How to determine how many characters `std::getline()` extracted?

Question

Let's say I read a std::string from std::istream by using std::getline() overload. How to determine how many characters extracted from the stream? std::istream::gcount() does not work as discussed here: ifstream gcount returns 0 on getline string overload

#include <iostream>
#include <sstream>
#include <string>

int main()
{
    std::istringstream s( "hello world\n" );
    std::string str;
    std::getline( s, str );
    std::cout << "extracted " << s.gcount() << " characters" << std::endl;
}

Live example

Note, for downvoters - length of the string is not the answer, as std::getline() may or may not extract additional character from the stream.

Answer 1

It would seem the way to do this is not completely straightforward because std::getline may (or may not) read a terminating delimiter and in either case it will not put it in the string. So the length of the string is not enough to tell you exactly how many characters were read.

You can test eof() to see if the delimiter was read or not:

std::getline(is, line);

auto n = line.size() + !is.eof();

It would be nice to wrap it up in a function but how to pass back the extra information?

One way I suppose is to add the delimiter back if it was read and let the caller deal with it:

std::istream& getline(std::istream& is, std::string& line, char delim = '\n')
{
    if(std::getline(is, line, delim) && !is.eof())
        line.push_back(delim); // add the delimiter if it was in the stream

    return is;
}

But I am not sure I would always want that.