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This guide dives deep into the intricacies of pointers in C and C++. It covers fundamental concepts, from defining pointers to dynamic memory allocation, pointer arithmetic, and the risks of improper pointer use. You'll learn how to manage memory effectively with `malloc`, `free`, and the usage of arrays of pointers. We also address common pitfalls such as segmentation faults and give tips on error handling with null pointers. This resource is essential for anyone looking to grasp pointer functionality and enhance their programming skills in C/C++.
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指標 Pointers
int a = 5; int* ptr; // pointer to int ptr = &a; // 取a位址 // int *ptr = &a; *ptr = 10; // 設定指向位址的值 printf(“a = %d\n”, a); //a = 10
int a = 5; int* ptr = &a; *ptr = 10; ptr 1304 a 5 10
小心 int* a, b; // X: b is int!! int*a, *b; // O
小心 int*ptr; … *ptr = 10; Segmentation fault ptr ??
好習慣 int*ptr=NULL; … if(ptr==NULL){ /* error */ } else *ptr = 10;
指標&陣列 char s[16]; char *ptr = s; // 相當於ptr = &s[0]; strcpy(s, “OAO”); printf(“%s\n”, ptr); // OAO putchar(ptr[1]); // A
動態記憶體配置 C style: malloc(), free() int *ptr = malloc(100*sizeof(int)); for(inti=0; i<100; i++) ptr[i] = i; free(ptr); C++ style: new[], delete[] int *ptr = newint[100]; for(inti=0; i<100; i++) ptr[i] = i; delete [] ptr;
小心 intarr[100]; memset(arr, 0, sizeof(arr)); // sizeof(arr) == sizeof(int[100]) // (400 bytes) int *ptr = new int[100]; memset(ptr, 0, sizeof(ptr)); // sizeof(ptr) == sizeof(int*) // (4 bytes)
struct struct XD { int a, b; }; XD xd; xd.a = 2; XD *ptr = &xd; ptr->b = 3; // or (*ptr).b
你可以… void my_strcpy(char *dst, char *src) { for(; *src!=‘\0’; dst++, src++) *dst = *src; } char s1[16], s2[16]; scanf(“%s”, s2); my_strcpy(s1, s2); // my_strcpy(&s1[0], &s2[0])
pointer to pointer int**pp, *p, a; p = &a; pp = &p;
array of pointers int*pa[20], n; pa[2] = &n;
pointerto array int(*pa)[20], arr[20]; pa = &arr;
你可以… int *mat[20], n; scanf(“%d”, &n); for(int i=0; i<20; i++) mat[i] = new int[n]; mat[3][n-2] = 8; for(int i=0; i<20; i++) delete [] mat[i];
你可以… int **mat, n, m; scanf(“%d%d”, &n, &m); mat = new int*[n]; for(int i=0; i<n; i++) mat[i] = new int[m]; mat[n-2][m-3] = 100; for(int i=0; i<n; i++) delete [] mat[i]; delete [] mat;
你不可以… void proc(int**mat){ mat[2][3] = 5; } int main() { … int mat[20][20]; proc(mat); // error } gcc: cannot convert ‘int(*)[20]’ to ‘int**’
你可以… void proc(int(*mat)[20]); void proc(int mat[20][20]); ***不建議***
結語 • 善用指標，可以讓程式更簡潔易懂、更好維護 • 亂用指標 → 輕鬆RE
