测试stream.good（）或！stream.eof（）读取最后一行两次 [英] Testing stream.good() or !stream.eof() reads last line twice

问题描述

可能重复：

我有以下代码：

ifstream f("x.txt");
string line;
while (f.good()) {
  getline(f, line);
  // Use line here.
}

但这会读取最后一行两次。为什么会发生这种情况，我该如何解决？

But this reads the last line twice. Why does this happen and how do I fix it?

发生类似的情况：

ifstream f("x.txt");
string line;
while (!f.eof()) {
  getline(f, line);
  // Use line here.
}

推荐答案

检查坏，eof和好。特别是对于eof（因为！stream.eof（）是一个常见的错误），当前处于EOF的流不一定意味着最后一个输入操作失败;相反，不是在EOF不意味着最后一个输入成功。

You very, very rarely want to check bad, eof, and good. In particular for eof (as !stream.eof() is a common mistake), the stream currently being at EOF does not necessarily mean the last input operation failed; conversely, not being at EOF does not mean the last input was successful.

所有流状态函数 - fail，bad，eof和good - 告诉你current而不是预测未来操作的成功。在所需操作之后检查流本身（相当于反向失败检查）：

All of the stream state functions – fail, bad, eof, and good – tell you the current state of the stream rather than predicting the success of a future operation. Check the stream itself (which is equivalent to an inverted fail check) after the desired operation:

if (getline(stream, line)) {
  use(line);
}
else {
  handle_error();
}

if (stream >> foo >> bar) {
  use(foo, bar);
}
else {
  handle_error();
}

if (!(stream >> foo)) {  // operator! is overloaded for streams
  throw SomeException();
}
use(foo);

要读取和处理所有行：

for (std::string line; getline(stream, line);) {
  process(line);
}

有人指出，good（）被错误命名，并不等同于测试流本身（上面的例子）。

Pointedly, good() is misnamed and is not equivalent to testing the stream itself (which the above examples do).

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