Qt signal for changed hashmap - c++

Question

I've got a hashmap ReferenceMap in a Qt serial terminal program.
The program receives a key to ReferenceMap in form of a reference followed by a value. It then uses the Qt hashmap insert function to insert the value. It's very neat and I would like to keep that part of the program.
The problem is that I would like to to do different things when different new values are coming in. I therefore would like to emit a signal when a value associated with a special place in the referencemap is changed.
According to the guide below, an object that emits a signal can easily be created.
http://qt-project.org/doc/qt-4.8/signalsandslots.html
I think I got a solution, however, and want to discuss wether it's a neat way or not.
I could create a class in like this
class Sfloat : public QObject
{
Q_OBJECT
public:
explicit Sfloat(QObject *parent = 0);
float Get();
void SetValue(float value);
private:
float MyFloat;
signals:
void ValueChanged(float newValue);
public slots:
};
The ReferenceMap could then be set to contain references to SFloat object like,
ReferenceMap(Key, &SFloat).
When I have the key, I could get the object reference, call SetValue() and a signal would be emitted. However, is this the best way to solve it?

Answer 1

I don't think that's a good idea. You are wrapping every float of your hash with a QObject which sounds very heavy to me.
You can hap the whole HashMap inside the QObject and emit the signal from there when the value you are interested in has changed.
class MyHashMap : public QObject
{
Q_OBJECT
public:
explicit MyHashMap(QObject *parent = 0);
float Get(const QString& key) const;
void SetValue(const QString& key, float value)
{
// only emit value changed if the value has actually changed
QHashMap<QString, float>::iterator it = myHashMap.find(key);
if(it != myHashMap.end()){
if(it.value != value){
myHashMap[key] = value;
emit ValueChanged(key, value);
}
}
}
private:
QHashMap<QString, float> myHashMap;
signals:
void ValueChanged(const QString& key, float newValue);
};
According to your comment, you want to update a label when a value has changed. Simply add all the labels to a QHashMap, then connect the ValueChanged signal to a slot that updates the right label:
class MyWidget : public QWidget
{
Q_OBJECT
public:
explicit MyWidget(QWidget *parent = 0)
{
...
...
myLabels["label1"] = myLabel1;
myLabels["label2"] = myLabel2;
QObject::connect(myLabels, SIGNAL(ValueChanged(const QString&, float)),
this, OnValueChanged(const QString&, float));
}
public slots:
void OnValueChanged(const QString& key, float newValue){
QHashMap<QString, QLabel*>::iterator it = myLabels.find(key);
if(it != myLabels.end()){
it.value()->setText(QString::number(newValue));
}
}
private:
QHashMap<QString, QLabel*> myLabels;
};