Precedenţa şi asociativitatea operatorilor
Limbajul C este unul dintre cele mai complexe limbaje când vine vorba de precedenţa operatorilor. Exista 15 de nivele diferite de precendenţă .
Precendenţa se referă la ordinea de evaluare a expresiilor şi depinde de doua lucruri:
– Operatorii au nivele de precendeţă diferite.
– Operatorii au asociativitate diferită de stânga sau de dreapta.
| Operator | Associativity |
| ( ) [ ] -> . ++’postfix’ –’postfix’ | Left |
| ! ˜ ++ — + – (type) * & sizeof | Right |
| * / % | Left |
| +’binary’ -’binary’ | Left |
| << >> | Left |
| < <= > >= | Left |
| == != | Left |
| & | Left |
| ˆ | Left |
| | | Left |
| && | Left |
| || | Left |
| ?: | Right |
| assignments | Right |
| , | Left |
Exemplu:
<br />#include "stdio.h" /* printf */<br />#include "string.h" /* strcat */<br />#include "stdlib.h" /* strtol */<br /><br />unsigned char *p;<br />unsigned char *ps;<br />unsigned char a;<br />unsigned char b;<br />unsigned char c;<br /><br />unsigned char x;<br />unsigned char y;<br />unsigned char z;<br />typedef struct<br />{<br /> unsigned char h;<br /> unsigned short r;<br />} ms;<br />ms *m;<br />unsigned char result;<br />unsigned char temp;<br />unsigned char flag;<br />unsigned char low;<br />unsigned char high;<br /><br />const char *byte_to_binary(int x)<br />{<br /> static char b[9];<br /> b[0] = ' ';<br /><br /> int z;<br /> for (z = 128; z > 0; z >>= 1)<br /> {<br /> strcat(b, ((x & z) == z) ? "1" : "0");<br /> }<br /><br /> return b;<br />}<br /><br />int main()<br />{<br /><br /> *p++; /* equivalent to *(p++); */<br /> /*(char)!-˜*++p; */ /* applied using an inside out order */<br /> /* m<- ps="">m;*/ /* ’->’ is an unary operator */<br /> a = b = c; /* equivalent to a = (b = c); */<br /> temp = x + y + z; /* equivalent to (x + y) + z */<br /> flag =2;<br /> if (flag & 2) /* clear meaning */<br /> {<br /> printf("flag = %dn", flag);<br /> }<br /> if (flag & 2 != 0) /* more explicit? */<br /> {<br /> printf("flag = %dn", flag);<br /> }<br /> if (flag & (2 != 0)) /* unfortunately:<br /> * ’!=’ binds stronger... */<br /> {<br /> printf("flag = %dn", flag);<br /> }<br /><br /> low = 1;<br /> printf("low = %sn", byte_to_binary(low));<br /> high = 8;<br /> printf("high = %sn", byte_to_binary(high));<br /><br /> result = high << 4 + low; /* hmm ... */<br /> printf("result = %sn", byte_to_binary(result));<br /><br /> low = 1;<br /> high = 8;<br /> result = high << (4 + low); /* probably not intended */<br /> printf("result = %sn", byte_to_binary(result));<br /><br /> low = 1;<br /> high = 8;<br /> result = (high << 4) + low; /* probably the<br /> * correct implementation */<br /> printf("result = %sn", byte_to_binary(result));<br /><br /> a = 3;<br /> a++;<br /> printf("a = %dn", a);<br /> ++a;<br /> printf("a = %dn", a);<br /><br /> b = 3;<br /> b--;<br /> printf("b = %dn", b);<br /> --b;<br /> printf("b = %dn", b);<br /><br />}<br />Rezultat:
<br /><b>/precendata.exe<br /><br />flag = 2<br /><br />result = 00000001<br /><br />result = 00001000<br /><br />result = 00000000<br /><br />result = 00000000<br /><br />result = 10000001<br /><br />a = 4<br /><br />a = 5<br /><br />b = 2<br /><br />b = 1<br /><br /></b><br />Regula de bază pentru a evita problemele este :
Fiecare subexpresie a unei expresii mai complicate ar trebui flancată de parenteze.
Exemplu:
r = (hi << 4) + (low >> 2); /* operatiile flancate de paranteze <br /> * sunt executate primele */<br />MISRA
E bine să ne bazăm şi pe cele două reguli MISRA:
MISRA-C:2004, Rule 12.1: Limited dependence should be placed on C’s operator precedence rules in expressions
MISRA-C:2004, Rule 12.2: The value of an expression shall be the same under any order of evaluation that the standard permits
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