Template class nested in template class

This is a discussion on Template class nested in template class within the C++ Programming forums, part of the General Programming Boards category; I'm trying to create a container class that has node-like objects. These need to be templates. I tried: Code: template ...

  1. #1
    Registered User
    Join Date
    Nov 2005
    Posts
    137

    Template class nested in template class

    I'm trying to create a container class that has node-like objects. These need to be templates. I tried:

    Code:
    template <class T>
    class H {
     private:
      template <class T> class K;
      //...
    };
    
    template <class T>
    class H<T>::K {
     //...
    };
    But that doesn't work. I tried some random things in hopes that I'd get the combination right, but none of them worked.

    What am I doing wrong? How do I fix this?

    Thank you

  2. #2
    Officially An Architect brewbuck's Avatar
    Join Date
    Mar 2007
    Location
    Portland, OR
    Posts
    7,352
    You can't reuse "T" as a template parameter. How could the inner class possibly distinguish between the two? Name it something else.
    Code:
    //try
    //{
    	if (a) do { f( b); } while(1);
    	else   do { f(!b); } while(1);
    //}

  3. #3
    Registered User
    Join Date
    Nov 2005
    Posts
    137
    I didn't want it to. The types always have to be the same. I guess this is achieved by having two separate variables anyways, though. Yes?

  4. #4
    Officially An Architect brewbuck's Avatar
    Join Date
    Mar 2007
    Location
    Portland, OR
    Posts
    7,352
    Quote Originally Posted by yahn View Post
    I didn't want it to. The types always have to be the same. I guess this is achieved by having two separate variables anyways, though. Yes?
    If the types have to be the same then the inner class does not need to be a template.
    Code:
    //try
    //{
    	if (a) do { f( b); } while(1);
    	else   do { f(!b); } while(1);
    //}

  5. #5
    Registered User
    Join Date
    Nov 2005
    Posts
    137
    Oh, that makes sense. How about when you want to return the nested type?

    I have:
    Code:
    template<class T>
    class H {
     private:
      class K;
      // ...
    };
    
    Class K {
     // ...
     public:
      K *doSomething();
      // ...
    };
    
    template<class T>
    H<T>::K *H<T>::K::doSomething() {
     // ...
    };
    That gives the error : "dependent name is not a type prefix with 'typename' to indicate a type"

    Do you know what I'm doing wrong?

    Thank you

  6. #6
    C++ Witch laserlight's Avatar
    Join Date
    Oct 2003
    Location
    Singapore
    Posts
    23,629
    That looks wrong. If K is nested in H, then I would expect you to define it as:
    Code:
    template<class T>
    class H<T>::K {
    public:
        K* doSomething();
    };
    though personally I would just define it within the definition of H. Anyway, once that is fixed, the next problem would be that in H<T>::K, the nested name K depends on the template parameter T. As such, it could, in theory, be interpreted not as a type name, but as an object name, and the rules say that it should be interpreted as a non-type name by default. As such, you should change to:
    Code:
    template<class T>
    typename H<T>::K* H<T>::K::doSomething() {
        // ...
    }
    Quote Originally Posted by Bjarne Stroustrup (2000-10-14)
    I get maybe two dozen requests for help with some sort of programming or design problem every day. Most have more sense than to send me hundreds of lines of code. If they do, I ask them to find the smallest example that exhibits the problem and send me that. Mostly, they then find the error themselves. "Finding the smallest program that demonstrates the error" is a powerful debugging tool.
    Look up a C++ Reference and learn How To Ask Questions The Smart Way

  7. #7
    Registered User
    Join Date
    Nov 2005
    Posts
    137
    Sorry, I had properly declared the class, I just didn't write it done correctly when making the example. But, you're suggested works. Thank you

    Now, I'm not getting any errors about the class, but when trying to compile I am getting unresolved external symbols to the functions of the class. What does that mean?

  8. #8
    C++ Witch laserlight's Avatar
    Join Date
    Oct 2003
    Location
    Singapore
    Posts
    23,629
    Quote Originally Posted by yahn
    Now, I'm not getting any errors about the class, but when trying to compile I am getting unresolved external symbols to the functions of the class. What does that mean?
    My guess is that you tried to separate the implementation of the class template into a source file, as would normally be done for classes. However, with templates, you generally have to put everything in the header file.
    Quote Originally Posted by Bjarne Stroustrup (2000-10-14)
    I get maybe two dozen requests for help with some sort of programming or design problem every day. Most have more sense than to send me hundreds of lines of code. If they do, I ask them to find the smallest example that exhibits the problem and send me that. Mostly, they then find the error themselves. "Finding the smallest program that demonstrates the error" is a powerful debugging tool.
    Look up a C++ Reference and learn How To Ask Questions The Smart Way

Popular pages Recent additions subscribe to a feed

Similar Threads

  1. linker error
    By tasha302 in forum C++ Programming
    Replies: 3
    Last Post: 12-13-2006, 11:00 PM
  2. Message class ** Need help befor 12am tonight**
    By TransformedBG in forum C++ Programming
    Replies: 1
    Last Post: 11-29-2006, 10:03 PM
  3. error: template with C linkage
    By michaels-r in forum C++ Programming
    Replies: 3
    Last Post: 05-17-2006, 08:11 AM
  4. help with template class using a template node
    By aciarlillo in forum C++ Programming
    Replies: 11
    Last Post: 06-02-2005, 05:46 PM
  5. Declare a template class as a friend?
    By AH_Tze in forum C++ Programming
    Replies: 11
    Last Post: 05-19-2004, 09:24 PM

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21