(C++) Writing and reading a dynamic array to a binary file

December 29, 2021

I’m new here so forgive me if I make some mistakes.
Anyway, I’ve got this code here:

class List{
    private:
        int dim;
        Reservations *res; //Dynamic array of reservations that composes "List"
    public:
        List();
        List(int dm, Reservations *resv);
        ~List();
        int getDim();
        void setDim(int dm);
        void readNwrite(List &ls);
};

This is a class and I need to write and read the Reservations array in a binary file. The Reservations class is composed of other types of data (two strings, an integer and also another class).
Here you can see the Reservations class:

class Reservations{
    private:
        DateTime dt;
        string name, phone;
        int codVisit;
    public:
        Reservations();
        Reservations(DateTime datT, string nm, string tel, int cod);
        ~Reservations();
        DateTime getDt();
        void setDt(DateTime datT);
        string getName();
        void setName(string nm);
        string getTel();
        void setTel(string tel);
        int getCodVisit();
        void setCodVisit(int cod);
        void read();
        void print();
};

And here’s the class DateTime:

class DateTime{
    private:
        Date d;
        int sec, min, hrs;
    public:
        DateTime();
        DateTime(int s, int m, int o, int d, int ms, int y);
        ~DateTime();
        void getDt();
        void setDt(int g, int ms, int y);
        int getSec();
        void setSec(int s);
        int getMin();
        void getMin(int m);
        int getOre();
        void getOre(int o);
        void print();
        void read();
        int validate();
        int compare(DateTime dt1);
    friend void Anglform(DateTime dt);
};

This is how I’ve done created and read the binary file in the List class:

void List::readNwrite(List &ls){
    ofstream file("list.dat", ios::binary);
    file.write(reinterpret_cast<char*>(res), sizeof(Reservations) * dim);
    file.close();

    ifstream fileF("list.dat", ios::binary);
    ls.setDim(dim);
    ls.res = new Reservations[ls.dim];
    fileF.read(reinterpret_cast<char*>(ls.res), sizeof(Reservations) * dim);
    file.close();
}

I’ve tried it but it isn’t working. I know that the second instance is getting the contents of the first one, but in the end the program always crashes…

>Solution :

To be able to read/write a structure to/from a file as a binary blob, that structure must be standard layout type (aka POD) and not contain any pointers within. Your structure obviously does not satisfy that requirement (std::string object is not a standard layout type and does contain a pointer(s) inside) so you have to write reading/writing methods to load/store data member by member or use a library designed for that purpose.

Note: your class Reservations violates the rule of 3/5/0
and you have memory leak right there on this line: