Question

I have a class containing many different variables, for example there’s a few multidimensional vectors in there.

I’ve heard that you can store and load data directly to a file, but to what extent?

For example if I create an instance of this class, fill it up, then save it to a file, can I load it the same way? Like how does that work? Do I just save it all in one go or do I have to split up the data somehow?

Answer 1

This is not exactly beginner’s topic in C++

C++ has no automated way to store / load your objects to / from a file. Either way you chose to go, you’ll have to implement it yourself.

You might chose to overload the << an >> operators to use with streams, or you might want to go with your own Load and Store methods (or whatever names you chose appropriate, like, Serialize / Deserialize). I personally prefer to create my own functions and not to use the operators, but it’s just me.

Here is a simple example (with overloaded << and >> operators):

#include <fstream>
#include <iostream>

using namespace std;

class MyClass
{
public:
    MyClass (int x) : m_x(x), m_y(x+1) {}

    friend istream& operator >> (istream& in, MyClass& obj);
    friend ostream& operator << (ostream& out, const MyClass& obj);

private:
    int m_x;
    int m_y;
};

istream& operator >> (istream& in, MyClass& obj)
{
    in >> obj.m_x;
    in >> obj.m_y;
    return in;
}

ostream& operator << (ostream& out, const MyClass& obj)
{
    out << obj.m_x << ' ';
    out << obj.m_y << endl;
    return out;
}

int main(int argc, char* argv[])
{
    MyClass myObj(10);
    MyClass other(1);
    cout << myObj;
    ofstream outFile ("serialized.txt");
    outFile << myObj;
    outFile.close();
    ifstream inFile ("serialized.txt");
    inFile >> other;
    inFile.close();
    cout << other;
    return 0;
}

It worth to mention that you should take care about the serialization format. In the example above it’s just text; but if you are going to store a lot of those objects, you might start thinking about serializing binary data (you’ll need to use ofstream::binary and ifstream:binary flags while opening the files, and will not need additional separators, like ' ' and endl in your serialization stream).

Typically, when you think of serialization, you also want to think of the versioning of your stream — and this is another separate topic.