Writing/reading objects from a file

Question

I was trying to write X objects in a file, to read it later, but I don't understand much about I/O in c++. X would be a class with a string and a number, just as an example. (I used a string, but it is not supposed to me just a letter)

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

class X { 
    int _number;
    std::string _letter;
    public:
    X() { _number = 0; _letter = "anonimo"; }
    X(int number, std::string letter) { _number = number; _letter = letter; }
    int getnumber() { return _number; }
    std::string getletter() { return _letter; }
    void setNumber(int number) { _number = number; }
    void setLetter(std::string letter) { _letter = letter; }

};

std::istream& operator>>(std::istream& is, X &x) {
    std::string str;
    std::string letter;
    int number;
    std::getline(is, str);
    sscanf(str.c_str(), "%[^,], %d", letter, &number);A
    X c(number, letter);
    return is;
}
std::ostream& operator<<(std::ostream& out, X &x) {
    out << x.getLetter() << ", " << x.getNum();
    return out;
}

int main(int argc,char *argv[]) {
    X c1 = X(2, "w");
    X c2 = X(6, "c");
    X c3, c4;
    std::ofstream outFile;
    if (!(argc > 1)) return -1;
    outFile.open(argv[1]);
    std::stringstream str;
    str << c1 << std::endl << c2 << std::endl;
    std::string xs = xs.c_str();
    outFile << xs.c_str();
    outFile.close()
    std::ifstream infile;
    infile.open(argv[1]);
    infile >> c3;
    infile >> c4;
    return 0;
   }

I know I'm doing it wrong, and I wanted to know the best way to do it, using std::stringstream to save it in a file, and creating 2 new X's, c3 and c4, using information from c1 and c2.

Community · Accepted Answer · 2017-05-23 12:03:24Z

The problem you are describing is that of serialization. From the C++ FAQ:

It [serialization] lets you take an object or group of objects, put them on a disk or send them through a wire or wireless transport mechanism, then later, perhaps on another computer, reverse the process: resurrect the original object(s). The basic mechanisms are to flatten object(s) into a one-dimensional stream of bits, and to turn that stream of bits back into the original object(s).

Also from the C++ FAQ, your specific situation appears to match: [36.7] How do I serialize objects that aren't part of an inheritance hierarchy and that don't contain pointers to other objects?

There are lots of questions on SO with respect to serialization. For a broad discussion of the topic, see How do you serialize an object in C++?

Finally, if you choose to go with a text format for serialization, you might consider formatting that text as JSON or XML. For JSON serialization, see Converting C++ class to JSON. For XML serialization, see Minimal XML library for C++?, what's the easiest way to generate xml in c++? and a third-party tool such as CodeSynthesis XSD.

odedsh · Accepted Answer · 2013-11-08 12:32:28Z

1

I would suggest using boost::serialization

http://www.boost.org/doc/libs/1_54_0/libs/serialization/doc/index.html

The discussion in these help documents also covers a lot of the considerations around saving objects in files.

answered Nov 8, 2013 at 12:32

odedsh

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