1. C++ Programming: review on C: Pointer and malloc
2018, Spring
Pusan National University
Ki-Joune Li

2. Pointer 와 Address Address 또는 Pointer Pointer Variable
Main Memory의 위치를 지정
각 Variable의 값을 저장하는 위치
&n : m이 저장된 위치 (0x20AC)
Pointer Variable
Address를 값으로 하는 변수
Main memory
int n,m; n=20; m=30;
0x20AC
n = 20
pn = 20AC
int n,*pn; pn=&n; *pn=30;

3. Pointer와 Function Parameter
Call-By Value를 통한 Function내의 값 변경
int m,n;
. . .
m=20; m=50; swap(&m,&n);
void swap(int *a, int *b) {
int m; m=*a; *a=*b; *b=m; }
int m,n;
. . .
m=20; m=50; swap(m,n);
void swap(int a, int b) {
int m; m=a; a=b; b=m; }

4. Pointer와 Array Pointer 와 Array의 유사성과 차이점 Main memory int values[100];
values: values array의 처음 주소
int *pvalues; pvalues=values;  pvalues[0]와 values[0]은 서로 호환 가능
차이점: values는 상수
0x20AC=values=&values[0]
values[0] …
values[99]

5. Pointer와 Array Pointer와 Array Index
int *pvalues; pvalues=values; /* pvalues[0]==values[0] */ ++pvalues; /* *pvalues==values[1] */
Example int sum=0; int sum=0; int i, values[10]; int i,values[10],*pvalues; /* ... value[0], ... value[9] */ pvalues=values; for(i=0;i<10;i++) for(i=0;i<10;i++) sum=sum+values[i]; sum=sum+*values++; */
int sum=0; int sum=0; int i, values[10]; int i,values[10],*pvalues; pvalues=values; for(i=0;i<10;i++) sum=sum+pvalues[i];

6. Array Arithmetic Increment and Decrement
pvalues가 0x1000일 때 pvalues++ 한 후 pvalues != 0x1000 pvalues == 0x sizeof(type of pvalues, int의 경우 4)
Multiplication과 Division은 Pointer에 대하여 사용 불가능
int *pvalue;
int value;
pvalue=&value;
pvalue=pvalue*2; /* Error */
pvalue=pvalue+2; /* OK */

7. Character Pointer Character Pointer Exchangeable with Character Array
Example
char *string1=“Hello World\n”; char string2[]=“Hello World\n”;
void strcpy(char *s, char *d) {
while((*s++=*d++)!=‘\0’); }
void strcpy(char s[], char d[]) {
int i=0; while((s[i]=d[i++])!=‘\0’); }

8. Character Pointer Pointer to String and Double Pointer
Example char *stringArray[2];
char **pstr;
stringArray[0]="Hello first\n";
stringArray[1]="Hello second\n";
pstr=stringArray;
printf("%s",*pstr++);
printf("%s",*pstr);
Parameters of main function
void main(int argc, char *argv[])
pstr
stringArray[0]
“Hello first”
stringArray[1]
“Hello second”

9. Pointer Array and Multidimensional Array
Pointer Array: Array of Pointer
Example int values[10][20]; int *pvalues[10]; pvalues[0]=values[0]; pvalues[0]++; /* ? */ pvalues[0][1]; /* ? */
Main memory
values =0x1000
values[0] = 0x1000
values[0][0]
values[0][0]
values[0][1] …
values[1] =0x1050
values[1][0] … …
values[9][19]

10. Double Pointer Example If single pointer would be used?
#include <stdlib.h> void main() { char *string = "Hello, world!"; char *copystr; if(allocstr(strlen(string), &copystr))strcpy(copystr, string); else fprintf(stderr, "out of memory\n"); } int allocstr(int len, char **retptr) { char *p = malloc(len + 1); /* +1 for \0 */ if(p == NULL) return 0; *retptr = p; return 1; }
If single pointer would be used?

11. malloc Dynamic memory allocation (in run time) typedef struct {
int year;
char name[20];
float score;
} Student;
int main(int argc,char **argv){
Student Astudents[100];
Student *Bstudents;
Bstudents=(Student *)malloc(sizeof(Student)*100);
. . . }

12. malloc and function parameter passing
int main(int argc,char **argv){
Student *students;
...
InitializeStudents(student, 100); ... }
void InitializeStudents(Student *ptr, int n){
int i;
ptr=(Student *)malloc(sizeof(Student)*100); for(i=0;i<n;i++) ptr++.year=0; }

13. malloc and function parameter passing
Call-by value
students=cccc
ptr=cccc  8000 ?
8000
*student
Memory allocated by
malloc
*(student+1) …

14. malloc and function parameter passing - solution
int main(int argc,char **argv){
Student *students;
...
InitializeStudents(&student, 100); ... }
void InitializeStudents(Student **ptr, int n){
int i;
*ptr=(Student *)malloc(sizeof(Student)*100); for(i=0;i<n;i++) ptr++.year=0; }

15. malloc and function parameter passing - solution
Call-by value
4000
students=cccc
ptr=4000 ?
*ptr=malloc(…)
8000
*student
*(student+1) …