Specifying generics with nested classes

This is a discussion on Specifying generics with nested classes within the C++ Programming forums, part of the General Programming Boards category; I am using a public outer class and a private inner class: Code: class List { // public outer class ...

  1. #1
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    Specifying generics with nested classes

    I am using a public outer class and a private inner class:

    Code:
    class List {	// public outer class
    
    	class Node {	// private inner class
    
    		/*
    		 * private instance variables
    		 */
    
    		string dbName;
    		Node *link;	// reference
    
    	public:
    
    		....
    	};
    
    	Node *head;	// private instance variable
    
    public:
    
    	List(){
    		head = NULL;
    	}
    
    	List::Node *addToStart(string new_dbName){
    		head = new Node(new_dbName, head);
    
    		return head;
    	}
    
    	.....
    };
    I would like to define a generic template

    Code:
    template <class T>
    class List {
       ....
    }
    With a single class or function, template definition and use I understand. However, nested class use is generating compiler errors related to

    Code:
    List::Node *addToStart(string new_dbName){
             head = new Node(new_dbName, head);
    
    	 return head;
    }
    Error specifies C++ ANSI forbids Node not having a type. I do not understand this error as Node is a type itself.

  2. #2
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    Code:
    List<T>::Node *addToStart(string new_dbName){
             head = new Node(new_dbName, head);
    
    	 return head;
    }

  3. #3
    C++ Witch laserlight's Avatar
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    Quote Originally Posted by Imanuel
    However, nested class use is generating compiler errors related to
    Are you defining this as a member function within the class template definition? If so, just changing List::Node to Node should work. If you are implementing this member function outside of the class template definition, then I would expect something like:
    Code:
    template<typename T>
    typename List<T>::Node* List<T>::addToStart(string new_dbName){
        head = new Node(new_dbName, head);
    
        return head;
    }
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  4. #4
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    I defined the function within the outer class and not as a prototype so I will use Node as return type and it should compile fine.
    Last edited by Imanuel; 09-30-2011 at 01:09 PM.

  5. #5
    C++まいる!Cをこわせ! Elysia's Avatar
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    You cannot refer to anything within a template class with unknown compile-time parameters (such as unknown types, T, etc) without specifying that it is a type.
    Hence, inside the class, Node is fine.
    Outside the class, it must be typename List<T>::Node, where T is an unknown type (ie a template parameter or the like). If T is known, then typename is not required.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

  6. #6
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    You guys are so smart; a veritable living repository of knowledge!

    Before the transition from

    Code:
    List::Node
    to

    Code:
    Node
    I had more errors than lines of code ... C++ is an awesome language.

  7. #7
    C++まいる!Cをこわせ! Elysia's Avatar
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    But it is also very tricky, especially when it comes to templates. It requires a lot of experience to do it right. Keep that in mind.
    Also, this might be of help in the future, [35] Templates ..Updated!.., C++ FAQ
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

  8. #8
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    Thank you for all the replies.

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