Invalid use of incomplete type and forward declaration

Question

I was trying to resolve this issue but there must be some misunderstanding in how I undestand forward declaration. I am getting following error: ``` src/algorithm.cpp: In constructor ‘Algorithm::Algorithm(MainWindow*)’: src/algorithm.cpp:22:20: error: invalid use of incomplete type ‘struct Ui::MainWindow’ src/mainwindow.h:23:10: error: forward declaration of ‘struct Ui::MainWindow’ ``` I have these files (i ommited some lines and files and pasted only relevant code): algorithm.cpp ``` #include "algorithm.h" #include "mainwindow.h" Algorithm::Algorithm(MainWindow *mainWindow) { this->mainWindow = mainWindow; QAction *action = new QAction(this); action->setObjectName(QStringLiteral("action")); action->setText(this->getName()); mainWindow->m_ui->menuAlgorithms->addAction(action); mainWindow->connect(action, SIGNAL(triggered()), this, SLOT(this->start())); } ``` algorithm.h ``` #ifndef ALGORITHM_H #define ALGORITHM_H #include <QObject> #include "graphwidget.h" #include "edge.h" #include "…

Accepted Answer

The problem in your code is that you're trying to use the `m_ui` member of `MainWindow` class in your `Algorithm` class, and the type of `m_ui` is `Ui::MainWindow`, but your `Algorithm` class is not aware of such type. You can solve this by including the `ui_mainwindow.h` in your `algorithm.cpp`.

But this is still poor design. `m_ui` should be a private member of `MainWindow` class. You should not be accessing it from other classes. Instead create functions in your `MainWindow` class that will allow you to do what you want.

For example, create a public function like this:

```
void MainWindow::addCustomAction(QAction *action)
{
    m_ui->menuAlgorithms->addAction(action);
}
```

Then call this function from your `Algorithm` class:

```
Algorithm::Algorithm(MainWindow *mainWindow)
{
   this->mainWindow = mainWindow;

QAction *action = new QAction(this);
   action->setObjectName(QStringLiteral("action"));
   action->setText(this->getName());

mainWindow->addCustomAction(action);

connect(action, SIGNAL(triggered()), this, SLOT(start()));
}
```

Invalid use of incomplete type and forward declaration

Solution

Problem