Conditional expression behavior incorrect

Question

I would expect that the two conditionals would yield the same answer (nEntries = 0). I don't understand why it doesn't.

# include <stdlib.h>
# include <iomanip>
# include <iostream>

using namespace std;

    int main() {
       int nEntries = 1;
       nEntries = (nEntries)? nEntries--: nEntries;
       cout << "nEntries = " << nEntries << endl;   // nEntries = 1
       nEntries = 1;
       if (nEntries) nEntries--;                    // nEntries = 0
       cout << "nEntries = " << nEntries << endl;
       return 0;
}

It looks like the conditional expression is doing:

if (nEntries) tmp = nEntries--;

art

possible duplicate of [What is the difference between ++i and i++](http://stackoverflow.com/questions/24853/what-is-the-difference-between-i-and-i) — Axalo, Sep 27 '15 at 22:16

score 4 · Answer 1 · edited May 23 '17 at 12:14

nEntries = (nEntries)? nEntries--: nEntries; // statement 1
// ...
if (nEntries) nEntries--;  // statement 2

1 and 2 are not equivalent statements, so there is no reason to expect them to behave equivalently.

// statement 3, equivalent with statement 1
if (nEntries)
     nEntries = nEntries--; // Assignment and decrement are unsequenced modifications.
                            // The behaviour is undefined.
else
     nEntries = nEntries; // this is pointless...

As you can see, statement 1 and it's equivalent 3 have undefined behaviour, so you cannot expect it to behave in any particular way.

In case you aren't familiar with unsequenced modification being undefined, see Undefined behavior and sequence points

Thanks for the answer. I am unfamiliar with unsequenced behavior and will attempt to get smarter. Thanks. — lostbits, Sep 27 '15 at 22:46

Conditional expression behavior incorrect

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