目录
1.指针
1.1指针三要素:
1.2修饰结构体struct
1.3 Pointers of Pointers
1.4constant修饰 pointer
2.指针和数组
2.1.数组的地址是连续的
2.2pointer arithmetic:指针的代数运算
2.3指针和数组的不同
3.内存分配:
1.指针
1.1指针三要素:
- 运算符&:可以取对象或基本类型变量的地址。
- 运算符*:可以获取指针指向的内容
int main(){
//指针三要素
int num = 10;
int *p1 = NULL , *p2 = NULL; //1.声明指针变量,必须初始化(NULL=整数0)
p1 = # //2.取num地址,赋值给指针
p2 = #
cout << "p1的值:"<<p1 <<endl;
cout<<"num = "<<num<<endl;
*p1 = 20; //3.利用指针修改num的值
cout<<"num = "<<num<<endl;
*p2 = 50; //利用指针修改num的值
cout<<"num = "<<num<<endl;
cout << "p1的值:"<<p1 <<endl;
cout << "p2的值:"<<*p2 <<endl;
return 0;
}
1.2修饰结构体struct
指针指向结构体:
Student zhaoyan = { "yan",2000,true}; //实例化结构体
Student *pStu = &zhaoyan; //取这个结构体的地址
strncpy(pStu->name,"Li",4); //将pStu->name修改为Li
pStu->born = 2021;
(*pStu).born = 2002;
pStu->male = false;
cout << zhaoyan.name << " was born in " << zhaoyan.born
<< ". Gender: " << (zhaoyan.male ? "male" : "female") << endl;
//打印指针地址
printf("address of zhaoyan: %p\n",pStu); //C style p:输出指针地址
cout<< "addres of zhaoyan: "<< pStu <<endl; //C++ style
cout<< "addres of zhaoyan: "<< &zhaoyan <<endl; //C++ style
cout<< "addres of zhaoyan.born: "<< &(zhaoyan.born) <<endl; //C++ style
cout<<"sizeof(pStu):" << sizeof(pStu) <<endl;
return 0;
1.3 Pointers of Pointers
指针也是变量,也有地址:任何指针变量都占4个字节
int num = 10;
int *p = #
int **pp = &p; //指针的指针
*(*pp) =20;
cout<< "num=:"<<num<<endl;
return 0;
1.4constant修饰 pointer
int main()
{
int num = 1;
int another = 2;
//You cannot change the value that p1 points to through p1
const int * p1 = # 指向的地址不变,即num不变
*p1 = 3; //error 不能利用指针修改指针指向的值
num = 3; //okay
//You cannot change value of p2 (address)
int * const p2 = # 内存里地址的值不变 ,即(*a)不变
*p2 = 3; //okay 只能通过指针修改指针指向的值
p2 = &another; //error
//You can change neither
const int* const p3 = #
*p3 = 3; //error
p3 = &another; // error
return 0;
}
例子:
为了函数求和等,传入只读的数
int foo(const char * p)
{
// the value that p points to cannot be changed
// play a trick?
char * p2 = p; //syntax error
//...
return 0;
}
2指针和数组
2.1.数组的地址是连续的
Student stu[128];
Student *p0 = &stu[0]; //主要中括号的优先级比&高
Student *p1 = &stu[1];
Student *p2 = &stu[2];
Student *p3 = &stu[3];
printf("p0=%p\n",p0);
printf("p1=%p\n",p1);
printf("p2=%p\n",p2);
printf("p3=%p\n",p3);
//相同的操作
stu[1].born = 2000;
p1->born =2000;
将数组名可以看作一个指针:
Student students[128];
printf("&students = %p\n", &students);
printf("students = %p\n", students);
printf("&students[0] = %p\n", &students[0]);
Student * p = students;
p[0].born = 2000;
p[1].born = 2001;
p[2].born = 2002;
printf("students[0].born = %d\n", students[0].born);
printf("students[1].born = %d\n", students[1].born);
printf("students[2].born = %d\n", students[2].born);
2.2pointer arithmetic:指针的代数运算
- p+num or num+p points :指向数组p的第num个元素
- p - num :指向第-num个数组
给参数加()是为了优先级:更安全
//用宏来打印数组:宏是不能换行的
#define PRINT_ARRAY(array,n)\
for(int i=0;i<(n);i++)\ //最好加一个()防止优先级
cout<<"array["<<i<<"] = " <<(array)[idx] <<endl;
int main(){
int numbers[4] = {0,1,2,3};
PRINT_ARRAY(numbers,4);
int *p = numbers+1; //指向值为1的元素
p++; //指向值为2的元素
cout << "numbers = " << numbers << endl;
cout << "p = " << p << endl;
*p =20; //change 2 to 20
*(p-1) = 20; //change 1to 10
p[1] = 30; //change 3 to 30
PRINT_ARRAY(numbers,4);
return 0;
}
注意:指针不能去了不该去的地方!这种错误最难找
小心越界:有时候会报错,有时不报错
int num = 0;
int * p = #
p[-1] = 2; //out of bound
p[0] = 3; //okay
*(p+1) = 4; //out of bound
2.3指针和数组的不同
- 数组是const指针
- sizeof运算符指向指针将返回地址的大小(4或8)
- 数组中所有元素的总大小可以通过sizeof运算符获得
int numbers[4] = {0, 1, 2, 3};
int * p = numbers;
cout << sizeof(numbers) << endl; //4*sizeof(int)
cout << sizeof(p) << endl; // 4 or 8
cout << sizeof(double *) << endl; // 4 or 8
3.内存分配:
在head上可以申请很大的内存使用,
void* malloc(size_t size)
int *p1 = (int*)malloc(4)
int *p1 = (int*)malloc(3)
//申请3个字节,但是int*需要4个字节
//但是不会报错,因为内存至少分配16个字节
内存的释放:
void free(void* ptr);
p = (int *) malloc(4 * sizeof(int));
// ...
p = (int *) malloc(8 * sizeof(int));
// ...
free (p);
内存泄漏:
案例:
void foo()
{
int* p = (int*) malloc(sizeof(int));
return;
}
上述的函数没有释放就return,在内存就找不到申请指针p的地址;必须在函数内释放
内存泄漏: 没有变量保留第一个地址。内存管理系统不会自动解除分配。浪费内存!
int main()
{
int * p = NULL;
p = (int *) malloc(4 * sizeof(int));
// some statements
p = (int *) malloc(8 * sizeof(int));
// some statements
free (p);
// the first memory will not be freed
for(int i = 0; i < 1024; i++)
{
p = (int *) malloc(1024 * 1024 * 1024);
}
printf("End\n");
return 0;
}
C++中申请释放内存:
new:
//allocate an int, default initializer (do nothing)
int * p1 = new int;
//allocate an int, initialized to 0
int * p2 = new int();
//allocate an int, initialized to 5
int * p3 = new int(5);
//allocate an int, initialized to 0
int * p4 = new int{};//C++11
//allocate an int, initialized to 5
int * p5 = new int {5};//C++11
//allocate a Student object, default initializer
Student * ps1 = new Student;
//allocate a Student object, initialize the members
Student * ps2 = new Student {"Yu", 2020, 1}; //C++11
//allocate 16 int, default initializer (do nothing)
int * pa1 = new int[16];
//allocate 16 int, zero initialized
int * pa2 = new int[16]();
//allocate 16 int, zero initialized
int * pa3 = new int[16]{}; //C++11
//allocate 16 int, the first 3 element are initialized to 1,2,3, the rest 0
int * pa4 = new int[16]{1,2,3}; //C++11
//allocate memory for 16 Student objects, default initializer
Student * psa1 = new Student[16];
//allocate memory for 16 Student objects, the first two are explicitly initialized
Student * psa2 = new Student[16]{{"Li", 2000,1}, {"Yu", 2001,1}}; //C++11
cout << psa2[1].name << endl;
cout << psa2[1].born << endl;
delete:
//deallocate memory
delete p1;
//deallocate memory
delete ps1;
//deallocate the memory of the array
delete pa1;
//deallocate the memory of the array
delete []pa2;
//deallocate the memory of the array, and call the destructor of the first element
delete psa1;
//deallocate the memory of the array, and call the destructors of all the elements
//调所有的析构函数
delete []psa2; //最好用这种方法,删除数组结构体里所有内容