Template Type is undefined

Question

I'm learning C++ and now I'm working with Template.

I'm trying to implement a Linked List:

ListElement.hpp

#ifndef LIST_ELEMENT_HPP_
#define LIST_ELEMENT_HPP_

template <class Type> class SingleLinkedList;

template <class Type>
class ListElement
{
public:
    ListElement(const Type element);
    ~ListElement(void);
public:
    Type val;
    ListElement* next;
};

#endif

ListElement.cpp:

#include "ListElement.hpp"

ListElement<Type>::ListElement(const Type element)
{
     *next = NULL;
     val = element;
}


ListElement<Type>::~ListElement(void)
{
}

I'm getting an Error on ListElement.cpp releated to Type: Type is undefined.

I have found a lot of examples about how to implement a Linked List but none using a separated hpp and cpp.

Do you know how can I fix this error?

The final code (class and implementation after preprocessing) all has to appear in the same file if you're not using specific instantiations in the implementation. — chris, May 27 '13 at 12:12
By not using separate source and header files? It doesn't really work when you are using templates, because the user of your template class needs the _whole_ class definition, and that includes the functions as well. — Some programmer dude, May 27 '13 at 12:13
Also, your initialization of `next` in the constructor doesn't do what you expect it to! It assigns `NULL` to a _random_ location. You want `next = nullptr;` (or `next = 0;`) which sets the _pointer_ to `NULL`. — Some programmer dude, May 27 '13 at 12:14
@JoachimPileborg, Technically, you really want `: val(element), next(nullptr)` — chris, May 27 '13 at 12:16
@JoachimPileborg What amount of things you can learn here! Thanks. — VansFannel, May 27 '13 at 12:17

score 2 · Accepted Answer · edited May 23 '17 at 10:25

2

First problem:

You need to fix the way you are defining the member functions of your class template:

template<typename Type> // <== ADD THIS!
ListElement<Type>::ListElement(const Type& element)
//                                       ^
//                                       And perhaps also this?
//                                       (don't forget to modify the
//                                       corresponding declaration if 
//                                       you change it)
{
     *next = NULL;
     val = element;
}

Second problem:

You should move those definitions to the same header file that contains the definition of the class template, or the linker will complain about undefined references. For more information, see this Q&A on StackOverflow.

Third problem:

In your constructor, you are currently causing undefined behavior by dereferencing an uninitialized pointer. You shouldn't be doing:

*next = NULL;
^^^^^^^^^^^^^
Undefined Behavior! next is uninitialized and you are dereferencing it!

But rather:

next = NULL;

Or even better (using constructor initialization lists and C++11's nullptr):

template<typename Type>
ListElement<Type>::ListElement(const Type& element) :
    val(element),
    next(nullptr)
{
}

edited May 23 '17 at 10:25

Community

1
1

answered May 27 '13 at 12:13

Andy Prowl

124,023
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I have another question: If you set `next(nullptr)`, why do you set `*next = NULL;`? – VansFannel May 27 '13 at 12:20
@VansFannel: Sorry, that was the result of a bad copy-paste. You don't need to in fact, I edited the answer ;) – Andy Prowl May 27 '13 at 12:21
@AndyProwl, It could if `Type` is not default-constructible, or if that's expensive :) – chris May 27 '13 at 12:24
@chris: But the order of initialization is the order in which the members appear in the class definition anyway ;) – Andy Prowl May 27 '13 at 12:25
@chris: Oh, forget, I think I misunderstood your point - I see what you mean :) – Andy Prowl May 27 '13 at 12:26
I don't understand why I have to add `&` here: `ListElement::ListElement(const Type element)`. Any advice? – VansFannel May 27 '13 at 17:31
@VansFannel: You don't have to, but it makes sense. Passing by reference would save an unnecessary copy – Andy Prowl May 27 '13 at 17:41

ForEveR · Answer 2 · 2013-05-27T12:21:37.927

1

Firstly - in general you cannot split declaration and implementation of template class in different files. Secondly - before implementation should be template decl.

template<typename Type>
ListElement<Type>::ListElement(const Type element)
{
     next = NULL;
     val = element;
}

edited May 27 '13 at 12:21

answered May 27 '13 at 12:12

ForEveR

55,233
2
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Ah, I didn't know that I can't split declaration and implementation of template classes. Thanks. – VansFannel May 27 '13 at 12:13

score 0 · Answer 3 · answered May 27 '13 at 12:13

At first try to add

template<class Type>

before each function in .cpp file

It wouldn't work. (Linker errors) So move all your implementation to .h file.

Then perhaps you should change

ListElement(const Type element);

to

ListElement(const Type &element);

Template Type is undefined

3 Answers3