Using post increment in relational operators

Question

I am a complete beginner in programming and just started learning C++ about 2 days ago. There is a strange problem that i dont really understand.

int main()
{
   int value1;
   int value2;
   cin >> value1 >> value2;
   value1 = (value1 > 5) ? value1++ : value1;
   value2 = value2++;
   cout << value1 << " " << value2 << endl;
}

Well lets say value1 = 6 and value2 = 2. Well the display is 6 3. As far as i understand the post increment operator adds one to its operand but the value of the current expression that it is used in is still the "un-incremented" value, which makes sense for value2's case. value2 is assigned to the expression "value2++" and value2++ has value 2, once it is assigned the effects of the ++ takes place and adds 1 to the value of value2. But how about for value1's case? Why wansnt value1's value incremented after the post increment operator was used in the codes above?

That looks like undefined behaviour. The same as saying `value1 = value1++`. See [this SO question](http://stackoverflow.com/questions/4176328/undefined-behavior-and-sequence-points) for more on that. — juanchopanza, Nov 10 '13 at 11:42

score 0 · Accepted Answer · answered Nov 10 '13 at 11:44

The error is that your assignment of value1 is effectively value = value1++;... so you want to read the current value of value1, 6, and then increment it but also store that 6 somewhere else (that overwrites where the incrementation occurred).

Use value1 = (value1 > 5) ? value1 + 1 : value1; instead.

Using post increment in relational operators

1 Answers1