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，也许强调编程规范的重要性也在这把。规范了就easy避免一些问题。

程序是这种

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

差别出来了，按我的理解，应该C-free输出的对，可是还是探究了下，下面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，有兴趣的能够在这里继续探究非常多C++出现不确定行为的情况。

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