Wrong evaluating increment and decrement exp in visual studio

Question

#include <iostream>
using namespace std;

int main() {

    int x = 10;
    int y = 11;
    int z = 12;

    cout << x++ + --x + y++ - y-- + z;

    return 0;
}

The output here is 31

http://ideone.com/tp58av

The output usign Visual Studio 2005 is 30

What is wrong !

Undefined behaviour. Been asked at least a thousand times before. — NPE, Nov 01 '13 at 18:42
What is wrong? Nothing! Compilers are free to do whatever they want when you use ++ or -- twice on the same variable in the same statement. — Gort the Robot, Nov 01 '13 at 18:51

score 3 · Answer 1 · answered Nov 01 '13 at 18:45

3

cout << x++ + --x + y++ - y-- + z;

This is undefined behavior.

You cannot access a single variable in a single sequence to both read and write to it multiple times.

As with all undefined behavior, it may work as you expect, it may reformat your hard drive, it may order me a pizza ... or it could start the self destruct sequence for the entire planet. All would be acceptable behavior as what you attempted to do is undefined.

answered Nov 01 '13 at 18:45

Zac Howland

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Blowing up the planet might break some laws, so I'm not sure you can call it "acceptable", but it would conform to the C++ standard. – Gort the Robot Nov 01 '13 at 18:52
"acceptable" in terms of the C++ standard. The human impact obviously would not be acceptable (except for nihilists). – Zac Howland Nov 01 '13 at 19:15

Wrong evaluating increment and decrement exp in visual studio

1 Answers1