C++（10）：類(lèi)基礎(chǔ)

2019-11-10 19:20:41

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面向?qū)ο缶幊淌且环N特殊的設(shè)計(jì)程序的概念性方法，C++通過(guò)一些特性改進(jìn)了C語(yǔ)言，使得應(yīng)用這種方法更容易。下面是最重要的OPP特性：抽象封裝和數(shù)據(jù)隱藏多態(tài) 繼承代碼的可重用性

為了實(shí)現(xiàn)這些特性并將它們組合在一起，C++最重要的改進(jìn)是提供了類(lèi)。

1，類(lèi)的設(shè)計(jì)和使用

1.1，類(lèi)的聲明，頭文件(.h)

在類(lèi)聲明中一般包括兩部分，一般情況下，數(shù)據(jù)（PRivite）,成員函數(shù)（public）。成員函數(shù)可以調(diào)用私有數(shù)據(jù)，是接口。

// stock00.h -- Stock class interface（用戶接口）// 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();}; // 冒號(hào)#endif

1.2,實(shí)現(xiàn)類(lèi)成員函數(shù)，源文件（.cpp）

對(duì)于類(lèi)的成員函數(shù)在源文件中實(shí)現(xiàn)，如果成員函數(shù)很短，可以在類(lèi)聲明里實(shí)現(xiàn)，這樣的話就會(huì)變成內(nèi)聯(lián)函數(shù)，如void set_tot()。

// stock00.cpp -- implementing the Stock class（實(shí)現(xiàn)類(lèi)成員函數(shù)）// version 00#include <iostream>#include "stock00.h"http://使用類(lèi)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，創(chuàng)建一個(gè)程序，使用創(chuàng)建的類(lèi)

// 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;}

三個(gè)文件編譯后結(jié)果

2，類(lèi)的構(gòu)造函數(shù)和析構(gòu)函數(shù)

2.1構(gòu)造函數(shù)

使用C++類(lèi)的目標(biāo)之一是讓使用類(lèi)對(duì)象就像使用標(biāo)準(zhǔn)類(lèi)型一樣。然而以上并不能像初始化int類(lèi)型那樣初始化Stock對(duì)象。不能初始化的原因在于，數(shù)據(jù)的狀態(tài)是私有的，意味著程序不能直接訪問(wèn)數(shù)據(jù)成員。因此，C++提供了也特殊的成員函數(shù)，類(lèi)構(gòu)造函數(shù)。它專(zhuān)門(mén)用于構(gòu)造新對(duì)象，將值賦給它們的數(shù)據(jù)成員。

2.2析構(gòu)函數(shù)

用構(gòu)造函數(shù)創(chuàng)建對(duì)象后，程序負(fù)責(zé)跟蹤該對(duì)象，直到過(guò)期為止，此時(shí)程序自動(dòng)調(diào)用析構(gòu)函數(shù)完成清理工作，因此很有必要。如果構(gòu)造函數(shù)用new來(lái)分配內(nèi)存，則析構(gòu)函數(shù)將用delete來(lái)釋放這些內(nèi)存。Stock的構(gòu)造函數(shù)沒(méi)有使用new,則析構(gòu)函數(shù)也沒(méi)什么任務(wù)。

析構(gòu)函數(shù)的名稱(chēng)：在類(lèi)名前加~,沒(méi)有返回值和聲明類(lèi)型。增加構(gòu)造函數(shù)析構(gòu)函數(shù)后的三個(gè)文件如下：

2.3 構(gòu)造函數(shù)與析構(gòu)函數(shù)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"http:// 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指針

當(dāng)成員函數(shù)設(shè)計(jì)兩個(gè)指針的時(shí)候可能會(huì)用到this指針，如比較兩個(gè)對(duì)象某個(gè)數(shù)據(jù)成員的大小時(shí)。 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; //添加比較兩個(gè)對(duì)象大小的聲明 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，返回對(duì)象}

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