C++：将动态分配的数组打印到控制台，行为异常

Question

I am having weird behavior with my print function but I am wondering if it is an IDE issue or an issue with the fact that my code and target/executable is stored on an external hard drive. OR if it is my code. it will randomly print things like '/323' '2u3m' that kind of stuff. Then I run it again and it prints totally fine.

int WordList::print() const {
    
    if (m_list != nullptr) {
        for(int i=0; i<(m_count-1); i++){
            cout << m_list[i] << " ";
        }
    cout << m_list[m_count-1] << endl;
    return m_count;
    }
    else
    
    return -1;
}

Also the computer I am using has 4gb of RAM and is often kinda slow... I am using Xcode

I realize you do not see the rest of my code and that their may be some kind of issue there.

I haven't been coding very long so this kind of behavior maybe a sign of something but I haven't had the experience to know what it is so I am hoping someone here can help me out!

Thanks!

Member variables:

class WordList {
.
.
.
private:

    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

};

Adding where I think a problem would lie if there is one:

int WordList::add(const char word[]) {
    if (strlen(word)==0) {
          return -2;
        }
    if (m_list == nullptr ){
        m_list = new char*[1];
        m_list[m_count] = new char (strlen(word));
        strcpy (m_list[m_count], word);
        m_count++;
        m_max ++;
        return -2;
        }
    if (m_count == 0 && m_list != nullptr ) {
        m_list[m_count] = new char (strlen(word));
        strcpy (m_list[m_count], word);
        m_count++;
        return 0;
        }
    if (m_count < m_max) {
        m_list[m_count] = new char (strlen(word));
        strcpy (m_list[m_count], word);
        m_count++;
        return 0;
        }
    if (m_count == m_max) {
        m_list[m_count] = new char (strlen(word));
        strcpy (m_list[m_count], word);
        m_count++;
        m_max ++;
        return 1;
        }
else
    return -2;
}

So I am using this to store char arrays, I have to only use things in the cstring library hence the reason I am doing everything the hard way.

This is a project for school if it isn't obvious and I know there are details I may have left out as to the greater scope of the project but I think it should make sense with the information I have given.

Answer 1

This line, and lines that are similar to this:

m_list[m_count] = new char (strlen(word));

are wrong for two reasons.

First, that line does not allocate strlen(word) chars dynamically. What it does is it creates a single character dynamically, with the value of that character equal to strlen(word) .

Second, even if you allocated correctly, it does not allocate enough memory to hold the terminating null character when you call strcpy later on.

Thus your program had memory overwrites and undefined behavior.

The fix for this line (and other similar lines is):

m_list[m_count] = new char [strlen(word) + 1];

Note the [ ] and the addition of the +1 to make room for the null terminator.

---

Other issues that are suspicious is your usage of m_count and m_list[m_count] . Where do you actually increase the size of m_list to hold more items?

---

Having said all this, why are you not simply using std::string , so that these types of mistakes are not committed? Also, why not simply std::vector<std::string> m_list instead of raw pointers?