Undefined Reference to

Question

When I compile my code for a linked list, I get a bunch of undefined reference errors. The code is below. I have been compiling with both of these statements:

g++ test.cpp

as well as

g++ LinearNode.h LinearNode.cpp LinkedList.h LinkedList.cpp test.cpp

I really do not understand why I am getting these errors because I am really rusty on classes in C++. I could really use some help.

LinearNode.h:

#ifndef LINEARNODE_H
#define LINEARNODE_H
#include<iostream>

using namespace std;

class LinearNode
{
    public:
        //Constructor for the LinearNode class that takes no arguments 
        LinearNode();
        //Constructor for the LinearNode class that takes the element as an argument
        LinearNode(int el);
        //returns the next node in the set.
        LinearNode* getNext();
        //returns the previous node in the set
        LinearNode* getPrevious();
        //sets the next element in the set
        void setNext(LinearNode* node);
        //sets the previous element in the set
        void setPrevious(LinearNode* node);
        //sets the element of the node
        void setElement(int el);
        //gets the element of the node
        int getElement();

    private: 
        LinearNode* next;
        LinearNode* previous;
        int element;        
};//ends the LinearNode class

#endif

LinearNode.cpp:

#ifndef LINEARNODE_cpp
#define LINEARNODE_cpp
#include<iostream>
#include"LinearNode.h"

using namespace std;

//Constructor for LinearNode, sets next and element to initialized states
LinearNode::LinearNode()
{
    next = NULL;
    element = 0;
}//ends LinearNode default constructor

//Constructor for LinearNode takes an element as argument.
LinearNode::LinearNode(int el)
{
    next = NULL;
    previous = NULL;
    element = 0;
}//ends LinearNode constructor

//returns the next element in the structure
LinearNode* LinearNode::getNext()
{
    return next;
}//ends getNext function

//returns previous element in structure
LinearNode* LinearNode::getPrevious()
{
    return previous;
}//ends getPrevious function

//sets the next variable for the node
void LinearNode::setNext(LinearNode* node)
{
    next = node;
}//ends the setNext function

//sets previous for the node
void LinearNode::setPrevious(LinearNode* node)
{
    previous = node;
}//ends the setPrevious function

//returns element of the node
int LinearNode::getElement()
{
    return element;
}//ends the getelement function

//sets the element of the node
void LinearNode::setElement(int el)
{
    element = el;
}//ends the setElement function

#endif

LinkedList.h:

#ifndef LINKEDLIST_H
#define LINKEDLIST_H
#include<iostream>
#include"LinearNode.h"

using namespace std;

class LinkedList
{
    public:
        LinkedList();
        void add(int element);
        int removie (int element);

    private:
        int count;
        LinearNode *contents;
};//ends the class linked list

#endif

LinkedList.cpp:

#ifndef LINKEDLIST_CPP
#define LINKEDLIST_CPP

#include<iostream>
#include"LinearNode.h"
#include"LinkedList.h"

using namespace std;

//linkedlist constructor for an empty linked list
LinkedList::LinkedList()
{
    count = 0;
    contents = NULL;
}//ends the constructor

//adds an element to the front of the linked list
void LinkedList::add(int element)
{
    int found = 0, current = 0;

    while( (found == 0) && (current !=count) )
    {
        if (contents.getElement() == element)
            found = 1;
        else    
        {
            contents = contents.getNext();
            current++;
        }//ends the else statement
    }//ends the while loop

    if (found == 0)
    {
        LinearNode node = new LinearNode(element);
        node.setNext(contents);
        contents.setPrevious(node);
        count++;
    }//ends the found == 0 if statment
}//ends the add function

//this function removes one element from the linked list.
int LinearNode::remove(int element)
{
    int found = 0;

    if (count == 0)
        cout << "The list is empty" << endl;
    else 
    {
        if (contents.getElement() == element)
        {
            result = contents.getElement();
            contents = contents.getNext();
        }//ends the contents.getElement() == element
        else 
        {
            previous = contents;
            current = contents.getNext();
            for (int index = 0; ( (index < count) && (found == 0) )index++)
                if (current.getElement() = element)
                    found = 1;
                else
                {
                    previous = current;
                    current = current.getNext();
                }//ends the else statement 

            if (found == 0)
                cout << "The element is not in the list" << endl;
            else
            {
                result = current.getElement();
                previous.setNext(current.getNext());
            }//ends else statement  

        }//ends the else stamtement

        count--;
    }//ends the else statement of count == 0
    return result;
}//ends the remove function

#endif

test.cpp:

#include<iostream>
#include"LinearNode.h"
#include"LinkedList.h"

using namespace std;

int main()
{

    LinearNode node1, node2, node3, move;
    LinkedList list;    

    node1.setElement(1);
    node2.setElement(2);
    node3.setElement(3);
}

First of all - don't put ".h" files as a parameter to the compiler. They're already included in your ".cpp" files. What's the missing references? What is the linker error you're getting? — littleadv, Mar 13 '11 at 22:58
I had a similar issue with getting the `undefined reference to` error, turns out I forgot to include the `.cpp` with code in the `.h`. — Avamander, Aug 30 '17 at 22:30
@Avamander That's the wrong way. You include your header files (i.e., `.h`) in your source files (i.e., `.cpp`), not the other way around. — stackprotector, Jun 10 '20 at 12:15

score 26 · Accepted Answer · answered Mar 13 '11 at 23:17

Usually headers guards are for header files (i.e., .h ) not for source files ( i.e., .cpp ).
Include the necessary standard headers and namespaces in source files.

LinearNode.h:

#ifndef LINEARNODE_H
#define LINEARNODE_H

class LinearNode
{
    // .....
};

#endif

LinearNode.cpp:

#include "LinearNode.h"
#include <iostream>
using namespace std;
// And now the definitions

LinkedList.h:

#ifndef LINKEDLIST_H
#define LINKEDLIST_H

class LinearNode; // Forward Declaration
class LinkedList
{
    // ...
};

#endif

LinkedList.cpp

#include "LinearNode.h"
#include "LinkedList.h"
#include <iostream>
using namespace std;

// Definitions

test.cpp is source file is fine. Note that header files are never compiled. Assuming all the files are in a single folder -

g++ LinearNode.cpp LinkedList.cpp test.cpp -o exe.out

now the include guards can be replaced by just `#pragma once` at the start of the file — Jean-François Fabre, Oct 10 '19 at 19:33

score 9 · Answer 2 · answered May 17 '20 at 22:34

9

I was getting this error because my cpp files was not added in the CMakeLists.txt file

answered May 17 '20 at 22:34

Deepak Sharma

582
8
10

Happened to me too – IronMan Jul 19 '22 at 03:05

score 5 · Answer 3 · answered Mar 13 '11 at 23:08

5

g++ test.cpp LinearNode.cpp LinkedList.cpp -o test

answered Mar 13 '11 at 23:08

karlphillip

92,053
36
243
426

1

This will solve linker dependencies that are generating `Undefined Reference to` errors. But this will only work if all the files are in the same directory. – karlphillip Mar 13 '11 at 23:09

score 3 · Answer 4 · answered Nov 19 '20 at 08:25

3

I had this issue when I forgot to add the new .h/.c file I created to the meson recipe so this is just a friendly reminder.

answered Nov 19 '20 at 08:25

Shauli

61
2

score 2 · Answer 5 · answered Aug 23 '17 at 18:34

Another way to get this error is by accidentally writing the definition of something in an anonymous namespace:

foo.h:

namespace foo {
    void bar();
}

foo.cc:

namespace foo {
    namespace {  // wrong
        void bar() { cout << "hello"; };
    }
}

other.cc file:

#include "foo.h"

void baz() {
    foo::bar();
}

score -3 · Answer 6 · answered Jan 06 '20 at 06:31

-3

Try to remove the constructor and destructors, it's working for me....

answered Jan 06 '20 at 06:31

MUKESH KUMAR BADIGINENI

1
2

Undefined Reference to

6 Answers6

Linked

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