I think you need to remove the static
keyword from your static function definition:
Wrong:
static DatabaseEngine& DatabaseEngine::instance()
Correct:
DatabaseEngine& DatabaseEngine::instance()
سؤال
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
المحلول
I think you need to remove the static
keyword from your static function definition:
Wrong:
static DatabaseEngine& DatabaseEngine::instance()
Correct:
DatabaseEngine& DatabaseEngine::instance()
نصائح أخرى
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
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;
}
};