Re: vc 6.0's bug?????
Momchil Velikov wrote:
Alf P. Steinbach wrote:
* fisker0303@126.com:
In vc 6.0:
#include <iostream>
using namespace std;
int main()
{
int a = 10;
int b = 20;
a = (a + b) - (b = a);
cout << "a=" << a << ",b=" << b << endl;
return 0;
}
Release output : a ,b=10
Debug output: a=10,b=10
If it were 'a' being modified, the behavior would not (IMO) be merely
unspecified, but undefined, which is much worse.
The behavior is *undefined*, because the expression "a = (a + b) - (b =
a);"
violates "5. Expressions" [#4]:
"... 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."
No, the behavior is merely unspecified - not undefined. In other words,
the variable "a" is certain to have either the value 20 or the value 10
after the expression is evaluated.
The expression would have to store a value in either "a" or "b" more
than once for it to have undefined behavior.
Greg
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