How does ifstream s eof work

Question

include  lt iostream gt   include  lt fstream gt   int main         std  fstream inf   ex txt   std  ios  in        while   inf eof               std  cout  lt  lt  inf get    lt  lt    n             inf close        inf clear        inf open   ex txt   std  ios  in        char c      while  inf  gt  gt  c             std  cout  lt  lt  c  lt  lt    n             return 0      I m really confused about eof   function  Suppose that my ex txt s content was   abc   It always reads an extra character and shows -1 when reading using eof    But the inf  gt  gt  c gave the correct output which was  abc   Can anyone help me explain this

User · Answer

eof   checks the eofbit in the stream state   On each read operation  if the position is at the end of stream and more data has to be read  eofbit is set to true  Therefore you re going to get an extra character before you get eofbit 1   The correct way is to check whether the eof was reached  or  whether the read operation succeeded  after the reading operation  This is what your second version does - you do a read operation  and then use the resulting stream object reference  which    returns  as a boolean value  which results in check for fail

User · Answer

The EOF flag is only set after a read operation attempts to read past the end of the file  get   is returning the symbolic constant traits  eof    which just happens to equal -1  because it reached the end of the file and could not read any more data  and only at that point will eof   be true  If you want to check for this condition  you can do something like the following   int ch  while   ch   inf get       EOF        std  cout  lt  lt  static cast lt char gt  ch   lt  lt    n

User · Answer

iostream doesn t know it s at the end of the file until it tries to read that first character past the end of the file   The sample code at cplusplus com says to do it like this   But you shouldn t actually do it this way     while  is good           loop while extraction from file is possible         c   is get             get character from file     if  is good          cout  lt  lt  c        A better idiom is to move the read into the loop condition  like so   You can do this with all istream read operations that return  this  including the  gt  gt  operator     char c    while is get c       cout  lt  lt  c

User · Answer

-1 is get s way of saying you ve reached the end of file  Compare it using the std  char traits lt char gt   eof    or std  istream  traits type  eof    - avoid -1  it s a magic number   Although the other one is a bit verbose - you can always just call istream  eof   The EOF flag is only set once a read tries to read past the end of the file  If I have a 3 byte file  and I only read 3 bytes  EOF is false  because I ve not tried to read past the end of the file yet  While this seems confusing for files  which typically know their size  EOF is not known until a read is attempted on some devices  such as pipes and network sockets   The second example works as inf  gt  gt  foo will always return inf  with the side effect of attempt to read something and store it in foo  inf  in an if or while  will evaluate to true if the file is  good   no errors  no EOF  Thus  when a read fails  inf evaulates to false  and your loop properly aborts  However  take this common error   while  inf eof        EOF is false here       inf  gt  gt  x          read fails  EOF becomes true  x is not set        use x          we use x  despite our read failing      However  this   while inf  gt  gt  x      Attempt read into x  return false if it fails          will only be entered if read succeeded      Which is what we want

[c++] How does ifstream's eof() work?

Examples related to c++

Examples related to ifstream

Examples related to eof