Question

I’m trying to create a linked list class in Eclipse but I can’t get it to compile properly.

Here is my .cc file (code snipet)

#include <iostream>
#include "list.h"

using namespace std;

template <class T>
bool List<T>::isEmpty()
{
    return (firstNode == NULL);
}

and here is my list.h file (code snipet)

#ifndef __LIST_H__
#define __LIST_H__

template <typename T>
class List {

public:

    bool isEmpty();

 private:
    struct node {
    node   *following;
    node   *previous;
    T      *contents;
    };

    node   *firstNode;
};

#include "list.cc"

#endif /* __LIST_H__ */

I try “Building All” in eclipse but I get the following error:

make all 
Building file: ../list.cc
Invoking: Cross G++ Compiler
g++ -O0 -g3 -Wall -c -fmessage-length=0 -MMD -MP -MF"list.d" -MT"list.d" -o "list.o"     "../list.cc"
../list.cc:13: error: redefinition of 'bool List<T>::isEmpty()'
../list.cc:13: error: 'bool List<T>::isEmpty()' previously declared here
make: *** [list.o] Error 1

Help please…thanks. I’ll be happy to provide any clarifications needed

EDIT: I was given the .h file so I know that it is correct. I also know that I am supposed to have a .cc file called list.cc (it is included at the end of the .h file)

Answer 1

You need to change the extension of the file with the implementation.

The compiler will process this file for compilation and will process it twice, since you’re including it in the header.

Your file looks like this:

#include <iostream>
#include "list.h"

using namespace std;

template <class T>
bool List<T>::isEmpty()
{
    return (firstNode == NULL);
}

which will become

#include <iostream>
#ifndef __DLIST_H__
#define __DLIST_H__

template <typename T>
class List {

public:

    bool isEmpty();

 private:
    struct node {
    node   *following;
    node   *previous;
    T      *contents;
    };

    node   *firstNode;
};

#include "dlist.cc"

#endif /* __DLIST_H__ */

using namespace std;

template <class T>
bool List<T>::isEmpty()
{
    return (firstNode == NULL);
}

which will in turn become

#include <iostream>
#ifndef __DLIST_H__
#define __DLIST_H__

template <typename T>
class List {

public:

    bool isEmpty();

 private:
    struct node {
    node   *following;
    node   *previous;
    T      *contents;
    };

    node   *firstNode;
};

template <class T>
bool List<T>::isEmpty()
{
    return (firstNode == NULL);
}

#endif /* __DLIST_H__ */

using namespace std;

template <class T>
bool List<T>::isEmpty()
{
    return (firstNode == NULL);
}

So the function isEmpty() is defined twice.

Rename the file to dlist.impl.