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  1. #include <stddef.h>
  2.  
  3. int main(void) {
  4.     char a[3][4] = { { 0 } };
  5.     size_t sz;
  6.     void * ptr;
  7.     char c;
  8.  
  9.     /*
  10.      * "Array decay"
  11.      *
  12.      * 'a' has type 'char[3][4]', as seen on line 4.
  13.      * 'a[0]' has type 'char[4]'.
  14.      * 'a[0][0]' has type 'char'.
  15.      *
  16.      * But when an expression with an array type appears
  17.      * outside of use as the immediate operand of the
  18.      * 'sizeof' operator or the unary '&' address operator,
  19.      * then the expressions are converted to yield a pointer
  20.      * value that points to the array's element type
  21.      *
  22.      * 'a' goes from 'char[3][4]' to 'char (*)[4]'
  23.      * 'a[0]' goes from 'char[4]' to 'char *'
  24.      */
  25.  
  26.     /* Below, 'a' has type 'char[3][4]' */
  27.     sz = sizeof a;
  28.  
  29.     /* Below, 'a[0]' has type 'char[4]' */
  30.     sz = sizeof a[0];
  31.  
  32.     /* Below, 'a[0][0]' has type 'char' */
  33.     sz = sizeof a[0][0];
  34.  
  35.     /* Below, 'a' yields a value with type 'char(*)[4]' */
  36.     ptr = a;
  37.  
  38.     /* Below, 'a[0]' yields a value with type 'char *' */
  39.     ptr = a[0];
  40.  
  41.     /* Below, 'a[0][0]' yields a value with type 'char' */
  42.     c = a[0][0];
  43.  
  44.     /* Below, '&a' yields a value with type 'char(*)[3][4]' */
  45.     ptr = &a;
  46.  
  47.     /* Below, '&a[0]' yields a value with type 'char(*)[4]' */
  48.     ptr = &a[0];
  49.  
  50.     /* Below, '&a[0][0]' yields a value with type 'char *' */
  51.     ptr = &a[0][0];
  52.  
  53.     /* Below, '*a' has type 'char[4]' */
  54.     sz = sizeof *a;
  55.  
  56.     /* Below, '*a[0]' has type 'char' */
  57.     sz = sizeof *a[0];
  58.  
  59.     /* Below, '*a' yields a value with type 'char *' */
  60.     ptr = *a;
  61.  
  62.     /* Below, '*a[0]' yields a value with type 'char' */
  63.     c = *a[0];
  64.  
  65.     return 0;
  66.   }
Success #stdin #stdout 0.02s 1716KB
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https://ideone.com/1PyNC
language:
C (gcc 8.3)
created:
12 years ago
visibility:
public

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