使用istream :: ignore（）函数时如何检查EOF条件？

Question

My test file contains just 2 characters: a and \\n :

$ hexdump -C a.txt
00000000  61 0a                                             |a.|
00000002

I want to test istream::ignore function:

(1) I use ifs.ignore(2) to skip a and \\n :

#include <iostream>
#include <fstream>
#include <limits>

using namespace std;

int main() {
    std::ifstream ifs("a.txt");

    while (!ifs.eof())
    {
        std::cout << "Not end of file\n";
        ifs.ignore(2);
    }   
    return 0;
}

Test result is like this:

$ ./a.out
Not end of file

Program only enter loop once, and it fits my expectation.

(2) I use std::numeric_limits<std::streamsize>::max(), '\\n' to skip a and \\n :

#include <iostream>
#include <fstream>
#include <limits>

using namespace std;

int main() {
    std::ifstream ifs("a.txt");

    while (!ifs.eof())
    {
        std::cout << "Not end of file\n";
        ifs.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
    }   
    return 0;
}

But this time the program enters loop 2 time:

$ ./a.out
Not end of file
Not end of file

I can't understand this. Per my understanding:

ifs.ignore(std::numeric_limits<std::streamsize>::max(), '\n');

should pass all characters, and ifs.eof() should return true the second time.

Answer 1

std::basic_istream::ignore will only set eofbit if it encounters the end of the stream. In your case, it instead encounters the \\n , discards this, then returns without examining the stream further . It has therefore not encountered the end of the stream in the first iteration, since it stops inspecting after extracting delim ; for all it knows, there could be more data after the \\n , because it hasn't tried to look at it yet.