised stock1:/n"; stock1.show(); cout << "Done/n"; } // std::cin.get(); return 0; }

3.this指针

当成员函数设计两个指针的时候可能会用到this指针，如比较两个对象某个数据成员的大小时。 1

//stock20.h...public: // Stock(); // default constructor Stock(const std::string & co, long n = 0, double pr = 0.0); ~Stock(); // do-nothing destructor void buy(long num, double price); void sell(long num, double price); void update(double price); void show()const; //添加比较两个对象大小的声明 const Stock & topval(const Stock & s) const;

//stock20.cpp...const Stock & Stock::topval(const Stock & s) const{ if (s.total_val > total_val) return s; else return *this; //解引用，返回对象}

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