Pointer 2_const

(1)a non-constant pointer to non-constant data

•存取權最高

•Data可透過pointer遭到更改

•Pointer可以改為只向別的物件

•Ex: pointer3.c
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/* Converting lowercase letters to uppercase letters
using a non-constant pointer to a non-constant data */
#include <stdio.h>
#include <ctype.h>
void convertToUppercase(char *ptr);
int main(void)
{
char string[]="characters and $32.25";
printf("The string before conversion is: %s",string);
convertToUppercase(string);
printf("\nThe string after conversion is:%s\n",string);
return 0;
}
void convertToUppercase(char *ptr)
{
while(*ptr !='\0')
{
if(islower(*ptr))
{
*ptr=toupper(*ptr);
}
++ptr;
}
}

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(2)a non-constant pointer to constant data

•Ex: pointer5.c

•若隨意更改指標指向的物件內容會有error

•編譯錯誤: assignment of read only location “ptr”

•明顯的問題出在ptr指向的是read only的位置,不能更改

•Tips: 陣列當引數呼叫function時，陣列會自動以傳參考呼叫function,而struct則是永遠以傳值的方式，傳遞整個副本過去會造成執行的負擔，因此必須將struct傳給函式時使用指向constant data的指標，一方面獲得傳參考呼叫的效率，一方面能保護資料。也就是說只要複製一份struct本身的位址就好。--一個時間/空間取捨(time/space trade-off)的例子

•Important: memory 不夠大且執行效率很重要時，用指標

•memory 很大且效率不那麼重要，用call by value以實行最小權限原則
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/* Attempting to modify data through a non-constant pointer to constant data */
#include<stdio.h>
void f(const int *ptr);

int main (void)
{
int y;
f(&y);
return 0;
}
/* ptr cannot be used to modify the value of variable to which it points */
void f(const int *ptr)
{
*ptr=100; //error: cannot modify a const object
}

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(3)a constant pointer to a non- constant data

•非常數指標的常數指標永遠會指向同一個記憶體位置，不過指標所指向的數值可以更改。

•Array名稱預設就是這種指標。

•Const的指標必須在宣告時指定初始值

•若指標是函式的參數，則初始值將設成傳入此函式的指標

•Ex:pointer6.c

Ptr是一個指向整數的常數指標，但後來又指向y,此時編譯器會error—assignment of read only variable “ptr”

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/* Attempting to modify a constant pointer  to a non-constant data */
#inlcude <stdio.h>
int main(void)
{
int x;
int y;
/* ptr is a constant pointer to an integer that can be modified through ptr,
but ptr always points to the same memory lacation */
int *const ptr=&x;
*ptr=7;
ptr=&y;

return 0;
}

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(4)a constant pointer to a constant data

•存取權最低
•Pointer永遠指向同一個記憶體位置，且該位置內容不能更改
•Ex: pointer7.c
  –Const int *const  ptr=&x;
  –Ptr是一個常數指標,指向一個整數常數
  –程式中藥更改指標與指標指向的內容都失敗 –Assignment of read only  location “ptr”  –Assignment of read only  variable “ptr”
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/* Attempting to modify a constant pointer to constant data */
#include<stdio.h>
int main(void)
{
int x=5;
int y;

/*ptr is a constant pointer to a constant integer.ptr always
points to the same location;the integer at that location
cannot be modified */
const int *const ptr=&x;
printf("%d\n",*ptr);
*ptr=7; //error : *ptr is const;cannot assign new value 
ptr=&y; //error:  ptr is const;cannot assign new address
return 0;
}

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