In this first example, with -=, this works as expected:
Image with result
int value = 3;
int *ptr = &value;
printf("%dn", ptr);
*ptr -= 1;
printf("%dn", ptr);
printf("%dn", value);
But in this other example, with –, the value of the pointer itself decrements, and the value of “value” stays the same:
Image with result
int value = 3;
int *ptr = &value;
printf("%dn", ptr);
*ptr --;
printf("%dn", ptr);
printf("%dn", value);
This is not a big deal of course, but I’m interested in hearing about the differences between two operations that many think are identical.
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C’s operator precedence rules say that postfix decrement binds more tightly than pointer dereferencing, so *ptr-- means *(ptr--), not (*ptr)--. If you write the latter, then it will do the same as -=.
For reference:
| Precedence | Operator | Description | Associativity |
|---|---|---|---|
| 1 | -- |
Suffix/postfix decrement | Left-to-right |
| 2 | * |
Indirection (dereference) | Right-to-left |
| 14 | -= |
Assignment by difference | Right-to-left |
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