c指针和动态内存分配.ppt

Lecture 9:Pointers and Dynamic Memory（指针和动态内存分配）,Learn C+through English and Chinese,2,Chapter Nine:Pointers and Dynamic Memory（指针和动态内存分配）,Variable address(变量的地址)Pointer variables(指针变量)The dereference operator*（解引用运算符*）Using const with pointers(使用const修饰指针变量)Pointers and one-dimensional arrays（指针和一维数组）Pointers and multi-dimensional arrays（指针和多维数组）,3,Pointers to structures(指向结构体的指针)Pointers to class objects(指向类对象的指针)Pointers as function arguments（指针变量作为函数实参）Dynamic memory allocation（动态内存分配）,Chapter Nine:Pointers and Dynamic Memory（指针和动态内存分配）,4,9.1 Variable address(变量地址),Every variable object used in a C+program is stored in a specific place in memory.Each location in memory has a unique address,in the same way that every house in a street has a unique address.(在C+程序中使用的每个变量和对象，都存储在内存中特定的存储单元中。每个存储单元都有唯一的地址，就像街道旁的每个房子都有唯一的地址一样。）,5,Variable address(变量地址),内存空间的访问方式通过变量名访问通过地址访问地址运算符：则&var 表示变量var在内存中的起始地址,6,#include#include using namespace std;void main()int var1=1;float var2=2;cout var1 has a value of var1 and is stored at,7,7.1.1 地址与指针,程序中:int i;float k;,内存中每个字节有一个编号-地址,i,k,为其分配内存单元,变量是对程序中数据及存储空间的抽象,8,9.2 Pointer variables(指针变量),A pointer variable is a variable that holds the address of another variable.（指针变量是存放另一变量地址的变量）data_type*variable_name;int*int_ptr;float*float_ptr;其中，“*”表示后面声明的变量是指针类型的变量。指针变量一旦被赋值，我们就说该指针变量有了指向。“数据类型”可以是任意类型，指的是指针所指向的对象类型，这说明了指针所指向的内存单元可以用于存放什么类型的数据，我们称之为指针的类型。,区分：int*p1,*p2;与 int*p1,p2;,9,Pointer variables(指针,声明例：int i;int*i_pointer=,注意事项 用变量地址作为初值时，该变量必须在指针初始化之前已说明过，且变量类型应与指针类型一致。可以用一个已赋初值的指针去初始化另一 个指针变量。,10,说明：在指针变量定义中，*是一个说明符，它表明其后的变量是指针变量，如在 int*p;语句中，p是指针变量,而不要认为“*p”是指针变量；指针变量定义时指定的数据类型不是指针变量本身的数据类型，而是指针变量所指向的对象（或称目标）的数据类型，指针变量只能指向定义时所规定类型的变量；定义后值不确定，而指针变量一旦被赋值，就有了有效的指向对象；指针变量并不固定指向一个变量，可指向同类型的不同变量；指针变量和普通变量的共同点是：它们都能存放数据，而又有自己的地址。不同的是：普通变量中直接存放通常意义下的数据，而指针变量中存放的是地址。,11,2000,指针,指针变量,变量的内容,变量的地址,指针变量和指针所指向的变量的区别：对于：int i=10,*i_pointer=,变量的地址,变量的内容,12,9.3 the dereference operator*(解引用运算符*),The dereference operator*is used to access the value of a variable,whose address is stored in a pointer.(解引用运算符*用于访问指针变量所指向的存储单元的内容。)int i=10,*i_pointer=,i_pointer=&i=&(*i_pointer)i=*i_pointer=*(&i),i_pointer-指针变量，它的内容是地址量*i_pointer-指针的目标变量，它的内容是数据&i_pointer-指针变量占用内存的地址,13,#include using namespace std;main()int var=1;int*ptr;ptr=,prt contains 0012FF88*ptr contains 1,14,#includeusing namespace std;int main()int*p1,*p2,*p,a,b;cinab;p1=,15,9.4 Using const with pointers(使用const修饰指针变量),When defining a pointer,the pointer itself,the value it points to or both can be made constant.The position of const in the definition determines which of these apply.(定义指针变量时，指针变量本身、指针变量所指向的数据都可以声明为常量。变量定义中的const的位置决定了谁被声明为常量。),16,int i=3,j=4;const int*p=/Illegal:p is a constant.,17,指针与常量 指向常变量的指针,不能通过指针来改变所指对象的值，但指针本身可以改变，可以指向另外的对象。例：const int n2=5;const int*pn=错,18,定义指向常变量的指针变量形式：const 类型名*指针变量名如：const char*ptr;（1）如果一个变量已经被声明为常变量，只能用指向常变量的指针变量指向它，而不能用一般的指针变量去指向它。如：const char c=“boy”;const char*pp=c;char*p2=c;/不合语法。,19,(2)指向常变量的指针变量除了可以指向常变量外，还可以指向未被声明为 const型的变量，此时不能通过指针变量改变该变量的值。char c1=a;const char*p;p=,20,指针与常量 常量指针,若声明指针常量，则指针本身的值不能被改变。但是它指向的整型变量是可以改变的。例：int n1=3;const int n2=5;int*const pn=错,21,例 指向常量的指针做形参,#includeconst int N=6;void print(const int*p,int n);void main()int arrayN;for(int i=0;iarrayi;print(array,N);,22,void print(const int*p,int n)cout*p;for(int i=1;in;i+)cout.*(p+i);coutendl;1 2 3 4 5 61.2.3.4.5.6,23,9.5 Pointers and one-dimensional arrays,Ponters and arrays are directly related to one another.In c+,the name of an array is equivalent to the address of the first element of the array.The name of an array,therefore,is a pointer to the first element of the array.(指针和数组是相互联系的。在C+中，数组名代表数组的首地址。因此，数组名即为指向数组第一个元素的指针。)int a5;a&a0 a+1&a1 As the name of an array is a pointer to the first element of the array,the dereference operator*can be used to access the elements of the array.,24,Pointers and one-dimensional arrays,指向数组元素的指针int a10;int*p;p=,25,Pointers and one-dimensional arrays,如果指针变量p已经指向数组中得一个元素，则P+1指向同一数组中得下一个元素。int array 10=1,2,3,4,5,6,7,8,9,10,*p;当p=array时下面的等价关系成立：p=array=/表示第i个数组元素的地址*(p+i)=*(array+i)=arrayi/表示第i个数组元素指向数组的的指针变量也可以带下标，所以p+i和pi是一样的、,26,引用数组元素,可以有三种方法：（1）下标法：数组名下标 a1（2）指针变量法：*指针变量*(P+i)（3）首地址法：*（首地址+位移量）*(array+i),27,如果先使指针变量p指向数组a的首地址（p=a）则：(1)p+使p指向下一个元素，即a1,用*p可以得到下一个元素a1的值，(2)*p+先得到p指向的变量的值，然后再使P得值加1，使p指向下一个元素，for（p=a;pa+10;）cout*p+;(3)*(p+)和*(+P)不同，后者是先使P加1，在求*P的值。(4)（*p）+表示P所指向的元素值加1.,28,#include using namespace std;main()int a5=10,13,15,11,6;for(int i=0;i 5;i+)cout Element i is*(a+i)endl;,Element 0 is 10 Element 1 is 13 Element 2 is 15 Element 3 is 11 Element 4 is 6,29,Although the name of an array is a pointer to the first element of the array,you cannot change its value;this is because it is a constant pointer.(虽然数组名是指向数组第一个元素的指针，但是由于它是一个常量指针，因此它的值是不能改变的。)int a10;float numbers100;a+;number+=2;,Pointers and one-dimensional arrays,30,You can assign the name of an array to a pointer variable of the same type.int a5;int*p;p=a;/valid:assiqnment of a constant to a variable a+;/Invalia:the value of a costant cannot change.p+;/Valid:p is a variable.p now points to element 1 of the array a.p-;/Valid:point to element 1 of the array a.p+=10;/Valid.But p is outside the range of the array a./so*p is underfined.A common error.p=a-1;/Valid,but p is outside the ranger of the array.,Pointers and one-dimensional arrays,31,A constant may be added to or subtancted from the value of a pointer.allowing access to different memory to locations.Hower,not all arithmeic operations are permissible on poniters.For example,the multiplication of two pointers is illegal,because the result would not be a valid memory address.(通过将指针变量加上或减去一个常量，可以使它指向不同的存储单元。但是，并非所有的算术运算符都可以应用于指针。例如，两个指针相乘是非法的，因为相乘的结果可能不是一个有效的内存地址。),Pointers and one-dimensional arrays,32,分别用三种方式访问数组中的所有元素void main()int i,array10=1,2,3,4,5,6,7,8,9,10,*p;coutNO1:;for(i=0;i10;i+)coutarrayi=arrayi;coutnNO2:n;for(i=0;i10;i+)cout*(array+“i)=*(array+i);coutnNO3(1)n;for(p=array,i=0;i10;i+)cout*(p+“i)=*(p+i);coutnNOs(2)n;for(p=array;parray+10;p+)cout*p+=“*p;coutn;,33,A two-dimensional array is stored as an array of arrays.This means that a is a one-dimensional array whose elements are themselves a one-dimensional arrays of integers.As with one-dimensonal array,the name of a two dimensional array is pointer to the first element of the array.Therefore a is equivalent to&a 0,a 0 is itself an array of two intergers,which means that a 0 is equivalent to&a00.,Pointers and multi-dimensional arrays,34,int a34；,a13*(a1+3)*(*(a+1)+3),35,#includeusing namespace std;int main()int a34=1,3,5,7,9,11,13,15,17,19,21,23;int*p;for(p=a0;pa0+12;p+)cout*p;coutendl;return 0;,指针与二维数组,36,9.7 Pointers to structures,In addition to defining a pointer to a variable of a built-in data type,it is also possible to define a pointer to a variable of a type defined by struct of classs.struct tag_name*variable_name;struct student int number;float score;stu;struct student*prt=,37,9.7 Pointers to structures,而使用结构体指针变量访问被指向的结构体变量有两种形式：(*结构体指针变量).成员名或者 结构体指针变量-成员名,例：struct stu date,*pstu=则：pstu-num 等价于(*pstu).num,38,指向结构体变量的指针,#includeusing namespace std;int main()struct student int num;string name;char sex;float score;,student stu;student*p=,39,（1）结构体变量名.成员名（2）（*p）.成员名（3）p-成员名-指向运算符P-n得到P指向的结构体变量中的成员n的值。p-n+得到p指向的结构体变量中成员n的值，用完改值后使它加1.+p-n得到p指向的结构体变量中的成员n的值，并使之加1，然后再使用它。,40,用指向结构体变量的指针作实参#include#includeusing namespace std;struct studentint num;string name;float score3;stu=12345,lifung,67.5,89,78.5;,int main()void print(student*);student*pt=,41,9.8 Pointers to class objects,Defining a pointer to a class object is similar to defing a pointer to a structure variable.class-name*variable-name;,42,指向类类型对象的指针,声明形式类名*对象指针名；例Point A(5,10);Piont*ptr;ptr=通过指针访问对象成员对象指针名-成员名,43,Class Time public:int hour;int minute;int sex;void get_time();Void Time:get_time()couthour“:”minute“:”sexendl;,44,在此基础上有以下语句：Time*p1;Time t1;p1=可以通过对象指针访问对象成员和对象(*p1).hour p1所指向的对象中的hour成员，t1.hourP1-hour p1所指向的对象中的hour成员，t1.hour(*p1).get_time 调用p1所指向的对象中的get_time 函数,t1.get_timeP1-get_time 调用p1所指向的对象中的get_time 函数,t1.get_time,45,例 对象指针应用举例,void main（）Point A(5,10);Point*ptr;ptr=,46,Example,#include#include#include bank_ac.h#include bank_ac.cppusing namespace std;main()bank_account ac;/ac is a bank_account object.bank_account*ac_ptr;/ac_ptr is a pointer to a bank_account.ac_ptr=,ac-ptr-deposit(100);(*ac_ptr).deopsit(100);,47,9.9 Pointers as funtion arguments,Like any data type,pointers can be used as function arguments.,48,#include using namespace std;void swap_vals(float*val1,float*val2);main()float num1,num2;cout num1;cin num2;if(num1 num2)swap_vals(,void swap_vals(float*ptr1,float*ptr2)float temp=*ptr1;*ptr1=*ptr2;*ptr2=temp;,Please enter two number:12.1 6.4The numbers in order are 6.4 and 12.1,49,9.10 Dynamic memory allocation,C+has the ability to allocate memory while a program is executing.This is done by using the memory allocation operator new.,50,Dynamic memory allocation,动态申请内存操作符 newnew 类型名T（初值列表）功能：在程序执行期间，申请用于存放T类型对象的内存空间，并依初值列表赋以初值。结果值：成功：T类型的指针，指向新分配的内存。失败：0（NULL）,51,new运算符的例子；new int；/开辟一个存放整数的存储空间，返回一个指向该存储空间的地址（即指针）new int（100）；/开辟一个存放整数的存储空间，并指定该整数的初值为100，返回一个指向该存储空间的地址（即指针）,new char10；/开辟一个存放字符数组的（包含10个元素）空间，返回首元素的地址。new int54;/开辟一个存放二维整型数组（大小为5*4）的空间，返回首元素的地址。float*p=new float（3.14159）；/开辟一个存放单精度数的空间，并指定该实数的初值是3.14159，将返回的该空间的地址赋给指针变量p。,52,释放内存操作符delete,delete 运算符使用的一般格式为：delete 指针P功能：释放指针P所指向的内存。P必须是new操作的返回值。delete p；new char10；如果把 new返回的指针付给指针变量pt，则用 delete pt；/在指针变量前面加一对方括号，表示是对数组空间的操作。,53,#include#includeusing namespace std;struct studentchar*name;int num;char sex;,int main()student*p;p=new student;p-name=wang Fun;p-num=10123;p-sex=m;coutnamenumsexendl;delete p;return 0;,54,例 动态存储分配举例,#include struct date int month;int day;int year;,55,void main（）int index,*point1,*point2;point1=,56,float*float_point1,*float_point2=new float;float_point1=new float;*float_point2=3.14159;*float_point1=2.4*(*float_point2);delete float_point2;delete float_point1;,57,date*date_point;date_point=new date;/动态分配结构体 date_point-month=10;date_point-day=18;date_point-year=1938;cout month day year n;delete date_point;/释放结构体,58,char*c_point;c_point=new char37;/动态分配数组 delete c_point;/释放数组 c_point=new charsizeof(date)+133;/动态分配数组 delete c_point;/释放数组 return 0;运行结果：The values are 77 77 173The values are 77 999 99910/18/1938,59,对象的动态建立和释放,如果已经定义了一个box类，可以用下面的方法动态的建立一个对象:new box;定义一个指向本类的对象的指针变量来存放地址box*pt;pt=new box;,60,在程序中就可以通过pt访问这个新建的对象，coutheighr;coutvolume();c+还允许在执行new时，对新建的对象进行初始化。可以使用delete运算符对新建的对象所占有的空间进行套释放。delete pt；在执行运算符时，自动调用析构函数。,61,例 动态创建对象举例,#includeclass Point public:Point（）X=Y=0;coutDefault Constructor called.n;Point(int xx,int yy)X=xx;Y=yy;cout Constructor called.n;Point（）coutDestructor called.n;int GetX（）return X;int GetY（）return Y;void Move(int x,int y)X=x;Y=y;private:int X,Y;,62,void main（）coutStep One:endl;Point*Ptr1=new Point;delete Ptr1;coutStep Two:endl;Ptr1=new Point(1,2);delete Ptr1;,运行结果：Step One:Default Constructor called.Destructor called.Step Two:Constructor called.Destructor called.,63,例 动态创建对象数组举例,#includeclass Point/类的声明同例6-15，略;void main（）Point*Ptr=new Point2;/创建对象数组 Ptr0.Move(5,10);/通过指针访问数组元素的成员 Ptr1.Move(15,20);/通过指针访问数组元素的成员 coutDeleting.endl;delete Ptr;/删除整个对象数组,运行结果：Default Constructor called.Default Constructor called.Deleting.Destructor called.Destructor called.,64,访问对象的数据成员,#include using namespace std;class Humanpublic:int get()constreturn i;void set(int x)i=x;private:int i;,int main()Human*p=new Human;p-set(1);coutget()endl;delete p;return 0;,运行结果：1,65,在构造函数中开辟空间,#include using namespace std;class Humanpublic:int get()constreturn*i;void set(int x)*i=x;Human();Human();private:int*i;Human:Human()cout构造函数执行中.n;i=new int(999);,int main()Human*p=new Human;coutget()set(1);coutget()endl;delete p;return 0;Human:Human()cout析构函数执行中.n;delete i;,构造函数执行中9991析构函数执行中,66,9.10.1 Allocating memory dynamically for an array,The new memory allocation operator can be used to allocate a contiguous block of memory for an array of any type,whether the data type is built-in or is a user-defined structure of class.pointer=new data_typesize;For example:/Allocate memory for 10 integers.int_ptr=new int10;/Allocate memory for 5 bank_account objects.ac_ptr=new bank_account5;When allocating memory for an array of class objects,there must be a default constructor for the class so that the elements of the array get initialised.(当为一个类对象数组动态分配内存时，这个类必须有自己的默认构造函数来初始化类对象数组元素。),67,/dynamic memory allocation for an array of integers.#includeusing namespace std;main()int*int_array;int no_els,i;cout no_els;int_array=new intno_els;for(i=0;i int_arrayi;for(i=0;i no_els;i+)cout“Element”i“is“*(int_array+i)endl;delete int_array;,Enter the number of elements 2Enter element 0:57Enter element 1:69Element 0 is 57Element 1 is 69,68,The elements of the newly allocated array can be accessed using either a pointer or an index.（可以使用指针或下标方式访问新分配内存的数组元素。）int_array1*(int_array+1)The allocated memory is freed using the delete operator.It is important to remember to include the square brackets when freeing memory for a previously allocated array.Without the square brackets,only the first element of the array will be deleted.(可以使用delete运算符释放已分配的内存。当为一个数组释放先前动态分配的内存时，必须用方括号。如果不加释放的仅是数组的第一个元素的内存。),69,9.10.2 Initialisation with new,When allocating memory for an array of class objects,the default constructor for the class initialises the elements of the array.(当为一个类对象的数组动态分配内存时，会自动调用类的默认构造函数对数组元素进行初始化。)Ac_ptr=new bank_account5;results in the default constructor for the bank account class being called five times.No initialisation is done for dynamically allocated arrays of built-in data types.(对内置数据类型的数组动态分配内存时无需对其初始化。)For example,int_ptr=new int10;results in the ten non-initialised integer elements.,70,Single instances of any data type(built-in,a user-defined structure or a class)can be initialised using a second form of the new operator.(对任意数据类型的单个实例，可以使用new运算符的另一种形式对其进行初始化。)pointer=new data_type(initial_value);Eg./allocate memory for an integer,with an initial value of 100.int_ptr=new int(100);/allocate memory for an integer,with no initial value.int_ptr=new int;/allocate memory for a bank_account object.The default constructor is/called to do the initialisation.ac_ptr=new bank_account;/allocate memory for a bank_account object.A constructor is called to/assign initial values to account number and balance.ac_ptr=new bank_account(1234,100);,初始值是可选的,71,#include#include#include“bank_ac.h”#include“bank_ac.cpp”using namespace std;main()int no_of_acs;cout no_of_acs;bank_account*accounts=new bank_accountno_of_acs;cout endl“Accounts:”endl;for(int i=0;i no_of_acs;i+)accountsi.display_balance();cout endl;,72,coutdisplay_balance();coutdisplay_balance();delete accounts;delete bank_ptr1;delete bank_ptr2;,运行结果Enter the number of bank accounts 2Account:Balance in Account 1 is 0.00Balance in Account 2 is 0.00bank_ptr1:Balance in Account 3 is 0.00bank_prt2:Balance in Account 123 is 100.00,73,9.10.3 allocating memory for multi-dimensional arrays,In C+,multi-dimensional a