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NewBie question : call back function.

Posted on 1998-07-11
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Last Modified: 2010-04-10
From the example of Booch's book :
Object Oriented Analysis and Design with Application (p.45,46)
There are two similar classes :
e.g. 1 :
typedef float temperature;
typedef unsigned int location;
class temperaturesensor{
public :
temperaturesensor(location);
~temperaturesensor();
void calibrate(temperature actual temperature);
temperature currenttemperature() const;
private :
.....
}


e.g. 2 :
class activetemperaturesensor{
public :
activetemperaturesensor(location,void (*f) (location, temperature));
~activetemperaturesensor();
void calibrate(temperature actualtemperature);
void establishsetpoint(temperature setpoint, temperature delta);
temperature currenttemperature() const ;
private :
....
}


The second one has a callback function and is so called a
more 'active' class.
(the first one is called a 'passive' one.)
Can someone explain it more detail?

many thanks.
rgds.
alanpong@hkstar.com
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Question by:alanpong
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alexo earned 150 total points
ID: 1167579
>> activetemperaturesensor(location,void (*f) (location, temperature));

The constructor for the second class takes a pointer to a user-function of the form:
    void function_name(location, temperature)

The supplied function presumably does something in a way that the designer of the class could not anticipate, so it is left to the creator (user) of the class to provide.

The constructor of the "active" class either invokes the supplied function directly
    f(some_location, some_temp);
or saves the supplied pointer in a private member variable and then has some other member function invoke it.

Example of usage:

    // Prototype:
    void MyFunc(location, temperature);
   
    // ...

    // Object creation:
    const unsigned int MY_LOCATION = 17;
    activetemperaturesensor mySensor(MY_LOCATION, MyFunc);

    // Implementation:
    void MyFunc(location, temperature)
    {
        // Do something...
    }

OK?


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Author Comment

by:alanpong
ID: 1167580
very happy!
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LVL 11

Expert Comment

by:alexo
ID: 1167581
Have fun!
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