error undefined reference to .. c++?

Question

so I have a class x that is being used by class y and it's also going to be used by other classes.

.h for class x

#pragma once
#include <string>
#ifndef X_H
#define X_H
class x
{
public:
    x();
    const std::string & getName() const;
    int getQuantity();

private:
    std::string name;
    int quantity;
};
#endif

.cpp for x

#include <string>
#include "x.h"
using namespace std;



x::x()
: name(),quantity(0)
{
}
const string & x::getName() const
{
return  name;
}
const string & x::getQuantity() const
{
return quantity;
}

this is the .h for class y

#pragma once
#include <string>
#include <array>
#include "x.h"

class y
{
public:
    static const size_t number = 20;

    y();
    float getTotal();

private:
    std::array<X*, number> arrayList;
};

and this is the .cpp for class y

#include "y.h"
#include "x.h"
#include <array>
#include <string>
#include <iostream>

using namespace std;

y::y()
: arrayList()
{
}

float y::getTotal()
{
    float total=0.0;
    for(int i=0; i< number; i++)
    {
        if(arrayList[i] != nullptr)
        {
            total += arrayList[i]->getQuantity();
        }
    }
}

methods in the y class uses an array of pointers to method y and I'm trying to use some methods from class x using the array members but I get an error saying:

undefined reference to `x::x(...)

I think it has something to do with the preprocessors or the headers.

Answer 1

This means that you forgot to define the default constructor X::X() or some other constructor with parameters ( what does x::x(...) mean?) of class X. You only declared it in the class definition. Or the other reason is that the module with the constructor definition was not included in the project build.

Answer 2

In class x you have explicitly declared the default constructor x() but you have not defined it. If you want to use the default constructor, remove its definition or define it with x::x():name(std::string()),quantity(0){}