c++ - 为什么在循环条件(即 `while (!stream.eof())`)内的iostream::eof被认为是错误的？

Question

我刚刚在this答案中找到一条评论，说在循环条件下使用iostream::eof是“几乎肯定是错误的”。我通常使用类似while(cin>>n)的东西-我猜它隐式检查EOF。

为什么显式使用while (!cin.eof())检查eof是错误的？

与在C语言中使用scanf("...",...)!=EOF有什么不同(我经常会毫无问题地使用它)？

Answer 1

因为iostream::eof仅在读取流的末尾后才返回true。它并不表示下次读取将是流的结尾。

考虑一下(假设下一个读取将在流的末尾):

while(!inStream.eof()){
  int data;
  // yay, not end of stream yet, now read ...
  inStream >> data;
  // oh crap, now we read the end and *only* now the eof bit will be set (as well as the fail bit)
  // do stuff with (now uninitialized) data
}

反对:

int data;
while(inStream >> data){
  // when we land here, we can be sure that the read was successful.
  // if it wasn't, the returned stream from operator>> would be converted to false
  // and the loop wouldn't even be entered
  // do stuff with correctly initialized data (hopefully)
}

关于第二个问题:

if(scanf("...",...)!=EOF)

是相同的

if(!(inStream >> data).eof())

和不是与

if(!inStream.eof())
    inFile >> data