Creating simple object in Qt 5.7 C++

Question

I have following problem:

I want to create Object from class inside main function. Looks like it is a linker problem. Do you have any ideas, what the reaason for this error message could be?

Its error message is:

main.obj:-1: error: LNK2019: Verweis auf nicht aufgelöstes externes Symbol ""public: __thiscall Test::Test(class QString)" (??0Test@@QAE@VQString@@@Z)" in Funktion "_main".

main.obj:-1: error: LNK2019: Verweis auf nicht aufgelöstes externes Symbol ""public: __thiscall Test::~Test(void)" (??1Test@@QAE@XZ)" in Funktion "_main".

debug\Wth.exe:-1: error: LNK1120: 2 nicht aufgelöste Externe

I have very simple Class Test:

test.h

#ifndef TEST_H
#define TEST_H
#include <QtCore/QObject>

class Test
{
public:
    Test(QString name);
   ~Test();

private:
    QString m_name;
};

#endif // TEST_H

then the .cpp file looks like this:

test.cpp

#include "test.h"

Test::Test(QString st) : m_name(st){}
Test::~Test(){}

very basic, in main function I have:

main.cpp

#include <QCoreApplication>
#include "test.h"


int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    Test t("lam");
    return a.exec();
}

sry, i already posted error messages, it says: Reference to unresolved external symbol — Dušan Tichý
Probably you should rebuild your code. Please run qmake and clean the project before rebuild it, — mohabouje

mohabouje mohabouje · Accepted Answer · 2016-09-16T12:33:38

Propably you are looking an example about a creation of an QObject class.

Let's extend your code:

#ifndef TEST_H
#define TEST_H
#include <QtCore/QObject>

class Test : public QObject
{
  // Allways add the Q_OBJECT macro 
  Q_OBJECT
public:
    // Use a null default parent
    explicit Test(const QString& name, QObject* parent = 0);
   ~Test();

private:
    QString m_name;
};

#endif // TEST_H

In your cpp file:

#include "test.h"

Test::Test(const QString& st, QObject* parent) : QObject(parent), m_name(st {}
Test::~Test(){}

No in your main.cpp

#include <QCoreApplication>
#include "test.h"


int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    Test t("lam");
    return a.exec();
}

This should work. If you have link problems, make some steps:

Run qmake (In your project folder use the contextual menu of the qt creator)
Clean your project
Rebuild it again

Creating simple object in Qt 5.7 C++

test.h

test.cpp

main.cpp

3 Answers