Error in post incement of a preincremented variable (++i)++?

Question

In the following code segment, I was expecting the answer to be 5 but it shows compile time error:

#include <stdio.h>
int main()
{
    int i = 4;
    printf("%d", (++i)++);
    return 0;
}

What is the reason?. Here ++i returns an l value right So we could increment it right?

Even if it were syntactically correct, it would be undefined behavior, so you could not expect 5. — Pascal Cuoq, Oct 26 '13 at 11:44
Not exactly a duplicate, but the answers there are better answers to the question here than the answers here. — Pascal Cuoq, Oct 26 '13 at 11:49

score 5 · Answer 1 · answered Oct 26 '13 at 11:11

5

(++i) returns an rvalue. This cannot be incremented.

answered Oct 26 '13 at 11:11

Igor Popov

is ++i returns lvalue? – san45 Oct 26 '13 at 11:13
1

@san45 Did you bother reading this one-liner? "(++i) returns an rvalue" -- so then does it return an lvalue? **No it doesn't. Period.** – Oct 26 '13 at 11:14
had a comment below saying that ++i returns lvalve that why – san45 Oct 26 '13 at 11:18
1

@san45 ++i operator returns a "value", but not the variable itself. So it's address can not be retrieved, which disregards it as an lvalue. – Igor Popov Oct 26 '13 at 11:20
Some useful links: http://stackoverflow.com/questions/16399485/condition-for-lvalues-and-rvalues-in-c , http://eli.thegreenplace.net/2011/12/15/understanding-lvalues-and-rvalues-in-c-and-c/ – Igor Popov Oct 26 '13 at 11:29

Arpit · Accepted Answer · 2013-10-26T13:02:08.317

4

postincrement\decrement or preincrement\decrement requires lvalue and returns rvalue. that's why you are getting an error.

++i =>returns rvalue

(rvalue)++ => error

In c++ the situation is different because of reference types.In c we don't have any reference type.

in c++

++i => returns reference to i that is lvalue.

lvalue++ => Way to go!

even it is working, It is a undefined behavior. You should not modify the same variable more than once in a same statement.

edited Oct 26 '13 at 13:02

answered Oct 26 '13 at 11:11

Arpit

3

It's not UB. It doesn't even compile. – Oct 26 '13 at 11:13
@H2CO3 Yeah! corrected. – Arpit Oct 26 '13 at 11:21
this is the same in C++ also right ? – san45 Oct 26 '13 at 11:26
@san45 **NO!** refrence is magic. – Arpit Oct 26 '13 at 11:27
You know what's magic? Undefined behavior. I think this should be mentioned instead of giving the impression that `(++i)++` is correct in C++. – Pascal Cuoq Oct 26 '13 at 11:48
@PascalCuoq Oh! yes. you are right. It was my first line when i answered it but then removed it because of C tag. And later while editing i forgot to mention it. :P. Copied from the draft. – Arpit Oct 26 '13 at 11:51
1

I think your statement, *"You can not modify the same variable more than one time in a same statement."* is misleading.... you ***can*** do it, it is allowed. You just cannot predict the results. – abelenky Oct 26 '13 at 12:25

2 Answers2