C++ Is std::ifstream::eof() a valid delimiter for std::ifstream::fget()?

Question

std::ifstream::get has as one of its signatures

istream& get (char* s, streamsize n, char delim);

Should I use file.eof() as a 'delimiter' for the whole file in

 const std::string_view arg1{argv[1]}; // not a copy
 std::ifstream file (arg1.data() ,std::ios::in|std::ios::ate);

 if (file) {
   auto file_size = file.tellg(); // get file size
   std::unique_ptr<char[]> buf(new char[file_size]{} );
   file.seekg(0);         // go to beg of file

   file.get(buf.get(), file_size, file.eof());
   std::string_view sv{buf.get()};  // does not copy
}

The code works fine with no error, and no leaks, but "looks suss" to use file.eof() in this way.

Indeed it doesn't. It must be bigger than `char`, because it has to fit all valid values for `char` and `EOF` on top of that. The valid way to read until eof is just not passing delimiter - `basic_istream& get( char_type* s, std::streamsize count );` This will read until `count` characters are read, eof happens or a failure occurs. — Yksisarvinen, Aug 26 '23 at 22:21

score 3 · Answer 1 · answered Aug 27 '23 at 01:13

std::ifstream::eof() is not a data value at all. It is a boolean status flag, used to indicate whether the stream has reached its end and there is no more data available to read.

For a data delimiter, you are thinking of std::ifstream::traits_type::eof() instead, which is a data value, but not a valid one that you can use as input to std::ifstream::get(). traits_type::eof() is an output value only, used to indicate when a stream has reached its end and the read operation has no valid character to return.

For what you are attempting to do, see How do I read an entire file into a std::string in C++? (for example, this answer is similar to what you are attempting).

score 0 · Answer 2 · answered Aug 26 '23 at 22:23

0

Is std::ifstream::eof() a valid delimiter for std::ifstream::fget()?

No, it is not a character. It is converted to character - most probably to byte 0xff with EOF being -1.

Use file.read(buf.get(), file_size).

answered Aug 26 '23 at 22:23

KamilCuk

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C++ Is std::ifstream::eof() a valid delimiter for std::ifstream::fget()?

2 Answers2