Template Inheritance issues - c++

Question

I have an Abstractqueue and I want to inherit it's members to another child class. When I try to create the child object in my main file I keep getting an undefined reference to `AbstractQueue::AbstractQueue()'
enter code here
#ifndef ABSTRACT_QUEUE_H
#define ABSTRACT_QUEUE_H
#include <iostream>
template <class Type>
class AbstractQueue
{
protected:
Type *items;
int front;
int back;
int capacity;
int count;
private:
// data goes here
public:
AbstractQueue(int s);
AbstractQueue(void);
~AbstractQueue(void);
bool empty();
int size();
Type frontele(); //throw(exception) {}
Type dequeue(); //throw(exception) {}
void enqueue ( Type e );
};
template <class Type>
AbstractQueue<Type>::AbstractQueue(int s){
items = new Type[s];
front = 0;
back = 0;
capacity = s;
count = 0;
std::cout << "made the abstract queue!" << std::endl;
}
template <class Type>
AbstractQueue<Type>::~AbstractQueue() {
delete[] items;
std::cout << "destructor called" << std::endl;
}
#endif
IncrementalQueue.h
#ifndef _INCREMENTALQUEUE_H
#define _INCREMENTALQUEUE_H
#include "Queue.h"
//#define SIZE = 10
#include <iostream>
template <class Type>
class IncrementalQueue : public AbstractQueue<Type>{
public:
//AbstractQueue(void);
//~AbstractQueue(void);
IncrementalQueue(int s);
bool empty();
int size();
Type frontele(); //throw(exception) {}
Type dequeue(); //throw(exception) {}
void enqueue ( Type e );
//~IncrementalQueue(void);
//AbstractQueue(void);
//AbstractQueue(int size);
//bool empty(void) ;
/*if (count == 0) {
return true;
}
else {
return false;
}*/
//int size(void);
//Type front throw(exception) {}
//Type dequeue(); //throw(exception) {}
//void enqueue ( Type e ) ;
//IncrementalQueue(int size);
};
template <class Type>
IncrementalQueue<Type>::IncrementalQueue(int s){
this->items = new Type[50];
this->front = 0;
this->back = 0;
this->capacity = 50;
this->count = 0;
std::cout << "made the incremental queue!" << std::endl;
}
#endif
main.cpp
#include "Queue.h"
#include "IncrementalQueue.h"
int main(){
IncrementalQueue<int> incqueue(50);
return 0;
}
I'm a little rusty with templates so I've been struggling at it for a couple of hours. does anybody have any clues on where my code can be failing?

Answer 1

IncrementalQueue::IncrementalQueue() constructor is calling the AbstractQueue::AbstractQueue() constructor and you have not defined it.
To make it compile you can just say AbstractQueue() = default; to have it generated by the compiler, if you are using C++11 or later, but that will not necessarily be correct (see the comments).

Answer 2

As it always happens with templates, you either need to put AbstractQueue::AbstractQueue() implementation (function body) to its header, or, if you know in advance all its possible template parameters, explicitly instantiate this function in cpp-file:
template class AbstractQueue<NeededType>;
In your case, it's impossible to know all possible template args in advance, so you need to put your function body to the header. That's the price of separate unit compilation.