[solved] Program crashes when comparing two integers

silviubogan

I have the following small program:

@#include <algorithm>
#include <QDebug>

QStringList list;

bool comp(const QString &a, const QString &b) {
qDebug() << a << b << list.indexOf(a) << list.indexOf(b);

if (list.indexOf(a) <= list.indexOf(b)) {
    return true;
}
return false;

}

int main () {
qDebug() << "begin";

list << "I" << "II" << "III" << "IV" << "V" << "VI" << "VII"
   << "VIII" << "IX" << "X" << "XI" << "XII" << "XIII";

QStringList list2;
list2 << "V" << "III" << "III" << "IX"
  << "XII" << "XI" << "IV";
std::sort(list2.begin(), list2.end(), comp);

qDebug() << "end";

return 0;

}
@

When I compile it with MSVC2012 (Debug) I get the following output text:
@begin
"III" "V" 2 4
"V" "III" 4 2
"III" "III" 2 2
"III" "III" 2 2 @

and the following error window: http://postimg.org/image/ec2hnxzk5/ .
In this window, If I press the Ignore button, the program somehow continues and after it closes, the output text is:
@begin
"III" "V" 2 4
"V" "III" 4 2
"III" "III" 2 2
"III" "III" 2 2
"IX" "III" 8 2
"IX" "V" 8 4
"XII" "III" 11 2
"XII" "IX" 11 8
"XI" "III" 10 2
"XI" "XII" 10 11
"XII" "XI" 11 10
"XI" "IX" 10 8
"IV" "III" 3 2
"IV" "XII" 3 11
"XII" "IV" 11 3
"IV" "XI" 3 10
"XI" "IV" 10 3
"IV" "IX" 3 8
"IX" "IV" 8 3
"IV" "V" 3 4
"V" "IV" 4 3
"IV" "III" 3 2
end@

If I compile it with MinGW (Debug) it is the same situation, but the program crashes without displaying an error message.

If I delete the following lines, the program runs without crashing:
@if (list.indexOf(a) <= list.indexOf(b)) {
return true;
}@

The program also runs without crashing when I build it in Release mode (either MinGW or MSVC2012).

If I debug the program, I find the source of the error message in the source code (it seems related to the algorithm header.

I really don't understand what is happening in that if condition and why the program doesn't work. I really need some help. Thank you!

hskoglund

Hi, it's because you're violating the contract with std::sort(), the comp() function should never return true when a and b are equal. (std::sort probably goes lost in your list and release mode is more forgiving)

Try change the comparison to:
@
if (list.indexOf(a) < list.indexOf(b)) {
return true;
}
@

silviubogan

I made the change you suggested and everything works as expected. Thank you!