Dynamic array of dynamic (array of struct)

Question

I have a struct named person as follows:

struct person {
    int height, weight;
};

I also created an array of person as follows:

struct Arrayofperson {
    int len;   //indicates the length of this array(its supposed to be dynamic)
    person *p; //this is supposed to be the dynamic array of person.
};

And I do this for an array of array of person as follows:

struct Array_2d_ofperson{
    int len;   //indicates the length of this array(its supposed to be dynamic)
    Arrayofperson *subarray; //this is supposed to be the dynamic 2d array of person.
};

This is my code:

#include <iostream>
#include "test.h"
using namespace std;

#define DLLEXPORT extern "C" __declspec(dllexport)

DLLEXPORT Arrayofperson create_arr_person(int len) {
    Arrayofperson arr_p;
    arr_p.len = len;
    arr_p.p = new person[len];
    //populate the array here:
    for (int a = 0; a < len; a++) {
        arr_p.p[a].height = a; //yes they're the same, but it doesn't matter for now.
        arr_p.p[a].weight = a; 
    };
    return arr_p;
}

DLLEXPORT void print_arr_person(Arrayofperson pp) {
    printf("length: %d\n", pp.len);
    for (int b = 0; b < pp.len; b++) {
        printf("height, weight %d, %d\n", pp.p[b].height, pp.p[b].weight);
    }; 
}

DLLEXPORT Array_2d_ofperson create_2darr_person(int len, int sublen) {
    Array_2d_ofperson arr_2d_person;
    arr_2d_person.len = len;
    arr_2d_person.subarray = new Arrayofperson[len];

    for (int a = 0; a < len; a++) {
        arr_2d_person.subarray[a].len = sublen;
        arr_2d_person.subarray[a].p = new person[sublen];
        for (int b = 0; b < sublen; b++) {          
            arr_2d_person.subarray[a].p[b].height = b;
            arr_2d_person.subarray[a].p[b].weight = b;
        }
    };

    for (int a = 0; a < len; a++) {
        for (int b = 0; b < sublen; b++) {  
            printf("(a, b): %d, %d", arr_2d_person.subarray[a].p[b].height, arr_2d_person.subarray[a].p[b].weight);
            printf("\n");
        }
    };
    return arr_2d_person;
    cin.get();
}

DLLEXPORT void print_2darr_person(Array_2d_ofperson pp) {
    int len = pp.len;
    int sublen = pp.subarray[0].len; //yes I haven't forgotten that it can change between different subarrays.

    for (int a = 0; a < len; a++) {
        for (int b = 0; b < sublen; b++) {  
            printf("(a, b): %d, %d", pp.subarray[a].p[b].height, pp.subarray[a].p[b].weight);
            printf("\n");
        }
    }; 
}

I intend to make a dll(the why is not important here) from the above code(it will have more code later on) and use it in python. So here are my questions:

1) It seems that when I do this on the python side:

from ctypes import *

test = CDLL('test.dll') //the dll from the code above, yes it works.

arr = test.create_arr_person(6)

test.print_arr_person(arr)

arr2 = test.create_2darr_person(2, 3)

#test.print_2darr_person(arr2)

raw_input('h')

I get garbage for printing the array of person and get an access violation error from windows when I try to print the 2d array.

So here are my questions, in order of importance(I don't want to use python api within the dll, because the dll could also be used by other languages)

1) How do I make it so that the memory dedicated to the array/ 2darray stays in memory so that I don't get access violation errors. I've tried doing static Arrayofperson, but it didn't work.

2) How is possible to make it easy to access person in the subarray of the 2d array instead of doing. pp.subarray[a].p[b]. (I want to do this: pp[a][b], where pp is 2darray of person). I believe it has something to do with overloading the [ ] operator, but I'm not familiar with making classes(thats why i made a struct now).

3) How do I access the array/2darray in python using the same way (I want to do this in python:

test = CDLL('test.dll')
array_of_person = test.create_arr_person(5)
print (array_of_person[0]) #something like this

I already know how to create the array, but I don't know how to keep the information inside it 'alive' once the function returns. — 7331skillz
– 7331skillz, Commented Dec 13, 2011 at 9:29

wolfgang · Accepted Answer · 2011-12-14 13:45:21Z

1

The problem here is that python does not know how to handle your struct. Check the documentation for ctypes, it has a list of supported python types that you can pass to C functions, and documentation on how to make it handle some more types.

The way you've written it, python thinks that all your functions return an int.

You need to read http://docs.python.org/library/ctypes.html

EDIT: If you do things right, you will probably end up returning an opaque pointer to your struct from your C function to python. Inside your struct, you can use all C++ features then, including the good stuff, like std::vector.

answered Dec 14, 2011 at 13:45

wolfgang

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Petr · Accepted Answer · 2011-12-13 12:50:22Z

0

I tried to compile your code on a Linux machine (gcc 4.4.3) and it works.

Have you considered using STL containers (vector) instead? You can use vectors of vectors to generate multidimensional arrays without having to worry about memory leaks.

answered Dec 13, 2011 at 12:50

Petr

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1 Comment

7331skillz Over a year ago

Vectors are c++, therefore I cannot export them using extern 'c'. I did try using vectors for it, but the parameters that I pass to the function always ends up being wrong. eg: my create_array_person function takes the 'int' parameter, but when I pass 5 to it, i get something like 2227068 on the dll side.

Petr · Accepted Answer · 2011-12-14 14:04:28Z

0

You can use the fact that the vector is guaranteed to be a continuous chunk of memory and return a pointer to the first element.

T * p = &v[0]

This pointer can be then accessed as an ordinary array and is safe across module boundaries. The same technique also works for std::strings that can be accessed via a raw pointer to the storage.

const char * p = s.c_str();

You just have to ensure the object that holds the storage does not accidentally go out of scope before you are done.

Multidimensional arrays can be always projected onto one dimension.

1 1 1

2 2 2

3 3 3

can be stored as:

1 1 1 2 2 2 3 3 3

edited Dec 14, 2011 at 14:04

answered Dec 14, 2011 at 13:33

Petr

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