为什么在c ++中会显示“找不到构造函数”？

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