LNK2019: unresolved error in singletone

Question

i need help to figure out what wrong about that code:

class DatabaseEngine
{
protected:
    DatabaseEngine();
    static DatabaseEngine* m_DatabaseEngine;
public:
    static DatabaseEngine& instance();
    void do_something();
};

cpp:

#include "databaseengine.h"

DatabaseEngine* DatabaseEngine::m_DatabaseEngine=nullptr;

DatabaseEngine::DatabaseEngine()
{
}


static DatabaseEngine& DatabaseEngine:: instance()
{
    if(m_DatabaseEngine==nullptr)
{
    m_DatabaseEngine=new DatabaseEngine;`enter code here`
}
return *m_DatabaseEngine;
}

void DatabaseEngine::do_something()
{

}

userwindow.cpp:

#include "databaseengine.h"
UsersWindow::UsersWindow(QWidget *parent) :
    QWidget(parent),
    ui(new Ui::UsersWindow)
{
    ui->setupUi(this);
    DatabaseEngine::instance().do_something();
}

UsersWindow::~UsersWindow()
{
    delete ui;
}

userswindow.obj:-1: error: LNK2019: unresolved external symbol "public: static class DatabaseEngine & __cdecl DatabaseEngine::instance(void)" (?instance@DatabaseEngine@@SAAAV1@XZ) referenced in function "public: __thiscall UsersWindow::UsersWindow(class QWidget *)" (??0UsersWindow@@QAE@PAVQWidget@@@Z)

userswindow.obj:-1: error: LNK2019: unresolved external symbol "public: void __thiscall DatabaseEngine::do_something(void)" (?do_something@DatabaseEngine@@QAEXXZ) referenced in function "public: __thiscall UsersWindow::UsersWindow(class QWidget *)" (??0UsersWindow@@QAE@PAVQWidget@@@Z)

thanks

Answer 1

I think you need to remove the static keyword from your static function definition:

Wrong:

static DatabaseEngine& DatabaseEngine::instance()

Correct:

DatabaseEngine& DatabaseEngine::instance()

Answer 2

declaration:

static DatabaseEngine& DatabaseEngine::instance();
   ^
only in declaration

definition:

DatabaseEngine& DatabaseEngine:: instance() {
   // code here
}

also make sure DatabaseEngine.cpp file is included in your project and is being compiled

Answer 3

You can use static variable in the static method instance to hold the unique instance and return the pointer. This is also a smart advice from Effective C++ I think. The example is untested but it should work

class DatabaseEngine
{
public:
    static DatabaseEngine& instance(){
        static DatabaseEngine db;
        return db;
    }
};