Saving C++ result in binary file for later use

Question

I am writing a C++ program that solves a PDE and saves the solution in a 2D array. Then it does something with the results of this PDE. However, solving the PDE accurately takes a lot of computations. Thus, the solving part has to be executed only once, but the results should be saved, such that I can get back to them later.

Therefore, I wrote two different codes:

1. solves the pde and saves results in matrix A. Matrix A is saved in a binary file.

2. Reads the values in the binary file and does computations with the results.

However, when I read the binary file in the second code, it only an empty array and does not contain the values that I saved here in the other code.

If I read it in code 1, it does function as desired.

What am I doing wrong? Is it even possible to read the values of a binary file in an other application? Should I approach this problem completely different?

I saved both codes in a different project, should I somehow link them? Or should both codes be in the same project?

I spend a few days searching the web for a solution, but I only find questions and answers on how to open and save binary files within the same code (for instance this and this). Does that mean that what I am trying to do is not possible or did I not find the correct sources?

I am a complete beginner in C++ and would be very happy with any help or directions.

Many thanks in advance!

This is code 1:

#include <iostream>
using namespace std;
#include <math.h>
#include <getopt.h>
#include <fstream>
#include <string>

// Returns a pointer-to-pointer to a newly created array
// of size [row : col].
double **Create2D(int row, int col)
{
    double **p = new double* [row];
    for (int j = 0; j < row; j ++)
        p[j] = new double[col];
    return p;
}

// Deletes an array pointed by 'p' that has 'row' number rows
void Delete2D(double **p, int row)
{
    for (int j = 0; j < row; j ++)
        delete [] p[j];
    delete [] p;
}

double PDE (double A_old, double A_old2, double dt, double s_int, double m, double ds, double R, double d){
    double A_next;
    A_next = A_old + dt*(s_int - 1)*m*A_old + (dt/ds)*((s_int-1)*(R*s_int-d)*(A_old2-A_old));
    return A_next;
}

// Start of main function

int main(int argc, char *argv[]) {

    // Set default values for input

    double t(1), m(0.5) , R (1.3) , d (0.8) ;

    // Solution by solving PDE for PGF

    double dt = 0.0003;
    const int tsteps = t/dt +1;
    const double ads = 0.01;
    const double ds = 0.01;
    const int ssteps = 1/ds +1;


    // Start Active Section
    double **A = Create2D(ssteps, tsteps);

    // Initial condition 1: f(s,0) = 1

    int i = 0;
    for (;i<ssteps;){
        A[i][0] = 1;
        i++;
    }

    // Initial condition 2: f(1,t) = 1

    int j = 0;
    for (;j<tsteps;){
        A[ssteps-1][j] = 1;
        j++;
    }

    // Calculate other matrix points
    int ii = ssteps-2;
    double as_int;
    as_int = 1-ads;

    for (;ii>=0;){
        int jj = 0;
        for (;jj<tsteps-1;){
            A[ii][jj+1] = PDE (A[ii][jj], A[ii+1][jj], dt, as_int, m, ds, R, d);
            jj++;
        }
        as_int = as_int-ds;
        ii--;
    }


    // Write A to a binary document

    ofstream out("valuesA", ios::out | ios::binary);
    if(!out) {
        cout << "Cannot open file.";
        return 1;
    }

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

    out.close();

    ifstream in("valuesA", ios::in | ios::binary);
    in.read((char *) &A, sizeof A);

    // see how many bytes have been read
    cout << in.gcount() << " bytes read\n";

    in.close();

    // Delete A from the heap to prevent memory leaking
    Delete2D(A, ssteps);

    return 0;
} // end main

This is code 2:

#include <iostream>
using namespace std;
#include <math.h>
#include <getopt.h>
#include <fstream>
#include <string>

double **Create2D(int row, int col)
{
    double **p = new double* [row];
    for (int j = 0; j < row; j ++)
        p[j] = new double[col];
    return p;
}

// Start of main function

int main(int argc, char *argv[]) {

    // Use output of PDE function
    double dt = 0.0003;
    const int tsteps = t/dt +1;
    const double ds = 0.01;
    const int ssteps = 1/ds +1;

    double **A = Create2D(ssteps, tsteps);

    ifstream in("valuesA", ios::in | ios::binary);
    in.read((char *) &A, sizeof A);

    // see how many bytes have been read
    cout << in.gcount() << " bytes read\n";

    in.close();

    return 0;
} // end main

Answer 1

One major problem is this:

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

Here you write the location of A, not the data it points to. Furthermore, since A is a pointer, then sizeof A will be the size of the pointer and not what it points to.

You need to loop and save each sub-array of A explicitly:

for (int i = 0; i < ssteps; ++i)
    out.write((char *) A[i], tsteps * sizeof(double));

Do the opposite when reading.

Answer 2

I think your problem is solved by @SomeProgrammerDude, but I suggest to have a look at boost serialization.

Thus, you can easily write and read the state of complete objects and you don't have to invent your own binary representation of your data.