Noob question about templates & inheritance

This is a discussion on Noob question about templates & inheritance within the C++ Programming forums, part of the General Programming Boards category; Hi all, I've got a noob question to post. I've got this class named "Vector": Code: using namespace std; template<class ...

  1. #1
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    Hi all,

    I've got a noob question to post. I've got this class named "Vector":

    Code:
    using namespace std;
    
    
    
    template<class T> class Vector;
    
    
    
    template<class T>
    
    ostream& operator<<(ostream&, const Vector<T>&);
    
    
    
    template <class T>
    
    class Vector
    {
    
          friend ostream& operator<< <>(ostream&, const Vector<T>&);
    
    protected:
    
          T* data;
    
          unsigned len;
    
    public:
    
          Vector(unsigned = 10);
    
          Vector(const Vector<T>&);
    
          virtual ~Vector(void);
    
          Vector<T>& operator =(const Vector<T>&);
    
          bool operator==(const Vector<T>&);
    
          T& operator [](unsigned);
    
    	  unsigned getLength(void) {return len;}
          (...)
    };
    I've also got a class named "AssociativeArrayInheritance" that descends form Vector, as you can see:

    Code:
    template<class KeyType, class ValueType>
    
    class AssociativeArrayInheritance : public Vector<Pair<KeyType, ValueType> >
    
    {
    
    public:
    
    	AssociativeArrayInheritance(unsigned size=0):Vector<Pair<KeyType, ValueType> >(size){}
    
    	ValueType& operator [](const KeyType);
    
    };
    
    
    
    template<class KeyType, class ValueType>
    
    ValueType& AssociativeArrayInheritance<KeyType, ValueType>::operator [](const KeyType key)
    
    {
    	unsigned i = Vector::getLength (); // HOW CAN I CALL THIS METHOD BY INHERITANCE? :| iCAN'T SEEM TO FIGURE OUT...
           (...)
    When compiling this file, the last line (line163) ("unsigned i = Vector::getLength ();") is getting me the following error:
    (all code is in file "vector2.h")
    vector2.h: In member function ‘ValueType& AssociativeArrayInheritance<KeyType, ValueType>:perator[](KeyType)’:
    vector2.h:163: error: ‘template<class T> class Vector’ used without template parameters
    Can anybody help me? I know it must be something stupid & basic but I've searched the web, talked with colleagues, and by now I should be delivering this to S teacher...
    :
    Last edited by blacknail; 10-24-2008 at 05:51 PM.

  2. #2
    and the Hat of Guessing tabstop's Avatar
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    Well, the error seems pretty straightforward -- you have a Vector, but it's a Vector of ... what? KeyTypes? ValueTypes? Something Elses? How do I know? And how do you expect to call getLength without an object? You have to have a specific Vector in mind to call a member function on it.

  3. #3
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    You have to have a specific Vector in mind to call a member function on it.
    With inheritance do I have to create an instance of the class to call a method in the super class?

  4. #4
    Frequently Quite Prolix dwks's Avatar
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    If it's a non-static method, yes. All methods need an object to operate on.

    But do you mean, "do I need a separate instance of the base class to call base class methods from within the derived class?" The answer to that question would be no. If you have permission to execute the method in the base class, you can call it from the derived class just by itself.
    Code:
    class base {
    public:
        void base_function() {}
    };
    
    class derived : public base {
    public:
        void derived_function() {
            base_function();
        }
    };
    [edit] Can you tell I didn't read the rest of the thread? Sorry for this rather useless post. [/edit]
    Last edited by dwks; 10-24-2008 at 06:03 PM.
    dwk

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  5. #5
    and the Hat of Guessing tabstop's Avatar
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    Okay, I missed that you wanted the inherited version. So then it's Vector<Pair<KeyType,ValueType> >::getLength.

    Is there a reason you need the base version and not the inherited version? (Did you override it, and now need the original?)

  6. #6
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    In first place, thanks to all. I was in the verge of despair and now I'm getting better.

    Second, tabstop, I only have getLength() defined in the superclass (Vector) and I want to call it in sub class (AssociativeArrayInheritance). As simple as that!

  7. #7
    and the Hat of Guessing tabstop's Avatar
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    Then do so. Everything in Vector is in AssociativeArrayInheritance.

  8. #8
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    That was just what I was thinking. I just thought that I could call the method by just typing getLength() in the subclass :S

    Anyway, thanks for all!

  9. #9
    Cat without Hat CornedBee's Avatar
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    You must type "this->getLength()". This is a rather complicated aspect of the name lookup rules in the face of templates. The C++ FAQ Lite describes why.
    All the buzzt!
    CornedBee

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  10. #10
    Algorithm Dissector iMalc's Avatar
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    Don't forget to make getLength a const method as well.
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