C++程序設計-華南理工大學 第5章-類3：初始化和清除

第五章類:初始化和清除面向?qū)ο蟪绦蛟O計(C++)5.3初始化和清除構造函數(shù)析構函數(shù)何時被調(diào)用默認構造函數(shù)與重載構造函數(shù)5.3.1安全性要求正確地初始化和清除對象是保證程序安全性的關鍵問題！intmain(){StashintStash;

intStash.initialize(sizeof(int));//初始化

for(inti=0;i<100;i++){

intStash.add(&i);}……;

intStash.cleanup();//清除}5.3.2構造函數(shù)5.3.2.1

構造函數(shù)：確保初始化類的特殊成員函數(shù)，編譯器在創(chuàng)建對象時自動調(diào)用該函數(shù)。通常做一些初始化動作。以保證同一個類的對象具有一致性。構造函數(shù)跟類同名，可以帶參數(shù)，沒有返回值。跟別的成員函數(shù)沒有名字沖突；編譯器總能知道調(diào)用哪一個函數(shù)；例：帶構造函數(shù)的StashclassStash{

intsize;//Sizeofeachspace…;public:

Stash(intsize);//構造函數(shù)，跟類同名,無返回值

int

add(void*element);…;voidcleanup();};Stash::Stash(int

sz){size=sz;quantity=0;storage=0;next=0;}自動初始化Stash對象intmain(){//創(chuàng)建對象時自動調(diào)用構造函數(shù)StashintStash1(2),intStash2(2);

for(inti=0;i<100;i++){intStash1.add(&i);}……;intStash1.cleanup();//清除}說明構造函數(shù)的名字必須與類的名字相同；構造函數(shù)不允許指明返回類型，也不允許返回一個值；構造函數(shù)應聲明為public（但不是必須），否則無法創(chuàng)建對象。classStash{

private:

Stash(int

sz){…};

…};voidmain(){

Stashs(10);//Error!can‘taccessprivatemember}說明(續(xù))可以重載構造函數(shù)；classStash{public:Stash(){size=1;…;}

Stash(int

sz){…}…};voidmain(){StashcharStash;StashintStash(2);StashintStash(10,2);//error,參數(shù)不匹配!}5.3.3析構函數(shù)5.3.3.1析構函數(shù)：確保清除析構函數(shù)：類的特殊成員函數(shù)，在撤銷對象時自動調(diào)用該函數(shù)。通常做一些撤銷對象前的回收工作。析構函數(shù)不帶參數(shù)，沒有返回值；不能夠重載。析構函數(shù)必須是public函數(shù)。例:String類ClassString{private:char*str;public:

String(char*s);//構造函數(shù)，跟類同名

String(unsigned

int

sz);//重載構造函數(shù)

…;

~String();//析構函數(shù)，類名前面加上一個~};

String::~String(){deletestr;}String::String(unsigned

int

sz){//不能有返回值；if(sz>0)str=newchar[sz];elsestr=newchar[100];}String::String(char*s){//不能有返回值；

int

len;

len=strlen(s);if(len<100)len=100;

str=newchar[len];

strcpy(str,s);}voidmain(){StringPaper_Tiger(“U.S.A.”);}//退出主程序前撤銷對象，調(diào)用析構函數(shù)5.3.4何時被執(zhí)行？5.3.4.1

何時被執(zhí)行？構造函數(shù)：當對象被創(chuàng)建時，調(diào)用構造函數(shù)；析構函數(shù)：當對象被撤銷時，調(diào)用析構函數(shù)。//此例說明何時調(diào)用構造函數(shù)和析構函數(shù)//類的定義classTree{

intheight;public:

Tree(int

initialHeight);//構造函數(shù)

~Tree();//析構函數(shù)

voidgrow(intyears);voidprintsize();};//類的實現(xiàn)Tree::Tree(int

initialHeight){height=initialHeight;printsize();}Tree::~Tree(){

cout<<"insideTreedestructor";

}voidTree::grow(intyears){height+=years;}voidTree::printsize(){

cout<<"Treeheightis"<<height<<endl;}intmain(){

cout<<"beforeopeningbrace"<<endl;{ Treet(12);//生成t

cout<<"afterTreecreation"<<endl; t.grow(4);

cout<<"beforeclosingbrace"<<endl;}//此時撤銷t;

cout<<"afterclosingbrace"<<endl;}///:~beforeopeningbraceafterTreeCreationbeforeclosingbraceinsideTreedestructor//執(zhí)行析構函數(shù)Treeheightis16afterclosingbrace5.3.5完整的類5.3.5.1

完整的類

完整的類通常具有的特征：數(shù)據(jù)抽象：數(shù)據(jù)成員&成員函數(shù)實現(xiàn)隱藏；(訪問控制)自動初始化和清除；(構造函數(shù)和析構函數(shù))例：類Stash//Withconstructors&destructors#ifndefSTASH2_H#defineSTASH2_HclassStash{

intsize;//Sizeofeachspace

intquantity;//Numberofstoragespaces

intnext;//Nextemptyspace

//Dynamicallyallocatedarrayofbytes:unsignedchar*storage;voidinflate(intincrease);public://:C06:Stash2.h

Stash(intsize); ~Stash();

int

add(void*element); void*fetch(intindex);

intcount();};#endif//STASH2_H///:~//Constructors&destructors#include"Stash2.h"#include"../require.h"#include<iostream>#include<cassert>usingnamespacestd;constintincrement=100;//:C06:Stash2.cppStash::Stash(int

sz){//構造函數(shù)

size=sz;quantity=0;storage=0;next=0;}int

Stash::add(void*element){

if(next>=quantity)inflate(increment);//Copyelementintostorage,//startingatnextemptyspace:

int

startBytes=next*size; unsignedchar*e=(unsignedchar*)element;

for(inti=0;i<size;i++)

storage[startBytes+i]=e[i]; next++;

return(next-1);//Indexnumber}void*Stash::fetch(intindex){ require(0<=index,"Stash::fetch(-)index");

if(index>=next)return0;//Toindicatetheend//Producepointertodesiredelement:return&(storage[index*size]);}int

Stash::count(){returnnext;//NumberofelementsinCStash}voidStash::inflate(intincrease){

require(increase>0,"Stash::inflatezeroornegativeincrease");

int

newQuantity=quantity+increase;

int

newBytes=newQuantity*size;

int

oldBytes=quantity*size;unsignedchar*b=newunsignedchar[newBytes];

for(inti=0;i<oldBytes;i++)

b[i]=storage[i];//Copyoldtonewdelete[](storage);//Oldstoragestorage=b;//Pointtonewmemoryquantity=newQuantity;}Stash::~Stash(){

if(storage!=0){

cout<<"freeingstorage"<<endl;

delete[]storage;}}///:~//:C06:Stash2Test.cpp#include"Stash2.h"#include"../require.h"#include<fstream>#include<iostream>#include<string>usingnamespacestd;intmain(){StashintStash(sizeof(int));

for(inti=0;i<100;i++)

intStash.add(&i);

for(intj=0;j<intStash.count();j++)

cout<<"intStash.fetch("<<j<<")="<<*(int*)

intStash.fetch(j)<<endl;constint

bufsize=80;StashstringStash(sizeof(char)*bufsize);

ifstreamin("Stash2Test.cpp");

assure(in,"Stash2Test.cpp");stringline;

while(getline(in,line))

stringStash.add((char*)line.c_str());

intk=0;char*cp;

while((cp=(char*)stringStash.fetch(k++