具有返回值的信号/插槽不起作用

Question

I made a signal slot in Qt and the program runs without error or warnings about the connect i made. The problem is that when i want to use the signal slot, it always returns NULL.

Main.cpp

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


  Game* game = new Game;

  Scrabble mainWindow;
  mainWindow.show();

  QObject::connect(&mainWindow,SIGNAL(getTurn()),game,SLOT(giveTurn()));

  return a.exec();
}

Game.h

class Game: public QObject
{
    Q_OBJECT
public:
    Game(QObject *parent = 0);
    ~Game();
private:
    int m_turn;
public slots:
    int giveTurn();

};

Game.cpp

Game::Game(QObject *parent)
    :QObject(parent)
{
    m_turn = 1;
}
Game::~Game()
{

}

int Game::giveTurn()
{
    return m_turn;
}

Scrabble.h

class Scrabble : public QMainWindow
{
    Q_OBJECT

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

private:
    Ui::Scrabble *ui;
signals:
    int getTurn();

};

when i use int turn = emit getTurn(); in Scrabble.cpp, turn will become 0 and not 1. Does anyone know what i'm doing wrong?

Answer 1

You're using signals and slots incorrectly. Signals cannot return value. See the Signals & Slots documentation page:

Signals are automatically generated by the moc and must not be implemented in the .cpp file. They can never have return types (ie use void).

Returning values from signals is not required when you use Qt features correctly. Maybe you should create another question and describe what you want to do and why you need such connection. You're definitely doing something wrong.

Answer 2

Signals/slots cant return any value. Possible solution:

Scrabble:

signal: void requestTurn();
public slot: receiveTurn(int);

Game:

public slot: onrequestTurn();
signal: sendTurn(int);

Answer 3

emit "keyword" is highly undocumented right now, but from Qt's source, it is only empty define, so your code

int turn = emit getTurn(); 

will be expanded to:

int turn = getTurn(); 

However, this is not covered in oficial documentation and it might change any time - so - don't use it!

Now, please note that turn variable is not getting value from slot , but from signal . There is nothing about passing return value from slot to signal - and it doesn't make sense (well, it may make sense in your sample, but what if I connect multiply slots to a single signal - what slot will return value, what if slots are executed asynchronously - should we wait for return value, etc.).

You can use regular function call (just call giveTurn() function: int turn = giveTurn() ).