Some simple questions

Some simple question, that require simple answers:

How can I force the implementation of methods in a class, like the Java's interfaces, if possible?
Plus, how do I declare an abstract class?
What is the relation between this and virtual functions?
And how do the protected fields behave (I read the tutorial but didn't understand well)?

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>>How can I force the implementation of methods in a class
Declare a function (method) as virtual or pure virtual, and overload it in a derived class (actually, if you declare an object of the derived class then I don't think you need to declare the function as virtual at all).

>>Plus, how do I declare an abstract class?
Any class with a pure virtual function is an abstract class.

>>What is the relation between this and virtual functions?
A virtual function is one that is meant to be overloaded. If you declare a pointer to a base class, then call the base class's virtual function, then the derived class's function will be called.

>>And how do the protected fields behave
Makes it so only the class itself and its derived classes can access the members declared as protected.

Declaring it as virtual does not mean it must be reimplemented. Declare it as pure virtual:

Code:

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class A {
  public:
    virtual void foo( ) = 0; // Pure virtual. Must be reimplemented in anything to be instantiated.
};

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Quote:

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Originally Posted by Zach L.

Code:

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    virtual void foo( ) = 0;

---

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So this is a pure virtual function, and must be implemented in derived classes, right? Without the "= 0", it would be simply a virtual function, and the class abstract...
But does it really have to be 0 or can be any number? What is that number suposed to do?

Quote:

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Without the "= 0", it would be simply a virtual function, and the class abstract...

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I don't believe the class would be abstract without the "= 0". And yes, it has to be 0, although I don't think it really does anything except tell you that it's a pure virtual function. Maybe 0 is symbolic for nothingness, i.e. there's no function definition.

The class would not be abstract (or rather, not an abstract data type [ADT]) without the '= 0'. A class with a pure virtual cannot be instantiated. And, yes, it must be 0.

just to note, " = 0" is nothing more than a tag that the compiler notices. If it sees this in a class, and sees you instantiating an object of that class, it's supposed to yell at you. I used to think that it would be declaring that function pointer to NULL, and when you define it in child classes, it would change the pointer for that class, but I was informed otherwise.

A final thing... (no need to make another thread ;) )

Imagine I have the following class:

Code:

---

template<typename T>
class Array{
private:
        T *array;
        uint _length;
public:
        Array(uint capacity = 0) :_length(capacity){        array = new T[capacity];        }
        ~Array(){        delete array;_length=0;        }

        T operator[](uint idx) const;
        //more operators
        uint length() const;
};

---

Can I declare an operator to make operation like this?
Array<int> a(3);
a[0] = 5;//<<--operator

You'd need to return a T& instead of just T.

I didn't understand well..

Code:

---

template<typename T>
T Array<T>::operator[](uint idx) const{
        return array[idx];
}

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I have this to do, e.g.:
Array<int> a(3);
int n = a[2];

But I want to add an operator to do also:
a[2] = n;

Code:

---

template<typename T>
T& Array<T>::operator[](uint idx){
        return array[idx];
}

---

Try that (and you have to change the function in the class declaration too).

That worked very well!! Thank you :D :D

No problem :) You do know how and why it works though, right?

Look at this thread for some explanation of why it works as it does: http://cboard.cprogramming.com/showthread.php?t=54632

I don't get it. How does that explain anything? Overloaded subscript operators, references and templates aren't even mentioned in that thread....

References sure are. And it is the same principle at work. :)

Quote:

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Originally Posted by Zach L.

References sure are. And it is the same principle at work. :)

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I did a search for "reference" on that thread... No hits :) Although there was one brief example, I'll give you that much... I think a link to the reference tutorial would have been better though, if you know what I mean :)

Can't think of any reference tutorials off the top of my head, but the first reply does address exactly the same issue.