Exam prep

I think for the exam, possibly in addition to the 2008 exam answers (other posting), maybe we can all come up with some ideas about what we might expect in this years exam. Maybe in question form, as if it was an actual exam question? I would think to also add the answer (or what we think is the answer!).
I don't want to fail this exam (as obviously not-one else does either duh!), and maybe this will just help prepare everyone a bit better.
Then we can all comment about what we think might or not be asked.

Question (3 marks)
In words (i.e. not code) explain the implementation of the Singleton Pattern
Answer
The object's constructor is declared either private or protected, and a static function (e.g. instance()) provides access to the constructor, either creating a new instance if one not already existing, or returning a pointer or reference to the current running instance if one already running.

Question (2 marks)
Explain the use of the Facade Pattern
Answer
A class using the Façade pattern provides a unified interface to a set of interfaces in a subsystem. Façade defines a higher-level interface that makes the subsystem easier and safer to use. A Façade is a class with a clear simple interface that encapsulates and hides a complicated set of classes and/or functions.

Here's a good question though (might come up):

What's the difference between the façade pattern and the adapter pattern?

Question
Name the 3 different types of design patterns, give 2 examples of each. (6 marks)

Answer
Creational (Abstract factory, Singleton)
Structural (Facade, Composite)
Behavioral (Visitor, Observer)

Fantastic Idea,

Unfortunately my knowledge is still too poor to add any useful contributions, however I am currently on the Monostate Pattern, so I ask this question -

Compare and contrast the Singleton and Monostate Pattern

The singleton pattern allows one instance of an object to exist, whereas the Monostate allows multiple instances of an object to be in the same state. Remember (if I am not mistaken in this) that the state of an object is determined by the values of its data members, and in this case the object exists as many instances, but they are all exactly the same state, i.e. clones?

Assuming you have the following:

Language: C++ (QT)
[singleton.h]
class Singleton : public QObject
{
    Q_OBJECT
public:
    static Singleton *getInstance();
    void setOtherParams();
    // ...
 
private:
    Singleton();
    static Singleton *instance;
};
 
 
[singleton.cpp]
#include “singleton.h”
Singleton *Singleton::instance = 0
 
Singleton *Singleton::getInstance()    //this is the singleton factory
{
    if( instance == 0 ) {
        instance = new Singleton();
    }
    return instance;
}

Now in the main file, how exactly do you create an object of Singleton above, and how do you call the other functions, like setOtherParams() above. Note I haven't put the implmentation of the function, just the definition.

I actually don't know the answer here, so if someone can help me.

Thanks.

A possible addendum to Shaun's first post re Singleton Vs Monostate.

Once an instance of a Singleton class has been created, any future attempts to create the same class will point to the first instance.

This is my understanding anyway

So only one instance of a Singleton with pointers for future creations and many possible instances of a monostate, but all posses the same state.

I am happy to be corrected.

@Shaun,
I think the following will work for main but I'm open to correction as I haven't tried it out.

Language: C++ (QT)
int main(int argc, char* argv[]) {
	Singleton *aRefToSingleton = Singleton::getInstance();
	aRefToSingleton->setOtherParams();
}

Also wrt the Exam 2008 Post Question 7.3 my answer would be:
Yes, the user/client knows that it is dealing with a Singleton because the constructor is private and the object cannot be created by merely calling the constructor. Rather the user/client has to call a function that returns a pointer to the already instantiated Singleton object.

A lot of oop terms in the 7.3 answer... I hope I haven't abused the terminology.

Thanks.
I have also updated the answer for the 2008 exam.

I'm not really a fan of using pointers everywhere in a program. I prefer to use references wherever possible instead. Qt makes this difficult, unfortunately.

One of the reasons I don't like pointers is that you have to worry about memory management. You have to make sure that everything that you've created with 'new' must be destroyed with 'delete'. In the example of the Singleton pattern above, there's no way to destroy the class once you've finished.

There's another way of creating Singleton classes that doesn't require pointers but has it's own set of problems.

Language: C++
[h file]
class Singleton{
public:
    Singleton &instance();
 
protected:
    static Singleton classInstance;
    Singleton();
    ~Singleton();
};
 
[cpp file]
Singleton Singleton::classInstance;  // automatic instance creation
 
Singleton::Singleton(){
// initialisation
}
 
Singleton::~Singleton(){
// destruction
}
 
Singleton &Singleton::instance(){
    return classInstance;
}

One of the main disadvantages of the Singleton pattern is that it's doesn't fit neatly into Object-oriented programming. An example of what I mean is that it's difficult to extend singletons by creating derived classes. This is because you can't create virtual static functions. This means that you'll always get the base class returned if you retrieve the instance.

A disadvantage of the variation of the implimentation that I've given is that you've got no control over the initial object creation. The instance is global (although only accessible via the class functions) but you've got no control over the order of creation. You'll be in a bit of a fix if your singleton requires interaction with other global data but the singleton gets created before it's dependent data does.

..and if they ask me about this in the exam, I'll be here re-doing this module again!

My guess is

1. Libraries
2. Signals and Slots. Show implementation and why is it useful
3. What pattern would be applied to this scenario. Positive this will be there
4. Visitor (because it's used everywhere), Observer (because Qt uses Signals and Slots), and Composite (QObject) patterns
5. Threads and processes (Like what are the differences or make this threadable)
6. Regex - Validators and/or example
7. Name an Anti-pattern (maybe)
8. Creational - Structural - Behavioural - one example each and/or define
9. XML (Don't know how much they'll ask but almost positive there will be something on it)
10. UML (Well that's what they said)
11. Something on Factory patterns. This is, what I feel, one of the "larger" patterns and maybe the most flexible in application, so it could be anything.

Again just my thoughts. Can predict anything, but if you don't know the work, you don't know the work.

----
"Flying is learning how to throw yourself at the ground and miss." - Douglas Adams
"Time is a great teacher, but unfortunately it kills all its pupils ..." - Louis Hector Berlioz
I think animal testing is a terrible idea; they get all nervous and give the wrong answers.

Yes, sounds good, unfortunately I don't know anything about XML, apart from what the structure of the file looks like, I don't know the theory/implementation of it.

There's been an xml question that asks the output of some xml code and if it does output or does it fail, etc

@Rev. A good list of possibilities.

I feel that because the exam will be an hour shorter (that is a whole lot of time to the previous three hour exam), that there are likely to be fewer questions than in the past for obvious time constraints. My money is on 6-8 (broad topics with sub questions), and more theoretical, so that they can get through more individual questions.

We can all hope. They may just give us less marks for the same amount of work

I really hope there's more theory, cause I've haven't done way enough practical.

I can probably implement almost all the design patterns in C++, but the Qt stuff worries me

----
"Flying is learning how to throw yourself at the ground and miss." - Douglas Adams
"Time is a great teacher, but unfortunately it kills all its pupils ..." - Louis Hector Berlioz
I think animal testing is a terrible idea; they get all nervous and give the wrong answers.

Here's my singleton:

Language: C++ (QT)
class Singleton{
	private:
	static Singleton* s;
	Singleton(){};
	~Singleton(){};
	public:	
	static Singleton* instance(){
		if (!s)
		s = new Singleton;
		return s;
	}
};

Language: C++ (QT)
Singleton* Singleton::instance() {
    static Singleton* s = 0;
    if (s == 0) {
        s = new Singleton();
    }
    return s;
}

http://cartan.cas.suffolk.edu/oopdocbook/opensource/singleton.html

----
"Flying is learning how to throw yourself at the ground and miss." - Douglas Adams
"Time is a great teacher, but unfortunately it kills all its pupils ..." - Louis Hector Berlioz
I think animal testing is a terrible idea; they get all nervous and give the wrong answers.

ShaunGVW Wrote:
-------------------------------------------------------
> Yes, sounds good, unfortunately I don't know
> anything about XML, apart from what the structure
> of the file looks like, I don't know the
> theory/implementation of it.

All you need to know about xml: http://www.w3schools.com/xml/default.asp

Thanks Rey

Are signals access modifiers always "public"?

Sorry Rick...my C++ vocab must be rusty but what do you mean?

Signals are public protected (Thanks Bini) (try make them private and see what happens) and slots can be private or public.

----
"Flying is learning how to throw yourself at the ground and miss." - Douglas Adams
"Time is a great teacher, but unfortunately it kills all its pupils ..." - Louis Hector Berlioz
I think animal testing is a terrible idea; they get all nervous and give the wrong answers.

Signals are protected, says so in TFM

Well, there's an example of it as public in TFM, Bini

test post. Database is giving errors!

Hmmm. me too. Trying to do a search on the forums internal server error 500 or something

----
"Flying is learning how to throw yourself at the ground and miss." - Douglas Adams
"Time is a great teacher, but unfortunately it kills all its pupils ..." - Louis Hector Berlioz
I think animal testing is a terrible idea; they get all nervous and give the wrong answers.

@Rick

Sorry were?

----
"Flying is learning how to throw yourself at the ground and miss." - Douglas Adams
"Time is a great teacher, but unfortunately it kills all its pupils ..." - Louis Hector Berlioz
I think animal testing is a terrible idea; they get all nervous and give the wrong answers.

Patterns I'm learning/learnt:
+Behavioral:
Command
Iterator
Observer
Visitor
Strategy (Thanks Duncan)

+Structural:
Adapter
Composite
Facade

+Creational:
Factory Method
Abstract Factory
Singleton

I'm I missing any that were in the book and may be asked?

----
"Flying is learning how to throw yourself at the ground and miss." - Douglas Adams
"Time is a great teacher, but unfortunately it kills all its pupils ..." - Louis Hector Berlioz
I think animal testing is a terrible idea; they get all nervous and give the wrong answers.

looks good Rey. The db seems to lock up when we try other functions. I'm trying to post a quote and code and that's when it bombs