Unresolved external symbol in singleton - c++

Question

I've got code like this:
#pragma once
#include "MV/stateSystem/StateSystem.hpp"
#include "MV/config/Config.hpp"
namespace mv
{
class Initializator
{
/* ===Objects=== */
public:
protected:
private:
static Initializator *instance;
/* ===Methods=== */
public:
//Inits the program
void init();
static Initializator& getInstance();
static void createInstance();
protected:
private:
Initializator();
Initializator(Initializator const& copy) = delete; // Not Implemented
Initializator& operator=(Initializator const& copy) = delete; // Not Implemented
};
}
and .cpp
#include "Initializator.hpp"
namespace mv
{
Initializator* Initializator::instance;
void Initializator::init()
{
StateSystem::readStatesFromFile("data/states/states.txt");
}
Initializator & Initializator::getInstance()
{
if (instance == 0)
Logger::Log(constants::error::singleton::SINGLETON_NOT_INITED, Logger::STREAM::BOTH, Logger::TYPE::ERROR);
return *instance;
}
void Initializator::createInstance()
{
if (instance == 0)
instance = new Initializator();
}
}
but my compiler has got problem:
Severity Code Description Project File Line Suppression State
Error LNK2019 unresolved external symbol "private: __thiscall mv::Initializator::Initializator(void)" (??0Initializator#mv##AAE#XZ) referenced in function "public: static void __cdecl mv::Initializator::createInstance(void)" (?createInstance#Initializator#mv##SAXXZ)
I can't understand it because i've got other class where code is really similar and compiler hasn't got problem with it. In the past, when i've got this problem, i had to declare static members in .cpp file (for example: Initializator* Initializator::instance; ) but now it doesn't help me.

Answer 1

You are declaring the ctor, but not defining it
You can define it within class declaration with
Initializator() = default;
Or
Initializator(){};
Or also in the cpp with
Initializator::Initializator(){}
As the error message is saying, Initializator::createInstance() is calling the ctor, which has to be defined even if it is private

Answer 2

Declaring your default ctor as private prohibits the creation of an implicilty defined default ctor. Thus, you get the unresolved external symbol linker error.
To get it working, provide a valid ctor definition in your code.
Either in the .cpp file, or in the .hpp file ( by provding an in-class definition or writing ctor() = default forcing the automatic generation of a default constructor by the compiler ).