Undefined reference to - c++

Question

I keep getting this error message every time I try to compile, and I cannot find out what the problem is. any help would be greatly appreciated:
C:\DOCUME~1\Patrick\LOCALS~1\Temp/ccL92mj9.o:main.cpp:(.txt+0x184): undefined reference to 'List::List()'
C:\DOCUME~1\Patrick\LOCALS~1\Temp/ccL92mj9.o:main.cpp:(.txt+0x184): undefined reference to 'List::add(int)'
collect2: ld returned 1 exit status
code:
//List.h
#ifndef LIST_H
#define LIST_H
#include <exception>
//brief Definition of linked list class
class List
{
public:
/**
\brief Exception for operating on empty list
*/
class Empty : public std::exception
{
public:
virtual const char* what() const throw();
};
/**
\brief Exception for invalid operations other than operating on an empty list
*/
class InvalidOperation : public std::exception
{
public:
virtual const char* what() const throw();
};
/**
\brief Node within List
*/
class Node
{
public:
/** data element stored in this node */
int element;
/** next node in list */
Node* next;
/** previous node in list */
Node* previous;
Node (int element);
~Node();
void print() const;
void printDebug() const;
};
List();
~List();
void add(int element);
void remove(int element);
int first()const;
int last()const;
int removeFirst();
int removeLast();
bool isEmpty()const;
int size()const;
void printForward() const;
void printReverse() const;
void printDebug() const;
/**
enables extra output for debugging purposes
*/
static bool traceOn;
private:
/** head of list */
Node* head;
/** tail of list */
Node* tail;
/** count of number of nodes */
int count;
};
#endif
//List.cpp I only included the parts of List.cpp that might be the issue
#include "List.h"
#include <iostream>
#include <iomanip>
using namespace std;
List::List()
{
//List::size = NULL;
head = NULL;
tail = NULL;
}
List::~List()
{
Node* current;
while(head != NULL)
{
current = head-> next;
delete current->previous;
if (current->next!=NULL)
{
head = current;
}
else
{
delete current;
}
}
}
void List::add(int element)
{
Node* newNode;
Node* current;
newNode->element = element;
if(newNode->element > head->element)
{
current = head->next;
}
else
{
head->previous = newNode;
newNode->next = head;
newNode->previous = NULL;
return;
}
while(newNode->element > current->element)
{
current = current->next;
}
if(newNode->element <= current->element)
{
newNode->previous = current->previous;
newNode->next = current;
}
}
//main.cpp
#include "List.h"
#include <iostream>
#include <string>
using namespace std;
//void add(int element);
int main (char** argv, int argc)
{
List* MyList = new List();
bool quit = false;
string value;
int element;
while(quit==false)
{
cin>>value;
if(value == "add")
{
cin>>element;
MyList->add(element);
}
if(value=="quit")
{
quit = true;
}
}
return 0;
}
I'm doing everything I think I'm suppose to be doing. main.cpp isn't complete yet, just trying to get the add function to work first. Any help will be greatly appreciated.

Answer 1

Describe your build process. It looks as though you're not building List.cpp, or else not linking it with main.cpp.

Answer 2

You're not compiling List.cpp. Add it to the command line.
In main.cpp, it's seeing (from List.h) "Hey, this class with this functionality will exist", but since you're not actually building/linking with List.cpp, it can't find the functions it's looking for.

Answer 3

Your command line should look something like g++ -o test.exe main.cpp List.cpp.
The key feature being to include both main.cpp and List.cpp.
There are other ways to do this, but this should get you started.

Answer 4

Your problem is not including all the different files in your command line arg compiler
Correct format:
get in correct directory
gcc -o list main.cpp List.cpp List.h
then you won't get anymore undefined references to functions
Good luck on building your 3 or 4 year old program...