Understanding class object

Question

I am trying to create object that will store inside vector using pointers

I can store object inside vector when I don t use pointers, but when I try to use pointers I can not do that

// created class that combine some attributes for file "datum" = date,     "vrijeme" = creation of file etc.

class datoteka{
    public:
      string datum;
      string vrijeme;
      string velicina;
      string ime;

    datoteka();
    datoteka(string datum, string vrijeme, string velicina, string ime)

    {
        this -> datum = datum;
        this -> vrijeme = vrijeme;
        this -> velicina = velicina;
        this -> ime = ime;
    }

    ~datoteka();
};


int main()
{
    vector <datoteka> fajlovi;

    string linija;
    string prva;
    int i = 0;
    datoteka * pokObjDatoteke;
    pokObjDatoteke = new datoteka();

    std::ifstream pisi("list.txt"); //file is open 

    while(getline(pisi,linija)) //get first line of file 
    {
        string vrijednost; 
        stringstream red;  //create stream from line 
        string datoteka[4]; // create array to store seperate information
        red << linija; 
        while(!red.eof() && i != 4) // since line containt lot of tabs i                          
                                    read just first for values
        {
            red >> vrijednost;
            datoteka[i]= vrijednost;
            i++;
            cout << vrijednost << " ovo je vrijednost" << endl;
        }
        pokObjDatoteke->datum = datoteka[0];
        pokObjDatoteke->vrijeme = datoteka[1];
        pokObjDatoteke->velicina = datoteka[2];
        pokObjDatoteke->ime = datoteka[3];

        fajlovi.push_back(*pokObjDatoteke);  /**** problem ****
    }

    return 0;
}

I want to store objects in vector, instead I get just some memory location, probably from pointer but how can I store object and not address

Answer 1

This:

fajlovi.push_back(*pokObjDatoteke);

stores a copy of the object pokObjDatoteke points to in the vector. The pointed-to object is then leaked, since you never delete it.

Do you really need pointers here though? Your vector contains value types, not pointers, so just use emplace_back to construct your objects directly in the vector:

while (getline(pisi, linija))
{
    string vrijednost; 
    stringstream red;
    string datoteka[4];
    red << linija; 
    while (!red.eof() && i != 4) {
        red >> vrijednost;
        datoteka[i]= vrijednost;
        i++;
        cout << vrijednost << " ovo je vrijednost" << endl;
    }
    fajlovi.emplace_back(datoteka[0], datoteka[1], datoteka[2],
                         datoteka[3]);
}

However, if you really want pointers in your vector, then declare it as such:

vector <datoteka*> fajlovi;

and then change the push_back call in your original code to:

fajlovi.push_back(pokObjDatoteke);

This will push the pointers into the vector. You need to remember to delete them when the vector is destroyed, or else all the objects will get leaked:

for (auto* obj : fajlovi) {
    delete obj;
}

Manual memory management like this is error-prone and a headache. Don't do it. Use smart pointers instead, like shared_ptr. Remove these lines in your original code:

vector <datoteka> fajlovi;

// ...

datoteka * pokObjDatoteke;
pokObjDatoteke = new datoteka();

and replace them with:

#include <memory>
// ...

vector <shared_ptr<datoteka>> fajlovi;
// ...

auto pokObjDatoteke = make_shared<datoteka>();

and then push_back with:

fajlovi.push_back(pokObjDatoteke);

No need to call new or delete.

However, again, it does not look like you need pointers at all. You can just use value types as shown in the beginning of this answer.

Answer 2

If you want to store objects, then why are you using pointers?

datoteka pokObjDatoteke;

while (...)
{
    ...
    pokObjDatoteke.datum = datoteka[0];
    pokObjDatoteke.vrijeme = datoteka[1];
    pokObjDatoteke.velicina = datoteka[2];
    pokObjDatoteke.ime = datoteka[3];

    fajlovi.push_back(pokObjDatoteke);
}

Why are you trying to use pointers when, like you said, not using pointers works?

Answer 3

While the OP didn't disclose an example of the input file, this line (besides all the issues pointed out in the other answers) seems suspicious

while(!red.eof() && i != 4) {...}

See e.g. Why is iostream::eof inside a loop condition (i.e. `while (!stream.eof())`) considered wrong?

A better pattern is to perform the extraction and then check if it was successful.

while( some_stream >> some_data ) {...}

OP's code could be rewritten into something like this

#include <iostream>
#include <fstream>
#include <string>
#include <sstream>
#include <vector>

class datum
{
    std::string a_, b_, c_, d_;
public:
    datum() = default;
    datum(std::string const &a, std::string const &b, std::string const &c, std::string const &d)
        : a_(a), b_(b), c_(c), d_(d)
    {}

    friend std::istream &operator>> (std::istream &is, datum &obj)
    {
        return is >> obj.a_ >> obj.b_ >> obj.c_ >> obj.d_;
    }
    friend std::ostream &operator<< (std::ostream &os, datum const &obj)
    {
        return os << "a: '" << obj.a_ << "', b: '"<< obj.b_
                  << "', c: '" << obj.c_ << "', d: '" << obj.d_ << '\'';
    }
};

int main()
{
    std::vector<datum> data;
    std::ifstream in_stream {"list.txt"};
    if ( !in_stream )
    {
        std::cout << "Error: unable to open input file.\n";
        return -1;
    }

    std::string line, junk;
    while(getline(in_stream, line))
    {
        // This won't consider an error to have empty lines in the file 
        if (line.empty())
            continue;

        std::istringstream iss(line);
        datum dd;
        iss >> dd;
        // Stops if there are more or less strings than required in the line
        if ( !iss  or iss >> junk)
            break;
        data.push_back(std::move(dd));
    }

    for (auto const & d : data)
        std::cout << d << '\n';
}

Testable here.