what happen to my code?

Question

I am new to C++. I am really confused between C and C++. I am familair with C and java, but not C++. Today I am going to write a linked list program using C++. But to what happened to my code ??? Thanks.

Raymond

the result: Unhandled exception at 0x00412656 in 09550978d.exe: 0xC0000005: Access violation writing location 0xcdcdcdcd.

#include <iostream>
#include <string> 
using namespace std;

struct word
{

    bool empty;
    string name;
    int count;
    word* next;
};
typedef struct word word;

word* create(word* theList)
{
    word* head = (word*)malloc(sizeof(word));
    head->empty = false;
    head->name = "";
    head->next = 0;
    return head;
}

void print(word* theList)
{
    word* current = theList;
    while(current!=0)
    {   cout << current->name << " : " << current->count << " \n" ;
        current = current->next;
    }

}

void add(string myString, word* theList)
{
    //word* newWord = (word*)malloc(sizeof(word));
    if( theList->empty == false )
    {
        theList->empty = true;
        theList->name = myString;
        theList->next = 0;
    }
    else
    {
        word* current = theList;
        while(current->next!=0)
        {
            current = current->next;
        }
        word* newWord = (word*)malloc(sizeof(word));
        newWord->empty = true;
        newWord->name = myString;
        newWord->next = 0;
        current->next = newWord;
    }
}

int main(void)
{
    word* theList = 0;
    theList = create(theList);
    add("Hello", theList);
    //add("world", theList);

}





















#include <iostream>
#include <string>
using namespace std;


class word
{
public:
    string name;
    int count;
    word *next;

word (string name);

};



word::word (string myName)
{
    name = myName;
    next = NULL;
    count = 1;
}

class List
{
public:
    bool isEmpty;
    word* theHead;



List();
List(word* aHead);
void print();
void add(string myString);
void search(string myString);
};



List::List()
{
    isEmpty = true;
}



List::List(word* aHead)
{
    isEmpty = false;
    theHead = aHead;
}



void List::add(string myString)
{
    word* newWord = new word(myString);
    if (isEmpty == true)
    {
        isEmpty = false;
        theHead = newWord;
    }
    else
    {
        word* current = theHead;
        if ( current->next == NULL)
        {
            if( myString.compare(current->name) == 0 )
            {
                current->count = current->count + 1;
                return;
            }
        }
        else
        {
            while ( current->next != NULL )
            {
                if( myString.compare(current->name) == 0 )
                {
                    current->count = current->count + 1;
                    return;
                }
                current = current->next;
            }
        }
        current->next = newWord;
    }
}
void List::print ()
{
    if (isEmpty)
    {
        cout << "nothing in the list";
    }
    else
    {
        word* current = theHead;
        while(current !=  NULL)
        {
        cout << current->name << " : " << current->count << " \n" ;
        current = current->next;
        }
    }
}

void List::search(string myString)
{
    if (isEmpty)
    {
        cout << "The word : " << myString << " is not in the List.\n";
    }
    else
    {
        word* current = theHead;
        while( current != NULL )
        {
            if( myString.compare(current->name) == 0 )
            {
                cout << "The word : " << myString << " is in the List.\n";
                return;
            }
            else
            {
                current = current->next;
            }
        }

        cout << "The word : " << myString << " is not in the List.\n";
    }
    return;
}


int main(void)
{
    List theList = List();
    string str1 = "Hello";
    string str2 = "world";
    theList.add(str1);
    theList.add(str2);
    theList.add(str1);
    theList.search("Hello");
    theList.search("You");

    theList.print();
    int i;
    scanf("%d", &i);
}

Answer 1

Most obvious problem: use new and not malloc to allocate new objects: malloc doesn't call constructor and one design principle of C++ is that constructors are called before any other operations on an object.

BTW, you code look like C using only the most basic C++ features. It would never be written like this by someone knowing C++ (word would have a constructor and private member, even for people using C++ as a "better C").

Answer 2

You should be using the new operator instead of malloc. See the difference here. Also why use structs and typedefs when c++ allows you to make a class

Here is my version of your code, its not bug free yet but it should illustrate how to use new and classes. I will try to fix it fully and update you.

Also note that within the class structure of c++ you automatically get a this pointer with member functions that acts as a pointer to the class, so you no longer have to pass in word* theList

Edit: I updated with working code, the only thing that doesn't work is the count aspect to the list. Otherwise notice that there are two classes, List interfaces with word to create a linked list, I have not included any memory management aspects to the code (which would not be so hard using the c++ destructor, if you need such facilities please indicate so in the comments, and I will be sure to add.

#include <iostream>
#include <string>
using namespace std;

class word
{
public:
    string name;
    int count;
    word *next;

word (string name);


};

word::word (string myName)
{
    name = myName;
    next = NULL;
    count = 0;
}

class List
{
public:
    bool isEmpty;
    word* theHead;

List();
List(word* aHead);
void print();
void add(string myString);


};

List::List()
{
    isEmpty = true;
}

List::List(word* aHead)
{
    isEmpty = false;
    theHead = aHead;
}

void List::add(string myString)
{
    word* newWord = new word(myString);
    if (isEmpty == true)
    {
        isEmpty = false;
        theHead = newWord;
    }
    else
    {
        word* current = theHead;
        while(current->next != NULL)
        {
            current = current->next;
        }
        current->next = newWord;
    }
}
void List::print ()
{
    if (isEmpty)
    {
        cout << "nothing in the list";
    }
    else
    {
        word* current = theHead;
        while(current !=  NULL)
        {
        cout << current->name << " : " << current->count << " \n" ;
        current = current->next;
        }
    }
}

int main(void)
{
    List theList = List();
    string str1 = "Hello";
    string str2 = "world";
    theList.add(str1);
    theList.add(str2);
    theList.print();
}

Edit: Here is the destructor to free the allocated memory, make sure to add the prototype ~List() in the class declaration:

List::~List()
{
    if (!isEmpty)
    {
        word* prev = NULL;
        word* current = theHead;
        while(current !=  NULL)
        {
            prev = current;
            current = current->next;
            delete prev;
        }
    }
}

Hope this helps.

Answer 3

For one, in Add function

if( theList->empty == false )
 {
     theList->empty = true;
     theList->name = myString;
     theList->next = 0;
 }

should be opposite - if list->empty == true, then set it to false.

As for the unhandled exception, a simple step by step 5 minute debugging session will both help you find your errors and will make you like and use the debugger. I mean it. DO TRY DEBUGGING!!!

my2c