Question

I am trying to use an external library in my Qt Creator project. I am building it with Visual C++ on Windows.

I added this to my qmake file:

# Include libspotify
INCLUDEPATH += C:\\libspotify\\include
LIBS     += -LC:\\libspotify\\lib -llibspotify

Then I went to use some typedef’d structs from the library:

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <libspotify/api.h>

#include <QMainWindow>

namespace Ui {
class MainWindow;
}

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();

private:
    Ui::MainWindow *ui;

    sp_session_config spConfig;
    sp_session spSession;
};

#endif // MAINWINDOW_H

Now in the IDE editor, the syntax highlight marks sp_session_config and sp_session in purple indicating the editor can find those typedefs fine (if it doesn’t find a definition it leaves the text black).

But when I build I get this:

mainwindow.h:32: error: C2079: 'MainWindow::spSession' uses undefined struct 'sp_session'

Now I know the compiler is finding the api.h file, because if I change it to a bogus filename it spits out a file not found error.

What am I doing wrong?

EDIT:
The header file defines the struct like this:

extern "C" {
typedef struct sp_session sp_session; ///< Representation of a session
}

Answer 1

You have a declaration for a typedef to struct sp_session but struct sp_session is an incomplete type. In order for the QMainWindow class to have a sp_session member, the type must be complete (ie., you need a declaration that also defines what members struct sp_session has).

If that’s not possible, you might be able to restructure things so that class QMainWindow has an sp_struct* or sp_struct& as a member instead.