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C++不确定行为

     一个简单的程序引发了一块让人纠结的领域,或许强调编程规范的重要性也在这把,规范了就容易避免一些问题。

程序是这样的

int Change(int& a)
{
   a = 4;
  return a;
}

int main()
{
  int a = 10;
  cout << Change(a) << a;
}

In C-Free : the output : 4 4

In VS2008 : the output : 4 10

区别出来了,按我的理解,应该C-free输出的对,但是还是探究了下,以下是我别人给我的回复:


Simply put, there is no rule that guarantees that "4 4" is right. Same goes for "4 10".

As others have mentioned in the comments you have a case of undefined behaviour here. Even if this would be defined behaviour, code like this is difficult to understand. So I recommend to do

cout << Change(a);
cout << a;

or

int b = a;
cout << Change(a);
cout << b;

whatever you really want to do.

另外,找到了C++不确定行为的一片天,有兴趣的可以在这里继续探究很多C++出现不确定行为的情况。

http://en.cppreference.com/w/cpp/language/eval_order

C++不确定行为