Templates and VC++ event handling and generics
December 1, 2005 Leave a comment
The difference between templates and generics is a hotly debated topic. I am trying to understand all of this and Bruce Eckel recently wrote about a form of ‘Curiously Recurring Template Pattern’ in his blog.
Let us look at Java code that uses this pattern
public class PropertySupportAspect<T extends PropertySupportAspect> {
PropertyChangeSupport support = new PropertyChangeSupport (this);
//Other methods are not shown
public void firePropertyChange( String property,
String oldval,
String newval) {
support.firePropertyChange( property,
( oldval == null ) ?
oldval :
new String(oldval),
new String(newval));
}
}
class Bean extends PropertySupportAspect{
private String name;
public String getName() {
return name;
}
public void setName( String name ) {
firePropertyChange( "name",
getName(),
name );
this.name = name;
}
}
Since ‘CRTP’ is usually associated with C++ templates I decided to try the same code with similar functionality using VC++.
This is what I could manage with VC++.
#include
template<T>
class PropertyChangeListener {
public:
void propertyChange( std::string oldValue, std::string newValue ) {
printf("Changed from " + oldValue + " to " + newValue );
}
void hookEvent(T* bean) {
__hook(&T::MyEvent, bean, &PropertyChangeListener::propertyChange);
}
void unhookEvent(T* bean) {
__unhook(&T::MyEvent, bean, &PropertyChangeListener::propertyChange);
}
};
class Bean : public PropertyChangeListener {
std::string name;
typedef PropertyChangeListener listener;
public:
__event void MyEvent(std::string value);
public:
void setName( std::string name ){
listener::hookEvent(this);
__raise this->MyEvent( name );
}
};
int main() {
Bean bean;
bean.setName( "Test" );
}
This code does not compile because methods ‘hookEvent’ and ‘unhookEvent’ have a problem. I tried the VC++ forum and I will update if there is a solution. I myself don’t have experience with VC++.
Update :
I corrected my code error based on the reply I received from the VC++ team in the MSDN forum. I hadn’t noticed the description of these errors.
1. C3713: ‘PropertyChangeListener::propertyChange’: an event handler method must have the same function parameters as the source ‘Bean::MyEvent’
class Bean : public PropertyChangeListener {
std::string name;
typedef PropertyChangeListener listener;
public:
__event void MyEvent(std::string oldValue, std::string newValue);
public:
void setName( std::string name1 ){
listener::hookEvent(this);
__raise this->MyEvent( name, name1 );
}
};
1. C3740: ‘PropertyChangeListener’: templates cannot source or receive events
Quote from the team:
We don’t support templates both in event source and event receiver. You can not hook an event to a member template function. Further more, we might cut Native Event support entirely in the future release. We really encourage you to write your own event codes.
MindSpace 