output of the code cout<<a+ a++ + --a; comes out to be 16 when a=5 what is the logic?

[umangbansal]

int a=5;
cout<< a+ a++ + --a ;
output is
16
logic behind the output??

[umangbansal]

@umangbansal
somebody reply please..

[code green]

I'm never comfortable with ugly looking code but it looks fairly simple to me

a=5
cout 5 ... 5
a++
a=6
cout 5 + 6 ... 11
a--
a=5
cout 5 + 6 + 5... 16

[weaknessforcats]

This code:

cout<< a+ a++ + --a ;

has indeterminate results.

The compiler is free to evaluate variables in any order within a sequence point (a statement). Therefore, if a variable is changed more than once within a statement you cannot be sure of the result.

[umangbansal]

@weaknessforcats
but this code always gives the same result..

[weaknessforcats]

Are you using the same compiler?

Visual C++ 2008 gives 12 as a result.

As I said, the results are indeterminate.

[Banfa]

but this code always gives the same result..

Accessing a variable more than once between sequence points where one of those accesses changes the variables value is undefined behaviour (as defined by the standard).

A program exhibiting undefined behaviour may do anything including but not limited to

• Exit without explanation
• Produce some sort of exception
• Appear to operate correctly
• Operate correctly the whole time you are testing it and only go wrong the first time it is critically important that it works
• Format your hard drive
• Make demons fly out of your nose

The compiler is under no constraints with the code it produces once undefined behaviour is invoked.

Trying to determine the output of a program exhibiting undefined behaviour is pointless because it is under no constraint to stick to whatever assumptions you have made while making that determination.

Your only real solution is to not write code that exhibits undefined behaviour.


I am beginning to feel like a scratched record on the subject a quick forum search would have thrown up several recent threads on this very subject.

[donbock]

banfa: "I am beginning to feel like a scratched record on the subject ..."

You mean a broken record, as in one where the stylus skips backwards and repeats the same audio over and over again.

It is true that a scratch might produce a backwards skip. However, in today's lingo, scratch refers to the effect made when a record is rotated back and forth through a small angle.

You're must be too young to remember vinyl records, but too old to be familiar with rap. ;-)

[Dheeraj Joshi]

Make demons fly out of your nose

Again demons started to fly. lol.

Regards
Dheeraj Joshi

[Banfa]

You're must be too young to remember vinyl records, but too old to be familiar with rap. ;-)

I think that would be a very narrow time frame. I do remember and in fact did own vinyl I just got my metaphors mixed :D