How to save an array into a file in C++?

Question

I've created a program that saves an array into a file and then reads the file to show the array on the screen. This is the code

#include <iostream>
#include <fstream>
using namespace std;

int main(void)
{
    int saveSize = 5;//size of the array
    int save [saveSize + 1] = {saveSize, 7, 1, 2 ,3, 4};  //creating an array which first element is his own size
    ofstream fileWrite; fileWrite.open ("File.dat", ios::out | ios::binary | ios::trunc);
    fileWrite.write((char *) &save, sizeof(save));  //saves the array into the file
    fileWrite.close();

    int sizeLoad;  //size of the array that will be loaded
    ifstream fileRead; fileRead.open("File.dat", ios::in | ios::binary);
    fileRead.read((char *) &sizeLoad, sizeof(int));  //it only reads the first element to know the size of the array
    int load[sizeLoad+1];  //creating the array
    fileRead.read((char *) &load, sizeof(int));
    fileRead.close();

    //showing the array
    for(int i = 1; i <= sizeLoad; i++) cout << "Element " << i << ": " << load[i] << endl;

    return 0;
}

The problem is that when I run the program, that´s the result:

Element 1: 2686224

Element 2: 1878005308

Element 3: 32

Element 4: 2686232

Element 5: 4199985

Not even close. Somebody knows why it shows that?

Answer 1

fileRead.read((char *) &sizeLoad, sizeof(int))
fileRead.read((char*)&load, sizeof(int));

You read sizeLoad which is 5.

In the second line you intend to read 5 integers, therefore it should be changed to

int load[sizeLoad];
fileRead.read((char*)&load, sizeLoad * sizeof(int));

You also need to change the loop, only go from 0 to sizeLoad

for(int i = 0; i < sizeLoad; i++) 
    cout << "Element " << i << ": " << load[i] << endl;

Alternatively you can use std::vector , then you don't have to save the number of items in advance. You can read integers one at a time and use std::push_back to add elements to the array. Example:

#include <iostream>
#include <fstream>
#include <vector>

int main()
{
    std::vector<int> save = { 7, 1, 2, 3, 4 };
    std::ofstream fileWrite("File.dat", std::ios::binary | std::ios::trunc);
    fileWrite.write((char*)save.data(), save.size() * sizeof(int));
    fileWrite.close();

    std::ifstream fileRead("File.dat", std::ios::binary);
    std::vector<int> load;
    int temp;
    while(fileRead.read((char*)&temp, sizeof(int)))
        load.push_back(temp);
    fileRead.close();

    for(int i = 0; i < load.size(); i++) 
        std::cout << "Element " << i << ": " << load[i] << std::endl;

    return 0;
}

Answer 2

I changed your code, and it works. Try following;

int saveSize = 5;   //size of the array
int save[saveSize + 1] = {saveSize, 7, 1, 2 ,3, 4};  //creating an array which first element is his own size
ofstream fileWrite("File.dat", ios::out | ios::binary | ios::trunc); 
fileWrite.write(reinterpret_cast<char*>(&save), sizeof save );  //saves the array into the file
fileWrite.close();

int sizeLoad[saveSize+1];  //size of the array that will be loaded
ifstream fileRead("File.dat", ios::in | ios::binary);
fileRead.read(reinterpret_cast<char*>(&sizeLoad), sizeof sizeLoad );  //it only reads the first element to know the size of the array
fileRead.close();

//showing the array
for(int i = 0; i < (sizeof(sizeLoad)/sizeof(sizeLoad[0])); i++) 
    cout << "Element " << i + 1 << ": " << sizeLoad[i] << endl;

return 0;

Edit : I try to explain unclear parts. In the 3.line you created ".dat" file and next line you put every number in the array to that file. ofstream.write function takes two parameters, the first parameters should be characters so I cast array to char, second parameter should be size of array (How many characters you will write). In your case your array contains 6 numbers and your array size is 24 (Each int value 4 byte). While writing array to console, you need to know your size of array so I simply divide the array size (24) to 1 array character (4). Sorry for my bad explanation and thanks for caution.

Answer 3

You have read one element,

fileRead.read((char *) &load, sizeof(int));   // load[0] was read

which was not printed

    for(int i = 1; i <= sizeLoad; i++) cout << "Element " << i << ": " << load[i] << endl;

You should read full array using

int load[sizeLoad];  //creating the array
fileRead.read((char *) &load, sizeof(load));

and print all elements in this way

   for(int i = 0; i < sizeLoad; i++) cout << "Element " << i << ": " << load[i] << endl;