Exer17_39.cpp

// This program uses data file ./data/story
#include <cstddef>
#include <cstdlib>
#include <iostream>
#include <fstream>
#include <string>
using std::size_t;
using std::cerr;
using std::endl;
using std::fstream;
using std::string;
int main(int argc, char* argv[])
{
    if (argc != 2)
        return -1;
    // open for input and output and preposition file pointers to end-of-file
    fstream inOut(argv[1], fstream::ate | fstream::in | fstream::out);
    if (!inOut) {
        cerr << "Unable to open file!" << endl;
        return EXIT_FAILURE; // EXIT_FAILURE see page 227
    }
    // inOut is opened in ate mode, so it starts out positioned at the end
    auto end_mark = inOut.tellg(); // remember original end-of-file position
    inOut.seekg(0, fstream::beg);  // reposition to the start of the file
    size_t cnt = 0;                // accumulate for the byte count
    string line;                   // hold each line of input
    // while we haven't hit an error and are still reading the original data
    while (inOut && inOut.tellg() != end_mark
           && getline(inOut, line)) { // and can get another line of input
        cnt += line.size() + 1;       // add 1 to account for the newline
        auto mark = inOut.tellg();    // remember the read position
        inOut.seekp(0, fstream::end); // set the write marker to the end
        inOut << cnt;                 // write the accumulated length
        // print a separator if this is not the last line
        if (mark != end_mark) inOut << " ";
        inOut.seekg(mark);            // restore the read position
    }
    inOut.seekp(0, fstream::end);     // seek to the end
    inOut << "\n";                    // write a newline at end-of-file
    return 0;
}