How to count lines of a file in C++?

Question

How can I count lines using the standard classes, fstream and ifstream?

Answer 1

kernel methods following @Abhay

A complete code I've done :

size_t count_line(istream &is)
{
    // skip when bad
    if( is.bad() ) return 0;  
    // save state
    std::istream::iostate state_backup = is.rdstate();
    // clear state
    is.clear();
    std::istream::streampos pos_backup = is.tellg();

    is.seekg(0);
    size_t line_cnt;
    size_t lf_cnt = std::count(std::istreambuf_iterator(is), std::istreambuf_iterator(), '\n');
    line_cnt = lf_cnt;
    // if the file is not end with '\n' , then line_cnt should plus 1  
    is.unget();
    if( is.get() != '\n' ) { ++line_cnt ; } 

    // recover state
    is.clear() ; // previous reading may set eofbit
    is.seekg(pos_backup);
    is.setstate(state_backup);

    return line_cnt;
}

it will not change the origin file stream state and including '\n'-miss situation processing for the last line.