c++ argument evaluation order for assignment?

Question

In c++, arguments evaluation of order is not guaranteed, but is the order of left/right sub expression of assignment expression is guaranteed? For example

#include <iostream>
#include <map>

int main()
{
    int i = 2;
    std::map<int, int> map;
    map[i++] = i--;
    return 0;
}

Is left expression i++ guaranteed to be executed before right expression i--?

@LightnessRacesinOrbit Well, you *do* have the dupe hammer in your hands too, you can use it :-) — Angew is no longer proud of SO, Jan 06 '15 at 17:14
@Angew: Requires being bothered to find the dupe for the 120th time this hour — Lightness Races in Orbit, Jan 06 '15 at 17:15
@LightnessRacesinOrbit I keep a few FAQ questions in my browser's bookmark toolbar, that helps. — Angew is no longer proud of SO, Jan 06 '15 at 17:15

R Sahu · Accepted Answer · 2015-01-06T17:19:55.897

1

You asked:

Is left expression i++ guaranteed to be executed before right expression i--?

No, it is not.

The line

map[i++] = i--;

could end up being

map[2] = 3;

or

map[1] = 2;

depending on which side of the assignment operator gets evaluated first.

However, since the line invokes undefined behavior, it could also be, as pointed out in the comment by @juanchopanza, :

map[42] = -999;

edited Jan 06 '15 at 17:19

answered Jan 06 '15 at 17:09

R Sahu

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Or it could end up being `map[42] = -999;` because it is undefined behaviour. – juanchopanza Jan 06 '15 at 17:10

c++ argument evaluation order for assignment?

1 Answers1