How Can I Detect That a Binary File Has Been Completely Consumed? - c++

Question

If I do this:
ofstream ouput("foo.txt");
output << 13;
output.close();
ifstream input("foo.txt");
int dummy;
input >> dummy;
cout << input.good() << endl;
I'll get the result: "0"
However if I do this:
ofstream ouput("foo.txt", ios_base::binary);
auto dummy = 13;
output.write(reinterpret_cast<const char*>(&dummy), sizeof(dummy));
output.close();
ifstream input("foo.txt", ios_base::binary);
input.read(reinterpret_cast<char*>(&dummy), sizeof(dummy));
cout << input.good() << endl;
I'll get the result: "1"
This is frustrating to me. Do I have to resort to inspecting the ifstream's buffer to determine whether it has been entirely consumed?

Answer 1

Regarding
How Can I Detect That a Binary File Has Been Completely Consumed?
A slightly inefficient but easy to understand way is to measure the size of the file:
ifstream input("foo.txt", ios_base::binary);
input.seekg(0, ios_base::end); // go to end of the file
auto filesize = input.tellg(); // current position is the size of the file
input.seekg(0, ios_base::beg); // go back to the beginning of the file
Then check current position whenever you want:
if (input.tellg() == filesize)
cout << "The file was consumed";
else
cout << "Some stuff left in the file";
This way has some disadvantages:
Not efficient - goes back and forth in the file
Doesn't work with special files (e.g. pipes)
Doesn't work if the file is changed (e.g. you open your file in read-write mode)
Only works for binary files (seems your case, so OK), not text files
So better just use the regular way people do it, that is, try to read and bail if it fails:
if (input.read(reinterpret_cast<char*>(&dummy), sizeof(dummy)))
cout << "I have read the stuff, will work on it now";
else
cout << "No stuff in file";
Or (in a loop)
while (input.read(reinterpret_cast<char*>(&dummy), sizeof(dummy)))
{
cout << "Working on your stuff now...";
}

Answer 2

You are doing totally different things.
The operator>> is greedy and will read as much as possible into dummy. It so happens that while doing so, it runs into the end of file. That sets the input.eof(), and the stream is no longer good(). As it did find some digits before the end, the operation is still successful.
In the second read, you ask for a specific number of bytes (4 most likely) and the read is successful. So the stream is still good().
The stream interface doesn't predict the outcome of any future I/O, because in the general case it cannot know. If you use cin instead of input there might now be more to read, if the user continued typing.
Specifically, the eof() state doesn't appear until someone tries to read past end-of-file.

Answer 3

For text streams, as you have written only the integer value and not even a space not an end of line, at read time, the library must try to read one character passed the 1 and 3 and hits the end of file. So the good bit is false and the eof is true.
For binary streams, you have written 4 bytes (sizeof(int)) assuming ints are 32 bits large, and you read 4 bytes. Fine. No problem has still occured and the good bit is true and eof false. Only next read will hit the end of file.
But beware. In text example, if you open the text file in a editor and simply save it without changing anything, chances are that the editor automacally adds an end of line. In that case, the read will stop on the end of line and as for the binary case the good bit will be true and eof false. Same is you write with output << 13 << std::endl;
All that means that you must never assume that a read is not the last element of a file when good it true and eof is false, because the end of file may be hit only on next read even if nothing is returned then.
TL/DR: the only foolproof way to know that there is nothing left in a file is when you are no longer able to read something from it.

Answer 4

You do not need to resort to inspecting the buffer. You can determine if the whole file has been consumed: cout << (input.peek() != char_traits<char>::eof()) << endl This uses: peek, which:
Reads the next character from the input stream without extracting it
good in the case of the example is:
Returning false after the last extraction operation, which occurs because the int extraction operator has to read until it finds a character that is not a digit. In this case that's the EOF character, and when that character is read even as a delimiter the stream's eofbit is set, causing good to fail
Returning true after calling read, because read extracts exactly sizeof(int)-bytes so even if the EOF character is the next character it is not read, leaving the stream's eofbit unset and good passing
peek can be used after either of these and will correctly return char_traits<char>::eof() in both cases. Effectively this is inspecting the buffer for you, but with one vital distinction for binary files: If you were to inspect a binary file yourself you'd find that it may contain the EOF character. (On most systems that's defined as 0xFF, 4 of which are in the binary representation of -1.) If you are inspecting the buffer's next char you won't know whether that's actually the end of the file or not.
peek doesn't just return a char though, it returns an int_type. If peek returns 0x000000FF then you're looking at an EOF character, but not the end of file. If peek returns char_traits<char>::eof() (typically 0xFFFFFFFF) then you're looking at the end of the file.