fstream read and write functions errors

Question

i am creating this program that stores some items into the (.dat) file.. the program seems to run smoothly but when i try to read the data from the file i see special character on my screen and none of the data that i was trying to save, was saved properly, note that when i run the similar code in main() it works and it actually shows me the correct output.. please help me on this i really don't know what to do here

void viewFile() {
    Product item;
    ifstream file("products.dat", ios::binary);
    if (!file.read(reinterpret_cast<char*>(&item), sizeof(Product))) {
        cout <<"failed to read";
        system("exit");
    }
    item.viewProduct();
    while (!file.eof()) {
        item.viewProduct();
        cout <<endl <<endl;
        file.read((char *)(&item), sizeof(Product));
    }
    file.close();
}
void addProductToInventory() {
    string name;
    int quantity;
    int pricePerItem;
    ofstream obj("products.dat", ios::out | ios::binary);

    Product item;
    int numberOfProducts;
    cout <<"enter the number of products you want to add in this inventory: ";
    cin >> numberOfProducts;

    for (int counter = 1; counter <= numberOfProducts; counter++) {
        Product product;        
        cout <<"enter the name of the object: ";
        cin >> name;
        cout <<"enter the quantity of the product: ";
        cin >> quantity;
        cout <<"enter the price of this product (per item): ";
        cin >> pricePerItem;

        product.setName(name);
        product.setQuantity(quantity);
        product.setPricePerItem(pricePerItem);

        if (!obj.write(reinterpret_cast<char*>(&product), sizeof(Product))) {
            cout <<"failed writing\n";
            system("exit");
        }
        cout <<"item added\n\n";
    }
    obj.flush();
    obj.close();
}

this is my code in main() which WORKS its exactly the same code.. i guess

ofstream file ("products.dat", ios::out | ios::binary | ios::trunc);
    Product p1("hammer", 12, 3);
    Product p2("screw driver", 43, 1);
    if (!file.write(reinterpret_cast<char*>(&p1), sizeof(Product))) {
        cout <<"failed to write";
        system("exit");
    }
    file.write(reinterpret_cast<char*>(&p2), sizeof(Product));
    file.close();
    ifstream file2("products.dat", ios::out | ios::binary);
    if (!file2.read(reinterpret_cast<char*>(&p1), sizeof(Product))) {
        cout <<"failed to read";
        system("exit");
    }
    while (!file2.eof()) {
        p1.viewProduct();
        cout <<endl <<endl;
        file2.read((char *)(&p1), sizeof(Product));
    }
    file2.close();
}

P.S i am really sorry if this comes out to be a messy question.. I have been debugging this for hours and now i can't even think straight.

You can't write and read non-trivial objects like that. Read about serialization. — molbdnilo, Apr 14 '20 at 17:11
You also need to read [Why is iostream::eof inside a loop condition (i.e. `while (!stream.eof())`) considered wrong?](https://stackoverflow.com/questions/5605125/why-is-iostreameof-inside-a-loop-condition-i-e-while-stream-eof-cons) — molbdnilo, Apr 14 '20 at 17:11
None of this code will work properly. Who gave the assignment to you to write these types of objects to binary files? — PaulMcKenzie, Apr 14 '20 at 17:13
The reason why it will not work is self-evident if you consider what `sizeof(Product)` is. That value will never change, since it is a compile-time constant. Now if `Product` contains members that are `std::string`, then `sizeof(Product)`, regardless of how long those strings are, will remain the same. So how will the file magically save, I don't know, 1000 characters if `sizeof(Product)` is, say 40 bytes? You want to save the *data* that `Product` represents, not the raw bytes of the `Product` object. That's what serialization is all about. — PaulMcKenzie, Apr 14 '20 at 17:18
so what if i used character arrays and restrict the user in a closed range of character.. the file wont have to store 1000 character if the size is 40 — usmancodes, Apr 14 '20 at 17:23
Yes, use character arrays. But my example is in theory why your solution could never work. Your `Product` must have members that are plain-old-data types and not contain pointers for any of this to work properly. — PaulMcKenzie, Apr 14 '20 at 17:25

score 1 · Accepted Answer · answered Apr 14 '20 at 17:54

Lets start by thinking about how pointers work.

char* a = new char[2];
a[0] = 'a';
a[1] = 'b';
char* b = a;
std::cout << a[0] << a[1]; //prints ab
std::cout << b[0] << b[1]; //prints ab
a[1] = 'c';
std::cout << b[0] << b[1]; //prints ac
delete[] a;
std::cout << b[0] << b[1]; //prints random garbage because the memory that b points at is deleted

know lets start writing these pointers to files

char* a = new char[2];
a[0] = 'a';
a[1] = 'b';
ofstream outFile("products.dat", ios::binary | ios::trunc);
outFile.write(reinterpret_cast<char*>(&a), sizeof(a));
outFile.close();
{
    ifstream inFile("products.dat", ios::binary);
    char* b;
    inFile.read(reinterpret_cast<char*>(&b), sizeof(b));
    std::cout << b[0] << b[1]; //prints ab
    a[1] = 'c';
    std::cout << b[0] << b[1]; //prints ac
}
delete[] a;
{
    ifstream inFile("products.dat", ios::binary);
    char* b;
    inFile.read(reinterpret_cast<char*>(&b), sizeof(b));
    std::cout << b[0] << b[1];//prints random garbage because the memory that b points at is deleted
}

std::string contains pointers to dynamically allocated memory.

You should do something like this.

void viewFile() {
    ifstream file("products.dat", ios::binary);
    while (!file.eof()) {
        int nameLength = 0;
        if (!file.read(reinterpret_cast<char*>(&nameLength), sizeof(nameLength))) {
            cout <<"failed to read";
            system("exit");
        }
        std::string name{ nameLength, '?' };
        if (!file.read(name.data(), sizeof(char) * nameLength) {
            cout <<"failed to read";
            system("exit");
        }
        int quantity;
        if (!file.read(reinterpret_cast<char*>(&quantity), sizeof(quantity)) {
            cout <<"failed to read";
            system("exit");
        }
        int pricePerItem;
        if (!file.read(reinterpret_cast<char*>(&pricePerItem), sizeof(pricePerItem)) {
            cout <<"failed to read";
            system("exit");
        }
        Product item{ std::move(name),quantity, pricePerItem };
        item.viewProduct();
    }
    file.close();
}
void addProductToInventory() {
    string name;
    int quantity;
    int pricePerItem;
    ofstream obj("products.dat", ios::out | ios::binary);

    int numberOfProducts;
    cout <<"enter the number of products you want to add in this inventory: ";
    cin >> numberOfProducts;

    for (int counter = 1; counter <= numberOfProducts; counter++) {   
        cout <<"enter the name of the object: ";
        cin >> name;
        cout <<"enter the quantity of the product: ";
        cin >> quantity;
        cout <<"enter the price of this product (per item): ";
        cin >> pricePerItem;

        int nameLength = name.size();
        if (!obj.write(reinterpret_cast<char*>(&nameLength), sizeof(nameLength))) {
            cout <<"failed writing\n";
            system("exit");
        }
        if (!obj.write(name.data(), sizeof(char) * nameLength) {
            cout <<"failed writing\n";
            system("exit");
        }
        if (!obj.write(reinterpret_cast<char*>(&quantity), sizeof(quantity))) {
            cout <<"failed writing\n";
            system("exit");
        }
        if (!obj.write(reinterpret_cast<char*>(&pricePerItem), sizeof(pricePerItem))) {
            cout <<"failed writing\n";
            system("exit");
        }
        cout <<"item added\n\n";
    }
    obj.flush();
    obj.close();
}

fstream read and write functions errors

1 Answers1