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can we declare a constructor of an abstract class?
if yes then why? if no then also why?(but it is allow in java)
Andrew Brock 19-Jan-11 22:52pm
Since you are asking a lot of questions regarding basics of C++ I would suggest reading a book or some tutorials
Sergey Alexandrovich Kryukov 19-Jan-11 23:41pm
You're absolutely right. We're doing fun exercise, where only systematic education really works. Good reading does the trick.
Besides, as I already said about previous question, it looks like OP is doing fine trolling to us, which is fine :-)
Andrew Brock 20-Jan-11 0:35am
If it wasn't fine, or I didn't like it I just wouldn't answer
Sergey Alexandrovich Kryukov 20-Jan-11 15:16pm

Abstract class just means that there is 1 or more methods that have no code defined.
It is perfectly legal to supply a constructor because any member variables of the class need to be initialised still (especially private ones)
class Graph {
	Graph(int nMyInt) : m_nMyInt(nMyInt) { }
	virtual void GraphFunc() = 0; //our virtual function
	int GetMyInt() { return m_nMyInt; }

	int m_nMyInt;

class Linear : public Graph {
    Linear() : m_nMyInt(20) { }
    virtual void GraphFunc() { /* some code for this func */ }
My 5 for your answer. I used your sample for additional notes.
Thank you.
Nuri Ismail 20-Jan-11 3:34am
Good answer. 5
Andrew's answer is very good but need some notes.

I would like to emphasize that the concept of abstract class is irrelevant to availability of constructor(s). That could cause some misunderstanding. See below.

As Andrew correctly explained us, "abstractness" of a class is recognized by one or more "pure virtual functions", his example: GraphFunc(). The compiler will not compile initialization if such class -- for a good reason (what should happen with the instance if someone calls a pure virtual function?). For more modern OOP languages a special keyword abstract is used to denote a class (or a method, a syntax used for the same purpose as C++ as pure virtual) as abstract. The sole purpose of the abstract class is to server as a base class for derived non-abstract classes.

Using declarations from Andrew's sample, we can add:

class RectangularGraph : public Graph {
    RectangularGraph(int nMyInt, int extraData) :
       Graph(nMyInt) { m_extraData = extraData; }
    virtual void GraphFunc() { /* some body */ }
    int m_extraData;


//Graph graph(1); //would not compile
RectangularGraph(2); //all correct

So, my point here is, if an abstract class cannot be "constructed" (a class instance can not be initialized), why it needs a constructor?

This is because a constructor, strictly speaking, has nothing to do with instance initialization -- it is designed to initialize part of the instance: see constructor RectangularGraph: it is composed of two parts: it's base is constructed first (came from Graph class), then the rest of the class RectangularGraph. Finally, it should be noted that a parameterless constructor of a base class (no matter abstract or not) is called automatically.

This way, not only constructors of abstract class is allowed -- it's very important. Look at the sample: how else m_nMyInt (see Andrew's sample) could be initialized, if it is not accessible for derived classes?
JF2015 20-Jan-11 0:16am
Good explanation! 5+
Andrew Brock 20-Jan-11 0:17am
My 5. Thanks for elaborating
Nuri Ismail 20-Jan-11 3:35am
Very good explanation indeed. Have a 5.
Both answers from Andrew Brock and SAKryukov are very good and I'm sure that they clarified a lot of things for you. I have a small addition about constructors.

In some cases it is handy to use the Virtual Constructor Idiom[^] in your abstract base classes. For additional details see this item[^] from C++FAQ[^]. :)

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