Why does it say "can't find constructor" in c++? - c++

Question

#include <iostream>
using namespace std;
template <class T>
class ring {
private:
T *m_values;
int m_size;
int m_index;
public:
class iterator;
public:
ring(int size):
m_size(size), m_index(0), m_values(NULL) {
m_values = new T[size];
}
~ring() {
delete [] m_values;
}
int size() const {
return m_size;
}
iterator begin() {
return iterator(0, *this);
}
iterator end() {
return iterator(m_size, *this);
}
void add(T value) {
m_values[m_index] = value;
m_index = (m_index + 1) % m_size;
}
T &get(int pos) {
return m_values[pos];
}
};
template<class T>
class ring<T>::iterator {
private:
int m_pos;
ring &m_ring;
public:
iterator(int m_pos, ring &m_ring): m_pos(m_pos), m_ring(m_ring) {};
iterator &operator++(int) {
m_pos++;
return *this;
}
iterator &operator++() {
m_pos++;
return *this;
}
bool operator!=(const iterator &other) {
return other.m_pos != m_pos;
}
T &operator*() {
return m_ring.get(m_pos);
}
};
Here is the c++ code. I am new to c++ and this codes at CLion gives me "Class iterator doesn't have a constructor iterator(int,ring)" at begin() and end() function. Can anone give me some hint about why this would happen even I have defined it?
BTW: This is from udemy "Learn Advanced C++ Programming" lecture 44.
UPDATE: Some comments find I didn't define and only declare. That's not the reason because I defined it in a separate .cpp file. I also tried to define it inline but still give me the same error message from CLion.

Answer 1

You declare the constructor here
iterator(int m_pos, ring &m_ring);
but there is no definition of this special member function. If you provide one, you're probably fine
iterator(int m_pos, ring &m_ring) : m_pos(m_pos), m_ring(m_ring)
{
/* Maybe some other stuff... */
}

Answer 2

You declared the interator(int, ring) but not defined it. That is the problem.
Declaration vs Definition:
A declaration provides basic attributes of a symbol: its type and its
name. A definition provides all of the details of that symbol--if it's
a function, what it does; if it's a class, what fields and methods it
has; if it's a variable, where that variable is stored. Often, the
compiler only needs to have a declaration for something in order to
compile a file into an object file, expecting that the linker can find
the definition from another file. If no source file ever defines a
symbol, but it is declared, you will get errors at link time
complaining about undefined symbols.
src: https://www.cprogramming.com/declare_vs_define.html