Binary I/O to file with 2 dimensional dynamic array

Question

I am having troubles with I/O to file with 2 dimensional dynamic array. It compiles good but it does not work as I want. Example, I save "map" of numbers 1 and after this I change number in source code to example 5 and compile it, now I load my "map" of numbers 1 but when it writes in cycle for at the end it, the output is 5 not 1. Please can somebody help to fix the code?

#include <iostream>
#include <fstream>
int main()
{
int ** array;

array = new int*[20];
for(int y=0;y<20;y++)
    array[y] = new int[30];

for(int y=0;y < 20;y++)
    for(int x=0;x < 30;x++)
        array[y][x] = 1;

int volba = 1;
std::cin >> volba;

if(volba)
{
    std::ifstream in("map",std::ios::in | std::ios::binary);
    if(!in.is_open())
        std::cout << "in map open error\n";
    in.read((char*)&array, sizeof(array));
    in.close();
    std::cout << "loaded\n";
}
else
{
    std::ofstream out("map",std::ios::out | std::ios::binary);
    if(!out.is_open())
        std::cout << "out map open error\n";
    out.write((char*)&array, sizeof(array));
    out.close();
    std::cout << "saved\n";
}

std::cout << "array\n";
for(int y=0;y < 20;y++)
{
    for(int x=0;x < 30;x++)
        std::cout << array[y][x] << " ";
    std::cout << std::endl;
}

for(int y=0;y<20;y++)
    delete [] array[y];
delete [] array;

return 0;
}

moooeeeep · Accepted Answer · 2012-05-22 09:28:15Z

1

The main problem is this: Hereby

array = new int*[20];

you allocate an array of pointers, and this will not become a two dimensional array as you later do:

array[y] = new int[30];

Note that there is a difference between this

// array of pointers to integer arrays
int ** array = new int*[20];
for(int y=0;y<20;y++)
  array[y] = new int[30];

and this

// two dimensional integer array
int array[20][30];

You cannot assume that your array of arrays will lie in contiguous memory.

Besides: Hereby

out.write((char*)&array, sizeof(array));

you just write out the pointer, not the actual data. Try to print the sizeof(array):

#include <iostream>

int main() {
  int * array = new int[10];
  std::cout << sizeof(array) << std::endl; // probably prints 4 or 8
  return 0;
}

Conclusion: Unless you need to implement this for educational purposes, std::vector will far more conveniently serve you memory management for free. Also have a look on Boost Serialization. It provides functionality for serialization of STL collections.

edited May 22, 2012 at 9:28

answered May 22, 2012 at 8:48

moooeeeep

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2 Comments

moooeeeep Over a year ago

@user1295618 - I have put an option in bold. You could alternatively use a static two-dimensional array[20][30].

moooeeeep Over a year ago

@user1295618 - of course not. The good programming way would be to use std::vector and probably Boost Serialization.

Alex Bakulin · Accepted Answer · 2012-05-22 08:41:12Z

0

The error is in the fact that sizeof(array) equals to the size of a pointer, not the size of dynamically allocated memory. As an effect, you read/write only 4 (or 8) bytes. Use actual array size (in this case 20*30) instead of sizeof(array).

answered May 22, 2012 at 8:41

Alex Bakulin

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2 Comments

c0ntrol Over a year ago

I tried to change sizeof(array) to sizeof(int) * 600 but I have still this problem: "Example, I save "map" of numbers 1 and after this I change number in source code to example 5 and compile it, now I load my "map" of numbers 1 but when it writes in cycle for at the end it, the output is 5 not 1."

Alex Bakulin Over a year ago

There is another problem in your code, see the answer by @moooeeeep.

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