关于c ++：为什么增量操作如“a [i] = i ++;”会导致未定义的行为？

Why does increment operation like “a[i] = i++;” result in undefined behavior?

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Possible Duplicate:
Undefined Behavior and Sequence Points

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#include <iostream>
using namespace std;

int main()
{
int x[3] = {};
int i=0;
x[i] = i++;
cout << x[0] <<"" << x[1] << endl;
return 0;
}

代码板告诉我：第9行：警告："i"上的操作可能未定义为什么操作未定义？

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这里解释清楚：C-FAQ

为什么这个代码：a[i] = i++;不起作用？

The subexpression i++ causes a side effect--it modifies i's value--which leads to undefined behavior since i is also referenced elsewhere in the same expression. There is no way of knowing whether the reference will happen before or after the side effect--in fact, neither obvious interpretation might hold; see question 3.9. (Note that although the language in K&R suggests that the behavior of this expression is unspecified, the C Standard makes the stronger statement that it is undefined--see question 11.33.)