当达到流的EOF时，seekg（）不会设置eofbit。 是设计使然吗？

Question

EDIT: there was a subtle error in my original test program code: the " char=" << aStream.peek() line (and probably the " input pos=" << aStream.tellg() , as well) modified the stream state-flags, so not the real state was reported. So those calls must be completely deleted from the code, otherwise we cannot see the real effect of seekg() on the state-flags. However the result is still the same: the eofbit is not set.

Original post:

I try to detect EOF of an std::istream by advancing the input pointer with 1 step by calling

seekg( 1, std::ios_base::cur )

However seekg() moves 1 position beyond EOF, when it sets the failbit of the stream. The eofbit is never set. See the output of this test program:

#include <iostream>
#include <sstream>

using namespace std;

void info( int aRelativePos, istream& aStream )
{
    cout << "POS=" << aRelativePos <<
            " input pos=" << aStream.tellg() <<
            " char=" << aStream.peek() <<
            "\tGood: " << aStream.good() <<
            " Eof: " << aStream.eof() <<
            " Bad: " << aStream.bad() <<
            " Fail: " << aStream.fail() << "\n";
}

int main()
{
    istringstream   input ("12");

    int i=0;
    while ( input.good() )
    {
        info( i, input );
        input.seekg( 1, std::ios_base::cur ); //advance 1 step forward
        ++i;
    }
    info ( i, input );

    return 0;
}

Output:

POS=0 input pos=0 char=49   Good: 1 Eof: 0 Bad: 0 Fail: 0
POS=1 input pos=1 char=50   Good: 1 Eof: 0 Bad: 0 Fail: 0
POS=2 input pos=-1 char=-1  Good: 1 Eof: 0 Bad: 0 Fail: 0
POS=3 input pos=-1 char=-1  Good: 0 Eof: 0 Bad: 0 Fail: 1

(Compiled by gcc 5.2 with -std=c++11 . You can run this code here: http://coliru.stacked-crooked.com/a/69f4d70e93359423 )

Moreover MS document on seekg ( https://msdn.microsoft.com/en-us/library/y2d6fx99(v=vs.120).aspx ) says that relative positioning in text files is not supported by the C++ Standard.

But I could not find such info in the Standard. Can you please give me the reference?

Answer 1

Well as long as you are okay with the standard says this is the behavior we have from [istream.unformatted]

basic_istream<charT,traits>& seekg(pos_type pos);

Effects: Behaves as an unformatted input function (as described in 27.7.2.3, paragraph 1), except that the function first clears eofbit , it does not count the number of characters extracted, and it does not affect the value returned by subsequent calls to gcount() . After constructing a sentry object, if fail() != true , executes rdbuf()->pubseekpos(pos, ios_base::in) . In case of failure, the function calls setstate(failbit) (which may throw ios_base::failure ).

So per the standard we will always clear the eofbit and on an failure only the fail bit is set. Trying to read past the end of failure is a failure so that is why it gets set. Just reaching the end of file is not a failure as the end is a valid position.

You can see in this example(modified from your code) that once we reach the end of file we are still good and then trying to read from there will not only set the eofbit but also the failbit as we are at the end of file and the extraction fails

input.seekg(0, std::ios::end);
info (input);
char ch;
input >> ch;
info (input);

Output:

Good: 1 Eof: 0 Bad: 0 Fail: 0
Good: 0 Eof: 1 Bad: 0 Fail: 1

Live Example