increment in C++

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Hi all, can any one help me?
I have this simple program:

#include<iostream.h>
void main()
{
int A,B= 8;
A = ++B + ++B;
cout<<A<<B;
}

I expected that the result of A will be 9+10 = 19 but the result was 20 and I tried many programs but I couldn't understand the reason of their results.

Can any one explain the reasons for me.
thank u for all

Posted 11-Oct-10 1:37am

Petra

Updated 4-Aug-17 0:01am

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5 solutions

Solution 1

Have you tried bracketing the formula ie. :

A = (B++) + (B++)

Posted 11-Oct-10 1:46am

Steven J Jowett

Solution 3

You are using the pre-increment, then the increments on your variable are done the same way as they are written on a separate statement, before than the current one. In other words, this line:

C++

int A, B = 8;
A = ++B + ++B;

is treated by the compiler in the same way than:

C++

int A, B = 8;
B = B + 1;
B = B + 1;
A = B + B;

Posted 11-Oct-10 2:00am

Sauro Viti

Solution 6

write A=(++B)+(++B);

or

:rose:

Posted 11-Oct-10 2:21am

Rohit from Delhi

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Aescleal · Accepted Answer · 2010-10-11T02:22:00

The C++ standard (1998) says:

"Between the previous and next sequence point a scalar object shall have its stored value modified at most once by the evaluation of an expression. Furthermore, the prior value shall be accessed only to determine the value to be stored. The requirements of this paragraph shall be met for each allowable ordering of the subexpressions of a full expression; otherwise the behavior is undefined."

As you're modifying the scalar object (B) twice before the next sequence point (the terminating semi-colon) what happens is undefined. When something is undefined all bets are off - the compiler can chuck out any old rubbish it wants. Sometimes the rubbish is what you want but quite often it's not.

Have a read of Angelika Langer's discussion[^] of sequence points for a bit more background.

Cheers,

Ash

Richard MacCutchan · Accepted Answer · 2010-10-11T02:16:00

You are breaking the rules of C++. Using increment/decrement multiple times in a single expression on the same variable gives results that cannot be guaranteed, as the compiler writer is free to evaluate the expression in any way they see fit. You should use a code block like:

C++

int A,B= 8;
A = ++B;    // A == 9
++B;        // B == 10
A += B;     // A == 19
}