C++ Simple Pointer Passing

Question

I'm pretty new to C++ and I have some problems with getting into all that pointer stuff. Basically I am passing a pointer to a Function, creating an Array at that pointer. Back in the main function I can't access this array.

Here's my code:

#include <iostream>

using namespace std;

void createArray(char** dict, int* arraysize)
{
    *arraysize = 26*26*26*26;

    delete dict;
    dict = 0;

    //Initialisiere character array of character
    //char **wortliste = 0;
    dict = new char*[*arraysize];
    for(int i = 0; i < *arraysize; i++)
        dict[i] = new char[5];

    int ctr = 0;

    //Erstelle Einträge (sortiert)
    for (char i = 'A'; i <= 'Z'; i++)
    {
        for (char j = 'A'; j <= 'Z'; j++)
        {
            for (char k = 'A'; k <= 'Z'; k++)
            {
                for (char l = 'A'; l <= 'Z'; l++)
                {
                    dict[ctr][0] = i;
                    dict[ctr][1] = j;
                    dict[ctr][2] = k;
                    dict[ctr][3] = l;
                    dict[ctr][4] = '\0';
                    ctr++;
                }
            }
        }
    }
}

int main(void)
{
    char** dict = 0;
    int arraysize;

    createArray(dict, &arraysize);

    cout << dict[0] << endl << dict[arraysize-1] << endl;

    return 0;
}

I can't figure out my error thank you very much in advance.

Answer 1

In C++ parameters are pass by value (unless explicitly marked as being reference parameters), so when you pass dict, a pointer (to a pointer to char) to createArray, the dict inside your function is a different object, albeit with the same initial value, as the dict in main. If you want to see changes to dict in main you would have to pass it by reference, or pass the address of it into a function taking a char ***.

E.g.

void createArray(char**& dict, int* arraysize)

or

void createArray(char*** pdict, int* arraysize)
{ // use (*pdict) instead of dict ...

and

// ...
createArray(&dict, &arraysize);

A more "C++" way to achieve what you want would be to have:

void createArray( std::vector<std::string>& dict );

and to simply have createArray resize the vector to the required size. Using standard containers like vector and string also frees you of the obligation to explicity deallocate that memory that you allocate which is currently missing from your code.

Answer 2

There are a couple of mistakes.

1. To delete an array:

   char **array = /* new with whatever */;
   /* do your work */
   for (i = 0; i < array_size; ++i)
       delete[] array[i];
   delete[] array;
   

2. To new an array:

   char **array = new char *[array_size];
   for (i = 0; i < array_size; ++i)
       array[i] = new char[array_size_2];
   

3. When deleteing, to make sure you don't iterate over a not-newed array, check it against NULL:

   for (i = 0; i < array_size; ++i)
   {
       if (array[i] != NULL)   /* Or simply if (array[i]) */
           delete[] array[i];
       array[i] = NULL;
   }
   if (array != NULL)
       delete[] array;
   array = NULL;
   

   alternatively, since delete makes a check for NULL anyway, you can simplify this to:

   if (array != NULL)
       for (i = 0; i < array_size; ++i)
           delete[] array[i];  /* no need to set to NULL after if going to delete the array */
   delete[] array;
   array = NULL;
   

Note: delete deletes a single object while delete[] deletes an array.

Answer 3

I can't imagine what you would do with such data, but you can at least use modern techniques.

#include <iostream>
#include <vector>
#include <string>

using namespace std;

vector<string> create() {
    vector<string> result;
    for (char i = 'A'; i <= 'Z'; ++i) {
        for (char j = 'A'; j <= 'Z'; ++j) {
            for (char k = 'A'; k <= 'Z'; ++k) {
                for (char l = 'A'; l <= 'Z'; ++l) {
                    result.push_back(string() + i + j + k + l);
                }
            }
        }
    }
    return result;
}

int main() {
    vector<string> data = create();
    cout << data.front() << endl << data.back() << endl;
}