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What is an undefined reference/unresolved external symbol error and how do I fix it?

Hey i am getting this error in my game. I have a header and .cpp file for Questions, so its obviously in that. but ive been looking through the header and .cpp file and i cannot find the problem. Here is the error:

Error   1   error LNK2019: unresolved external symbol "public: __thiscall Questions::Questions(void)" (??0Questions@@QAE@XZ) referenced in function "void __cdecl `dynamic initializer for 'questions''(void)" (??__Equestions@@YAXXZ)  C:\Users\Conor\Documents\College\DKIT - Year 2 - Repeat\DKIT - Year 2 - Semester 1 - Repeat\Games Programming\Millionaire\Millionaire\Main.obj  Millionaire

Here are the files:

Questions.h
#ifndef QUESTIONS_H
#define QUESTIONS_H
#include <string>
#include <algorithm>
#include <map>
using namespace std;

class Questions
{
public:
    Questions();
    Questions(string question,string correctAnswer, string wrongAnswer1,string wrongAnswer2,string wrongAnswer3);
    void shuffle(string *array, int n);
    string getQuestion();
    string getCorrectAnswer();
    string* getAnswers();
    bool checkAnswer(string answer);
    void questionStore();
    void addQuestion(int level, Questions *question);
    Questions* printQuestion(int level);


private:
    string question;
    string correctAnswer;
    string* pAnswers;
    multimap<int,Questions*> map;

};

#endif

Questions.cpp
#include "Questions.h"
using namespace std;

Questions::Questions(string question,string correctAnswer, string wrongAnswer1,string wrongAnswer2,string wrongAnswer3)
{
    this->question = question;
    this->pAnswers = new string[4];
    this->pAnswers[0]=wrongAnswer1;
    this->pAnswers[1]=wrongAnswer2;
    this->pAnswers[2]=wrongAnswer3;
    this->pAnswers[3] =correctAnswer;
    this->shuffle(this->pAnswers,4);
    this->correctAnswer = correctAnswer;
}

void Questions::shuffle(string *array, int n)
{
    random_shuffle(&this->pAnswers[0],&this->pAnswers[4]);
}

string Questions::getQuestion()
{
    return this->question;
}

string Questions::getCorrectAnswer()
{
    return this->correctAnswer;
}

string* Questions::getAnswers()
{
    return this->pAnswers;
}

bool Questions::checkAnswer(string answer)
{
    if(this->correctAnswer.compare(answer)==0)
    {
        return true;
    }
    return false;
}

void Questions::questionStore()
{
    Questions *q1 = new Questions("Whats the oldest known city in the world?", "Sparta" , "Tripoli" , "Rome", "Demascus");
    Questions *q2 = new Questions("What sport in the olympics are beards dissallowed?", "Judo", "Table Tennis" , "Volleyball", "Boxing");
    Questions *q3 = new Questions("What does an entomologist study?", "People" , "Rocks" , "Plants", "Insects");
    Questions *q4 = new Questions("Where would a cowboy wear his chaps?", "Hat" , "Feet" , "Arms", "Legs");
    Questions *q5 = new Questions("which of these zodiac signs is represented as an animal that does not grow horns?", "Aries" , "Tauris" , "Capricorn", "Aquarius");
    Questions *q6 = new Questions("Former Prime Minister Tony Blair was born in which country?", "Northern Ireland" , "Wales" , "England", "Scotland");
    Questions *q7 = new Questions("Duffle coats are named after a town in which country?", "Austria" , "Holland" , "Germany", "Belgium");
    Questions *q8 = new Questions("The young of which creature is known as a squab?", "Horse" , "Squid" , "Octopus", "Pigeon");
    Questions *q9 = new Questions("The main character in the 2000 movie ""Gladiator"" fights what animal in the arena?", "Panther" , "Leopard" , "Lion", "Tiger");

    addQuestion(1,q1);
    addQuestion(1,q2);
    addQuestion(1,q3);
    addQuestion(2,q4);
    addQuestion(2,q5);
    addQuestion(2,q6);
    addQuestion(3,q7);
    addQuestion(3,q8);
    addQuestion(3,q9);
}

void Questions::addQuestion(int level, Questions *question)
{
    map.insert(pair<int,Questions*>(level,question));
}


Questions* Questions::printQuestion(int level)
{
    multimap<int, Questions*>::iterator it;
    pair<multimap<int, Questions*>::iterator,multimap<int, Questions*>::iterator> ret;

    ret = map.equal_range(level);
    if(ret.first != ret.second)
    {
    size_t sz = distance(ret.first, ret.second);
    size_t idx = rand() % sz;
    advance(ret.first, idx);
    it =ret.first;
    return (*it).second;
    }
    else
    {
        return NULL;
    }
}

Answer 1

You’re declaring Questions::Questions() but didn’t define it.

Does it need a default constructor? Are you using the default constructor? Does it need any specific implementation?

In C++11, you can write

Question() = default;

in the class definition. In C++03, you can provide an empty implementation:

Question() { }