I try to write something from a class to a file but it has undefined refrence error c++

I try to write somthing from my class to a file bu it has this error

C:\Users\Lenovo\AppData\Local\Temp\ccaLsCIe.o:main.cpp:(.text+0xe7): undefined reference to `CoronaVaccine::CoronaVaccine(std::__cxx11::basic_string<char, std::char_traits, std::allocator >, std::__cxx11::basic_string<char, std::char_traits, std::allocator >, int, std::__cxx11::basic_string<char, std::char_traits, std::allocator >)’
collect2.exe: error: ld returned 1 exit status

and this is my code

#include <iostream>
#include <fstream>
#include <cstdlib>

using namespace std;

class CoronaVaccine{
    string name;
    string nationalID;
    int dose = 0;
    string nextDate;
    public:
    CoronaVaccine (string="", string="",int=0,string="");
    void setName(string a){
        name = a;
    }
    string getName() const{
        return name;
    }
    void setNatinalID(string b){
        nationalID = b;
    }
    string getNtinalID() const{
        return nationalID;
    }
    void setDoseDate(int c, string d){
        dose = c;
        if (dose == 1){
            nextDate = d;
        }else{
            nextDate = "Done";
        }
        
    }
    int getDose() const{
        return dose;
    }
    string getNextDte() const{
        return nextDate;
    }
};

int main(){
    char cont;
    string nameMain;
    string natinalIdMain;
    int doseMain;
    string nextDateMain;
    CoronaVaccine client;
    ofstream fp("coronaVaccine.txt");
    if (!fp){
        cout << "something goes wrong!";
        exit(0);
    }
    while (1)
    {
        cout << "Name natinalID dose date: \n";
        cin >> nameMain;
        cin >> natinalIdMain;
        cin >> doseMain;
        cin >> nextDateMain;
        client.setName(nameMain);
        client.setNatinalID(natinalIdMain);
        client.setDoseDate(doseMain,nextDateMain);
        cout << "do you want to countinue(y/n): ";
        cin >> cont;
        if (cont == 'n'){
            break;
        }
        fp.write((char *) &client,sizeof(CoronaVaccine));
    }
    cout << "\n==============================\n";
    fp.close();

    return 0;
}

>Solution :

The problem is that you have provided only a declaration for the parameterised constructor CoronaVaccine (string="", string="",int=0,string="");.

You can solve this by providing the corresponding definition as shown below:

//define the constructor. This uses constructor initializer list
CoronaVaccine::CoronaVaccine (string pname, string pnationalID,int pDose,string pnextDate)
            :name(pname), nationalID(pnationalID), dose(pDose), nextDate(pnextDate)
{
    
    
}

So the modified code should look like:

class CoronaVaccine{
    string name;
    string nationalID;
    int dose = 0;
    string nextDate;
    public:
    CoronaVaccine (string="", string="",int=0,string="");//THIS IS A DECLARATION

    //other members here
    
};
//THIS IS THE DEFINITION. This uses constructor initializer list
CoronaVaccine::CoronaVaccine (string pname, string pnationalID,int pDose,string pnextDate)
            :name(pname), nationalID(pnationalID), dose(pDose), nextDate(pnextDate)
{
    
    
}