CODEWITHSUNDEEP
c++c++11
Thomas Martin
Thomas Martin

Reputation: 112

I'm getting Read access violation while accessing objects from file C++.

In the below class error is in the init function where i load the class object I stored in the file to the vector Items.

class Item
{
std::string item_code;
std::string item_name;
std::string unit_name;
unsigned int price_per_unit;
double discount_rate;
static std::vector<Item> Items;
friend std::ostream& operator<< (std::ostream&, Item&);

public:
static void PrintAll();
static void Init();
~Item();
};

Default constructor is the one which reads data from user and writes into file. Below is the code of default constructor.

Item::Item(int a)
{
std::cout << "Item name : ";
std::getline(std::cin, item_name);
std::cout << "Unit (Kg/g/Qty) : ";
std::getline(std::cin, unit_name);
std::cout << "Price per unit : ";
std::cin >> price_per_unit;
std::cout << "Discount Rate : ";
std::cin >> discount_rate;
std::cin.ignore();
std::cout << "Product code (has to be unique) : ";
std::getline(std::cin, item_code);

std::ofstream outfile;
outfile.open("Files\\Items.txt", std::ios::out | std::ios::app);
outfile.write((char*)&(*this), sizeof(Item));
outfile.close();
}

Below is the Init() function for which read access violation is thrown at.

void Item::Init()
{
std::ifstream infile("Files\\Items.txt", std::ios::in);
if (!infile.is_open())
{
    std::cout << "Cannot Open File \n";
    infile.close();
    return;
}
else
{
    Item temp;
    while (!infile.eof())
    {
        infile.read((char*)&temp, sizeof(temp));
        Item::Items.push_back(temp);
    }
}
infile.close();
}

Even though i am checking for eof, read access violation is thrown. Please give me some advice on this issue.

Upvotes: 0

Views: 213

Answers (2)

tujiaw
tujiaw

Reputation: 91

std::string size is variable, you can try the following definition

char item_code[20];
char item_name[20];
char unit_name[20];

Upvotes: 1

David Schwartz
David Schwartz

Reputation: 182763

    infile.read((char*)&temp, sizeof(temp));

This fills the temp object with junk from the file. It's supposed to contain valid std::string objects and whatever is in the file, it can't possibly be a valid std::string object. If you don't see why, consider that creating a valid std::string object requires allocating memory to hold the string data -- that's what the std::string constructor does. Reading data from a file can't possibly do this.

A file is a stream of bytes. To write data to a file, you need to define some way to represent that data as a stream of bytes. You need to encode its length if it is variable length. To read it back in, you need to handle the variable length case as well. You need to convert the file data to an appropriate internal representation, such as std::string. This is called "serialization".

Upvotes: 2

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