File Input & Output File I/O File I/O

#include <fstream>  // file stream header
int main () {
  std::ofstream os {"squares.txt"};  // open file

  // if stream OK = can write to file 
  if (os.good()) {
    for (int x = 1; x <= 6; ++x) { 
      // write x space x² newline
      os << x << ' ' << (x*x) << '\n';
    }
  }
}  // file is automatically closed

#include <iostream>
#include <fstream>  // file stream header
int main () {
  std::ifstream is {"squares.txt"};  // open file
  // if stream OK = file readable
  if ( is.good() ) {
    double x, y;
    // as long as any 2 values readable
    while( is >> x >> y ) {
      // print pairs (x,y) 
      std::cout << x << "² = " << y << "\n"; 
    }
  }
}  // file is automatically closed

// we'll learn later what all this means…
std::uint64_t i = 0;
// read binary
is.read(reinterpret_cast<char*>(&i), sizeof(i));
// write binary
os.write(reinterpret_cast<char const*>(&i), sizeof(i));

#include <iostream>
#include <fstream>
#include <cstdint>

int main (int argc, char* argv[]) {
    if (argc < 3) {
        std::cerr << "usage: " << argv[0] << " <integer> <filename>\n";
        return 0;
    }
    std::string filename {argv[2]};
    {   // write binary
        std::uint64_t i = atoi(argv[1]);
        std::cout << "writing: " << i << " to " << filename << "\n";
        std::ofstream os {filename, std::ios::binary};
        if (os.good()) {
            os.write(reinterpret_cast<char const*>(&i), sizeof(i));
        }
    }
    {   // read binary
        std::uint64_t i = 0;
        std::ifstream is {filename, std::ios::binary};
        if (is.good()) {
            is.read(reinterpret_cast<char*>(&i), sizeof(i));
            std::cout << "read as: " << i << "\n";
        }
    }
}