Passing an argument to a slot

Maybe you can subclass QAction with an m_increase member variable.
Connect the triggered() signal to a slot on your new QAction subclass and emit a new signal (e.g. triggered(int number)) with the correct parameter.
e.g.

class MyAction:public QAction  
{  
public:  
    MyAction(int increase, ...)  
        :QAction(...), m_increase(increase)
    {  
        connect(this, SIGNAL(triggered()), this, SLOT(onTriggered()));  
    }  
protected Q_SLOTS:  
    void onTriggered()  
    {  
        emit triggered(m_increase);  
    }  

Q_SIGNALS:
    void triggered(int increase);   

private:  
    int m_increase;  
};

With Qt 5 and a C++11 compiler, the idiomatic way to do such things is to give a functor to connect:

connect(action1,  &QAction::triggered, this, [this]{ onStepIncreased(1); });
connect(action5,  &QAction::triggered, this, [this]{ onStepIncreased(5); });
connect(action10, &QAction::triggered, this, [this]{ onStepIncreased(10); });
connect(action25, &QAction::triggered, this, [this]{ onStepIncreased(25); });
connect(action50, &QAction::triggered, this, [this]{ onStepIncreased(50); });

The third argument to connect is nominally optional. It is used to set up the thread context in which the functor will execute. It is always necessary when the functor uses a QObject instance. If the functor uses multiple QObject instances, they should have some common parent that manages their lifetime and the functor should refer to that parent, or it should be ensured that the objects will outlive the functor.

On Windows, this works in MSVC2012 & newer.

The QObject::sender() function returns a pointer to the object that has signaled to the slot. You could use this to find out which action was triggered

Use QSignalMapper. Like this:

QSignalMapper* signalMapper = new QSignalMapper (this) ;
connect (action1, SIGNAL(triggered()), signalMapper, SLOT(map())) ;
connect (action5, SIGNAL(triggered()), signalMapper, SLOT(map())) ;
connect (action10, SIGNAL(triggered()), signalMapper, SLOT(map())) ;
connect (action25, SIGNAL(triggered()), signalMapper, SLOT(map())) ;
connect (action50, SIGNAL(triggered()), signalMapper, SLOT(map())) ;

signalMapper -> setMapping (action1, 1) ;
signalMapper -> setMapping (action5, 5) ;
signalMapper -> setMapping (action10, 10) ;
signalMapper -> setMapping (action25, 25) ;
signalMapper -> setMapping (action50, 50) ;

connect (signalMapper, SIGNAL(mapped(int)), this, SLOT(onStepIncreased(int))) ;