std::getline 在遇到 eof 时抛出

Question

std::getline在获取eof时抛出异常。 我就是这样做的。

std::ifstream stream;
stream.exceptions(std::ifstream::failbit|std::ifstream::badbit);
try{
  stream.open(_file.c_str(), std::ios_base::in);
}catch(std::ifstream::failure e){
  std::cout << "Failed to open file " << _file.c_str() << " for reading" << std::endl;
}
while(!stream.eof()){
  std::string buffer = "";
  std::getline(stream, buffer);
  //process buffer
  //I do also need to maintain state while parsing
}

在上面的代码中， getline在获取eof时抛出异常如何处理这种情况？

编辑

std::string buffer = "";
while(std::getline(stream, buffer)){
    //also causes getline to hit eof and throw
}

Answer 1

您在代码的最开头激活流的异常处理：

stream.exceptions(std::ifstream::failbit|std::ifstream::badbit);

现在，如果浮点值、整数或字符串等格式化数据的提取失败，它将设置 failbit：

eofbit    indicates that an input operation reached the end of an 
          input sequence;
failbit   indicates that an input operation failed to read the expected 
          characters, or that an output operation failed to generate the 
          desired characters.

虽然getline(stream,buffer)在到达文件末尾时确实会设置eofbit ，但它也会设置 failbit，因为无法提取所需的字符（一行）。

要么在循环周围包装另一个 try-catch-block，要么禁用 failbit 异常。

例子：

#include <iostream>
#include <fstream>

int main(){
  std::ifstream stream("so.cc");
  stream.exceptions(std::ifstream::failbit|std::ifstream::badbit);
  std::string str;

  try{
    while(std::getline(stream, str));
  }catch(std::ifstream::failure e){
    std::cerr << "Exception happened: " << e.what() << "\n"
      << "Error bits are: "
      << "\nfailbit: " << stream.fail() 
      << "\neofbit: " << stream.eof()
      << "\nbadbit: " << stream.bad() << std::endl;    
  }
  return 0;
}

结果：

Exception happened: basic_ios::clear
Error bits are:
failbit: 1
eofbit: 1
badbit: 0

请注意， eofbit和failbit都已设置。

也可以看看：

• std::ios_base::iostate
• § 27.5.3.1.5 中的表 124（见上文）

Answer 2

如您所说，当流当前位于文件末尾的空白行时调用 getline 将同时设置 eofbit （因为它到达文件末尾）和 failbit （因为无法提取任何内容，甚至行分隔符也不行） .

因此，如果您已将流设置为在设置 failbit 时抛出异常，则在读取该空白行时会抛出异常。

为防止这种情况，您可以在调用 getline 之前查看流中的下一个字符：

std::string buffer = "";
while(stream.peek() != EOF && std::getline(stream, buffer)){
    //do something with the buffer
}