List not a class or implementation - c++

Question

#ifndef __LISTA_H__
#define __LISTA_H__
template<typename VALUE_TYPE>
class List {
public:
virtual unsigned int size() const = 0;
};
#endif
#include <iostream>
using namespace std;
template<typename T>
unsigned int List :: size() const {
return 0;
}
int main() {
List<int> list;
return 0;
}
I am receiving a compile error stating that the list is not a class defined when its clearly defined above:
prog.cpp:16:14: error: 'List' is not a class, namespace, or
enumeration unsigned int List :: size() const {
^ prog.cpp:5:7: note: 'List' declared here class List {
^ 1 error generated.

Answer 1

List is indeed not a class. It is a class template.
List<T>, for some T, is a class.
Write this:
template<typename T>
unsigned int List<T>::size() const {
// ^^^
return 0;
}
Newer GCC gives better diagnostics here, literally telling you what to do:
main.cpp:16:14: error: 'template<class VALUE_TYPE> class List' used without template arguments
16 | unsigned int List :: size() const {
Once that's fixed, you'll need to create something to derive from List<T>, because that's an abstract class, and you can't instantiate (in main) one of them.
Again, GCC is very helpful here:
main.cpp: In function 'int main()':
main.cpp:21:15: error: cannot declare variable 'list' to be of abstract type 'List<int>'
21 | List<int> list;
| ^~~~
main.cpp:5:7: note: because the following virtual functions are pure within 'List<int>':
5 | class List {
| ^~~~
main.cpp:7:26: note: 'unsigned int List<VALUE_TYPE>::size() const [with VALUE_TYPE = int]'
7 | virtual unsigned int size() const = 0;

Answer 2

virtual unsigned int size() const = 0;
You have created an abstract class.
An abstract class is a class that has to be inherited (Or else simply not be used), used mainly for polymorphic inheritance systems. You cannot create an instance of an abstract class because it beats the whole point of its abstractness.
List<int> list;
Here, you are creating an instance of the declared abstract class.
The class is abstract due the fact that one of its methods, size() const = 0; is abstract (because of the = 0).
Furthermore, as the comments suggest, you should recheck your understanding of how classes and templates work, and my extra suggestion is to check what an abstract class is and make sure you understand when to use such classes.
There's another issue - if you want to declare a function as a template you should add template above the function declaration inside the class declaration too.

Answer 3

template<typename VALUE_TYPE>
class List
{
public:
virtual unsigned int size() const
{
return 0;
}
};
#endif
#include <iostream>
using namespace std;
int main() {
List<int> list;
return 0;
}
the problem is you leave size() as an abstract class for ( future inherited class ).
You have to make another class inherited from List and implement size()
Not to implement at List class