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Why does the expression a = a + b - ( b = a ) give a sequence point warning in c++?

Asked 2012-11-09T23:48:09.300
20

Following is the test code:

int main()
{
    int a = 3;
    int b = 4;
    a = a + b - (b = a); 

    cout << "a :" << a << " " << "b :" << b << "\n";    
    return 0;
}

Compiling this gives the following warning:

> $ g++ -Wall -o test test.cpp test.cpp: In function ‘int main()’:
> test.cpp:11:21: warning: operation on ‘b’ may be undefined
> [-Wsequence-point]

Why can the operation be undefined?

According to my understanding, first the subexpression (b = a) should be evaluated because of higher precedence of (), thus setting b = a. Then, since '+' and '-' have same precedence, the expression would be evaluated left-associatively. Thus, a + b should be evaluated next, and finally the result of (b = a) should be subtracted from a + b. I can't see any sequence-point rule being violated here.

c++ operators operator-precedence sequence-points
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1 Answer

-1

To solve it separate them in two different statements.

PS: Don't forget that humans may make mistakes performing arithmetic operations. Therefore is better to make the operations clearer by separating them in different statements. I hope I helped.

int main() 
{
   int a = 3;
   int b = 4;

   /* Two different Statements*/
   b = a;

   /* or a = a + b - a */
   a = a + b - b; 

   cout<<"a :"<<a<<" "<<"b :"<<b<<"\n";    
   return 0;
}
answered 2012-11-09T23:57:13.987

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