Cannot build project from QT wiki: Unresolved external symbol

Question

Error when attempting to build code I copy-pasted directly from the QT for beginners wiki page.

error

main.obj:-1: error: LNK2019: unresolved external symbol "public: __cdecl Window::Window(class QWidget *)" (??0Window@@QEAA@PEAVQWidget@@@Z) referenced in function main
debug\testempty.exe:-1: error: LNK1120: 1 unresolved externals

testempty.pro

TEMPLATE = app
TARGET = testempty
QT = core gui
greaterThan(QT_MAJOR_VERSION, 4): QT += widgets
SOURCES += \
    main.cpp \
    window.cpp
HEADERS += \
    window.h

window.h

#ifndef WINDOW_H
#define WINDOW_H

#include <QWidget>

class QPushButton;
class Window : public QWidget
{
public:
    explicit Window(QWidget *parent = nullptr);
private:
    QPushButton *m_button;
};

#endif // WINDOW_H

window.cpp

#include "window.h"
#include <QPushButton>

Window::Window(QWidget *parent) : QWidget(parent)
{
    setFixedSize(100, 50);

    m_button = new QPushButton("Hello World", this);
    m_button->setGeometry(10, 10, 80, 30);
}

main.cpp

#include <QApplication>
#include "window.h"

int main(int argc, char **argv)
{
    QApplication app(argc, argv);
    Window window;
    window.show();

    return app.exec();
}

As for things I have already tried, I was instructed to build->clean all and try building again, but that made no difference.

First, do not make screenshot of errors, copy/paste them. And second, which version of `Qt` are you using? — KernelPanic, Feb 24 '18 at 21:51
Is Qt installed correctly? which version? can you run other on it? — Mohammad Kanan, Feb 24 '18 at 22:25
QT is version 5.10.0 I think. I have successfully built other projects on it. — Corvus, Feb 24 '18 at 22:39
Possible duplicate of [File not found: mainwindow.obj](https://stackoverflow.com/questions/15864177/file-not-found-mainwindow-obj) — Mohammad Kanan, Feb 24 '18 at 22:45

score 1 · Accepted Answer · answered Feb 24 '18 at 22:51

Solution From a similar thread.

Right click on project > Clean
Right click on project > Run qmake
Right click on project > Build
Run

Why it works

The reason this worked is because Run qmake updates your Makefile. For some reason qt is not automatically updating paths when you make changes to your project such as adding or removing files. Running qmake forces qt to update the paths for your project which enables it to find the mainwindow.obj file. You probably could just run qmake and your problem would be solved.

Cannot build project from QT wiki: Unresolved external symbol

1 Answers1