what is the order of calculating i+v[i++]

Question

#include <iostream>
#include <vector>

using namespace std;

int main()
{
    vector<int> v{1,2,3,4,5};
    int i = 0;
    while (i < 5) {
        cout << i+v[i++] << endl;
    }

    return 0;
}

Why the output of the program is "2,4,6,8,10", instead of "1,3,5,7,9"?

`Why the output of the program is "2,4,6,8,10", instead of "1,3,5,7,9"` Undefined behavior. — kiran Biradar, Oct 23 '18 at 08:02
To expand on what @kiranBiradar says, you *aren't allowed* to do this expression. It'll compile, but the compiler can do whatever it wants. It'll probably try to do something sensible, but it might not be a good judge what "sensible" is, so you should avoid undefined behavior in almost all cases. — Daniel H, Oct 23 '18 at 08:06

score 5 · Accepted Answer · answered Oct 23 '18 at 08:07

5

This is undefined behaviour because the read of the "first" i and the other i++ are unsequenced. The output is meaningless.

Further reading: Undefined behavior and sequence points,

answered Oct 23 '18 at 08:07

M.M

138,810
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score 5 · Answer 2 · answered Oct 23 '18 at 08:09

Some compilers will warn you that you are doing something that leads to undefined behavior: Relying on the evaluation order of unsequenced expressions.

Example:

<source>:22:22: warning: unsequenced modification and access to 'i' [-Wunsequenced]
   cout << i+v[i++] << endl;

The evaluation order of expressions between sequence points is not defined. The only sequence point in the expression i+v[i++] is at the end of the expression, so the compiler is free to order the addition of i and the increment of i however it wants.

what is the order of calculating i+v[i++]

2 Answers2