c++ - Strange comma operator behaviour -

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this question has answer here:

void foo() {    int a[5], c = 2;    (int = 0; < 5; i++)       a[i] = 0;    int res = (c--, a[c]++, c++) + (c++, a[c]--, c--);    (int = 0; < 5; i++)       cout << << ": " << a[i] << endl; }

the code above print: 

0 : 0 1 : 1 2 : -1 3 : 0 4 : 0

instead of: 

0 : 0 1 : 1 2 : 0 3 : -1 4 : 0

this because operations order in generated code following:

// first parentheses c--; a[c]++; // second parentheses c++; a[c]--; // , last operation res = c++ + c--;

the question is: why operations not run expected (i.e. 3 operations in 1 parentheses , 3 operations in other)?

order of operations not guarenteed in between sequence points. sequence point found @ semi-colon.

wouldn't a[c-1]++; better (c--, a[c]++, c++) anyway? why write main ram new value c 4 times when don't keep value other calculation?


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