What's the difference?

[Atomic_Sheep]

Hi, I'm just wondering what the difference between the Qt documentation example:

signalMapper = new QSignalMapper(this);
     signalMapper->setMapping(taxFileButton, QString("taxfile.txt"));
     signalMapper->setMapping(accountFileButton, QString("accountsfile.txt"));
     signalMapper->setMapping(reportFileButton, QString("reportfile.txt"));
 
     connect(taxFileButton, SIGNAL(clicked()),
         signalMapper, SLOT (map()));
     connect(accountFileButton, SIGNAL(clicked()),
         signalMapper, SLOT (map()));
     connect(reportFileButton, SIGNAL(clicked()),
         signalMapper, SLOT (map()));

signalMapper = new QSignalMapper(this);
     signalMapper->setMapping(taxFileButton, QString("taxfile.txt"));
     signalMapper->setMapping(accountFileButton, QString("accountsfile.txt"));
     signalMapper->setMapping(reportFileButton, QString("reportfile.txt"));

     connect(taxFileButton, SIGNAL(clicked()),
         signalMapper, SLOT (map()));
     connect(accountFileButton, SIGNAL(clicked()),
         signalMapper, SLOT (map()));
     connect(reportFileButton, SIGNAL(clicked()),
         signalMapper, SLOT (map()));

To copy to clipboard, switch view to plain text mode 

connect(signalMapper, SIGNAL(mapped(QString)),
         this, SLOT(readFile(QString)));

connect(signalMapper, SIGNAL(mapped(QString)),
         this, SLOT(readFile(QString)));

To copy to clipboard, switch view to plain text mode 

As opposed to having something like this would be?:

button.h

class Button : public QWidget
{
    Q_OBJECT
public:
    explicit Button(QWidget *parent = 0);
 
    QString m_sName;
 
    Button(QString sName) : m_sName(sName)
    {}
 
signals:
    void ButtonClicked(QString m_sName);
}

class Button : public QWidget
{
    Q_OBJECT
public:
    explicit Button(QWidget *parent = 0);

    QString m_sName;

    Button(QString sName) : m_sName(sName)
    {}

signals:
    void ButtonClicked(QString m_sName);
}

To copy to clipboard, switch view to plain text mode 

otherfile.cpp

{
//somewhere init something like 20 button classes.
 
void class::class_sSlot(QString name)
{
    if(name == "a")
    {
        //do something
    }
    else if(name == "b")
    {
        //do something
    }
//...etc for remaining 18 comparisons
}

{
//somewhere init something like 20 button classes.

void class::class_sSlot(QString name)
{
    if(name == "a")
    {
        //do something
    }
    else if(name == "b")
    {
        //do something
    }
//...etc for remaining 18 comparisons
}

To copy to clipboard, switch view to plain text mode 

Would there be a speed advantage of doing it the way it's presented in the Qt documentation over the second example?

[mvuori]

I would guess that there is no speed difference.

Now, the difference is in maintenance. The QSignalMapper brings the mappings (!) into plain view, in a declarative stype that is easy to modify when needed. The alternative style is a mess of code, logic and symbols. In fact, every time that one sees such a structure with 20 comparisons, the first thought should be: how can I get rid of all this code and just describe these mappins in some neat way; in a table or a list perhaps... and a list is exactly what QSignalMapper makes possible.

[ChrisW67]

The signal mapper has two signal/slot calls to make: button to mapper, lookup, mapper to slot. The other approach has two also: button clicked to button slot emitting ButtonClicked() to end slot. Not much difference.

You should only worry about tweaking performance when the program works and you have a measurable performance bottleneck... and then you work the bottleneck. Your CPU could probably process the signal and call the slot in the order of a thousand of times per second... how slow is the actual work of the slot in comparison?

[Atomic_Sheep]

Thanks, makes sense.