read and write from files or from cin and cout

Question

my code work good when the argc is 1 but when I try to read and write from files (when argc is 3) the program not working well. Gcalc get the ostream (output file or cout) a

Answer 1

• Check that opening the files was done successfully.
• Check that the istream you read from doesn't have the failbit set after you've read from it. Since an istream in a boolean context checks badbit and failbit and that std::getline returns the same istream you gave it, replace your while (cin.good()) with:

  while(getline(cin, current_line)) {
      // ... only entered if badbit and failbit are false ...
  }
  

That said, it's usually better to create a separate function for reading/writing to generic istream/ostreams. This way you don't have to mess with the rdbufs of cin and cout.

#include <fstream>
#include <iostream>
#include <string>

#include "Gcalc.h"

void do_stuff(std::istream& is, std::ostream& os) {
    Gcalc gcalc;
    std::string current_line;

    while(getline(is, current_line)) {
        try {
            gcalc.implementCommand(os, current_line);
        } catch(Quit_Program& error) {
            break;
        } catch(const std::bad_alloc& error) {
            std::cerr << "Error: fatal error - " << error.what() << std::endl;
            break;
        } catch(Exception& error) {
            std::cout << error.what() << std::endl;
            // or, if you really want it:
            // os << error.what() << std::endl;
        }
    }
}

int main(int argc, char* argv[]) {
    if(argc == 1) {
        do_stuff(std::cin, std::cout);
    } else if(argc == 3) {
        std::ifstream input(argv[1]);
        std::ofstream output(argv[2]);
        if(input && output) do_stuff(input, output);
    }
}

If you want to give the user a prompt when the program is running in interactive mode, you could add a function that prints the prompt and then calls std::getline. You can combine this inside the while loop, but it looks messy, so I would suggest something like this:

std::istream& prompt(std::istream& is, std::string& line) {
    if(&is == &std::cin) std::cout << "Gcalc> ";
    return std::getline(is, line);
}

// ...

    while(prompt(is, current_line)) {
        // ...
    }