C++: Char pointer to char pointer array to char array - c++

Question

I'll do my best to be brief:
So I have an assignment where I am creating a 'Wordlist' class. In which I will store a list of words.
These are the member variables
class WordList
{ //...
unsigned int m_count; // Number of words currently in list
unsigned int m_max; // The total size of the list.
char** m_list; // The list storing the words
};
This is my constructor
WordList::WordList(const int max_words) {
if(max_words < 1){
m_list = nullptr;
m_max = 0;
m_count = 0;
}
else
m_list = new char*[max_words];
m_count = 0;
m_max = max_words;
for (int i = 0; i < max_words; i++) {
m_list[i] = new char[20];
}
}
And this is where I start to find problems.
The following add function is supposed to add a word in the form of a c-style string that is pointed to from the array of character pointers that is pointed to from **char m_list .
int WordList::add(const char word[]) {
if (m_count == 0 && m_list != nullptr ) {
strcpy (m_list[m_count], word);
m_count++;
return 0;
}
if (m_count < m_max) {
m_count++;
strcpy (m_list[m_count], word);
return 0;
}
if (m_count == m_max) {
m_count++;
m_max ++;
strcpy (m_list[m_count], word);
return 1;
}
if (strlen(word)==0) {
return -2;
}
if (m_list == nullptr ){
return -2;
}
else
return -2;
}
So the issue I am having is that I clearly not syntactically correct with my * because I am not getting an array of 5 pointers that point to full words rather I am getting the first letter saved to the final destination char but its not copying over everything like I want.
I'm sure I didn't translate my problem to English as well as I should have but hopefully thats a start. Thank you!
An example of how I will be calling my add function:
WordList *wordlist = new WordList(5);
wordlist->add("harry");
wordlist->add("ron");
wordlist->add("hermione");
And it should add to the bottom of the pointer array a pointer to each word
so
cout << wordlist->m_list[0][2] << endl; // Expect 'r'
cout << wordlist->m_list[1] << endl; // Expect "ron"
instead I get
r
printed out only

Answer 1

I don't see anything wrong with your use of double-pointers.
There are other issues, though:
in your WordList::add you should check for empty word or empty list first, and fail fast. Besides, in your code if the word was empty - you would already added it and returned form that function.
in if (m_count < m_max) block, you pre-increment m_count, leaving one element empty and risking to go out-of-bounds on the last entry.
in if (m_count == m_max) { you are CERTAINLY going out-of-bounds
Suggestion: instead of pre-allocating 20-character arrays, leave them nullptr; when you need to a word - use strdup(word); that would allocated a required space for you.
As for your I am getting the first letter saved - I am guessing you are not checking it right...

Answer 2

The problem is that you add the first word:
if (m_count == 0 && m_list != nullptr ) {
strcpy (m_list[m_count], word);
m_count++;
return 0;
}
Which increments m_count so now m_count is 1.
Then you add the second word:
if (m_count < m_max) {
m_count++;
strcpy (m_list[m_count], word);
return 0;
}
Which increments m_count BEFORE adding the word so the second word is at index 2 and index 1 is skipped altogether.
You need to always increment the count after copying the word because m_count is 1 based and the array is 0 based.