为什么在循环条件（即`while（！stream.eof（））`）内的iostream :: eof被认为是错误的？ [英] Why is iostream::eof inside a loop condition (i.e. `while (!stream.eof())`) considered wrong?

问题描述

我刚刚在此答案表示，在循环条件中使用 iostream :: eof 几乎肯定是错误的。我通常使用 while（cin>> n）之类的东西-我猜它隐式检查EOF。

I just found a comment in this answer saying that using iostream::eof in a loop condition is "almost certainly wrong". I generally use something like while(cin>>n) - which I guess implicitly checks for EOF.

为什么使用 while（！cin.eof（））显式检查eof是错误的？

Why is checking for eof explicitly using while (!cin.eof()) wrong?

与在其中使用 scanf（ ...，...）！= EOF 有何不同？ C（我经常会毫无问题地使用它）？

How is it different from using scanf("...",...)!=EOF in C (which I often use with no problems)?

推荐答案

因为 iostream :: eof 在读取流的结尾之后仅返回 true 。它不会不表示，下一个读取将是流的结尾。

Because iostream::eof will only return true after reading the end of the stream. It does not indicate, that the next read will be the end of the stream.

考虑一下（并假设下一个读取将位于流的结尾）：

Consider this (and assume then next read will be at the end of the stream):

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
}

对此：

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：因为

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

是相同的as

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

，不与

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

这篇关于为什么在循环条件（即`while（！stream.eof（））`）内的iostream :: eof被认为是错误的？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！