由指針

Question

功能swap2有兩個參數，int *a，和int *b的存儲器交換混淆。這些是指向整數值指針的指針。那麼爲什麼當你在swap2執行如int tmp = *a; *a = *b;這樣的行時，它是否會改變這些值的內存位置。 *不會參考在參數中聲明的指針嗎？

int main() 
{ 
     int x = 42; 
     int y = 9; 
     printf("x = %d, y = %d\n", x, y); 
     swap1(x,y); 
     printf("x = %d, y = %d\n", x, y); 
     x = 42, y = 9; 
     swap2(&x, &y); 
     printf("x = %d, y = %d\n", x, y); 
     printf("---\n"); 
     int z = 77, w = 33; 
     int *p = &z; 
     int *q = &w; 

     printf("*p = %d, *q = %d, p = %p, q = %p\n", *p, *q, p, q); 
     swap2(p,q); 

     z = 77, w = 33; 
     printf("*p = %d, *q = %d, p = %p, q = %p\n", *p, *q, p, q); 
     swap3(&p, &q); 
     printf("*p = %d, *q = %d, p = %p, q = %p\n", *p, *q, p, q); 

     printf("z = %d, w = %d\n", z, w); 

     return 0; 
} 

void swap1(int a, int b) 
{ 
     int tmp = a; 
     a = b; 
     b = tmp, 
     printf("a = %d, b = %d\n", a, b); 
} 

void swap2(int *a, int *b) 
{ 
     int tmp = *a; 
     *a = *b; 
     *b = tmp; 
     printf("*a = %d, *b = %d\n", *a, *b); 
} 

void swap3(int **a, int **b) 
{ 
     int *tmp = *a; 
     *a = *b; 
     *b = tmp; 
     printf("**a = %d, **b = %d\n", **a, **b); 
} 

Answer 1

更新

swap2 (int *a, int *b)不會改變任何內存位置，它改變什麼a和b指向，當你調用swap2(&x, &y)，你的內存架構將如下所示：

       int x 
[a (address of x)] -> [actual x = 42] 

          int y 
[b (address of y)] -> [actual y = 9] 

int tmp = * a;

[tmp = (deference address of x) = actual x = 42] 

* a = * b;

         int x 
[a (address of x)] -> [deference b => actual y => 9] 

* b = tmp;

        int y 
[b (address of y)] -> [tmp => actual x => 42] 

---

由於swap3(int ** a, int **b)以 「指針指向int」 的（和b）將看起來像內存架構如下：

      int * p 
[a (some address 1)] -> [another address A'] -> [real integer value A''] 

          int * q 
[b (some address 2)] -> [another address B'] -> [real integer value B''] 

所以，當你尊重一個，你實際上得到一個[另一個地址]，這就是爲什麼你實際上改變了內存位置而不是實際值。

當你執行INT * TMP = *一個，因爲你在右手邊的尊重一個，現在它的價值將是「另一個地址A`」，內存模式將變爲：

[tmp (another address A')] -> [real integer value A''] 

執行* A = * b，你實際上是這樣，請注意a的價值並沒有改變，只改變存儲它的指向：

       int * p 
[a (some address 1)] -> [another address B'] -> [real integer value B''] 

最後，* b = TMP，也注意到b的值沒有變化：

       int * q 
[b (some address 2)] -> [another address A'] -> [real integer value A'']