C++ dynamic array of string

Question

I am learning C++ coming over from Objective-C / C, and for a dummy project I want to load the words from the /usr/share/dict/words file stored on Mac OS X machines.

The idea is to load the file and get each word into an array, so I have an array of type string .

But I'm having trouble working correctly with dynamic memory with my arrays - using new and delete . I've added some of the code below, if anyone could help out...

And so I'm getting a memory error:

word:: A
word:: a
word:: aa
word:: aal
definitions(2758) malloc: *** error for object 0x100103b90: incorrect 
checksum for freed object - object was 
probably modified after being freed.
*** set a breakpoint in malloc_error_break to debug

Load words:

string* Definition::loadWords()
{
    int arrayLength = 0;

    arrayOfWords = new string[arrayLength];

    ifstream file;

    file.open("/usr/share/dict/words");

    if(file.is_open())
    {
        while(file.good()){
            string word;
            getline( file, word );
            this->addWord(word, arrayOfWords, &arrayLength);
        }

    }

    file.close();

    cout << endl << "There are " << arrayLength << " words" << endl;

    return arrayOfWords;
};

Add words to array:

void Definition::addWord(string newWord, string currentArray[], int* arrayLength)
{
    cout << endl << "word:: " << newWord;

    string *placeholderArray = new string[*arrayLength + 1];
    placeholderArray[*arrayLength + 1] = newWord;

    for(int i = 0; i < *arrayLength; i++){
        placeholderArray[i] = currentArray[i];
    }

    (*arrayLength)++;

    currentArray = placeholderArray;

    delete [] placeholderArray;
}

Answer 1

currentArray = placeholderArray;

This aliases placeholderArray to currentArray. So, when you call...

delete [] placeholderArray;

.. you are deleting what currentArray is pointing to.

Answer 2

Here you are simply assigning the pointer , as opposed to the values in the array:

currentArray = placeholderArray;

And here you free the space pointed to by said pointer:

delete [] placeholderArray;

The next time you to read from the freed memory space will result in undefined behavior.

---

Instead of using C-style arrays in C++, use std::vector and its resize() function. Better still, your application could simply invoke push_back() on each newWord , which will obviate the entire need for your addWord() function.

Answer 3

First thing I can see is this:

placeholderArray[*arrayLength + 1] = newWord;

You are adding an element past the end of the array. Arrays are indexed from 0. For example if the array length is 5 then the last element in the array is at index 4. So the line should be:

placeholderArray[*arrayLength] = newWord;

Then after that you are deleting your array with this:

currentArray = placeholderArray;

delete [] placeholderArray;

Since you are just setting currentArray to point to placeholderArray and then deleting it.

Also passing by reference is much better than passing by pointer. So rather than this:

void Definition::addWord(string newWord, string currentArray[], int* arrayLength)

Use this:

void Definition::addWord(string newWord, string currentArray[], int& arrayLength)

That you don't always have to use the * to get the value each time you want to use it.

Here's a tutorial on using references: http://www.learncpp.com/cpp-tutorial/73-passing-arguments-by-reference/

Also save yourself the time and effort and learn to use vectors and stl containers rather than arrays sooner rather than later.

Here's a tutorial for using vectors: http://www.codeguru.com/cpp/cpp/cpp_mfc/stl/article.php/c4027/C-Tutorial-A-Beginners-Guide-to-stdvector-Part-1.htm