Thread: an odd problem with a template class

I've been working with templated classes for years now...but we all have our brain farts, dont we? So...I was simply building a key-value map class, using a binary search tree as my backbone for the class.

Anyways, I am receiving some odd errors when I try to compile...and after doing an examination of my code I don't know what is wrong. The errors are as follows:

1. MAP_DTP is not a template type (line 8)
2. MAP_DTP is not a template (line 21)

The are more...but the basically follow the same pattern as those first 3. There is actually one more that I find quite funny. I will post it as comic relief:

3. confused by earlier errors, bailing out

haha...i love that...i made the computer confused.

Anyways...the code for the class declaration itself is quite short, so I shall post it:

Code:

#ifndef __MAP_DTP_H
#define __MAP_DTP_H

#include "BST.h" //where BST is contained
#include "sort240.h" //where simpleVector is contained

template <class TKey, class TValue>
class MAP_DTP
{

protected:
    
    //The actual binary-search-tree containing the keys and values
    BST myTreeMap;
    
    void OutputMap ( ostream & output, BSTNode *root );
    
public:

    //Constructors
    MAP_DTP ( );
    MAP_DTP ( const MAP_DTP<TKey, TValue> & otherMap );
    
    //Destructors
    ~MAP_DTP ( );
    
    //Accessors
    bool ContainsKey ( TKey myKey );
    bool ConstainsValue ( TValue myValue );
    bool IsEmpty ( void );
    
    int Size ( void );
    
    TValue & GetValue ( TKey myKey );    
    
    simpleVector <TKey> GetKeys ( void );
    simpleVector <TValue> GetValues ( void );
    
    void Test ( ostream & output );
    
    //Modifiers
    void Clear ( void );
    bool SetValue ( TKey myKey, TValue myValue );
    bool RemoveKey ( TKey myKey );
    
    //Operators
    TValue & operator [] ( const TKey & index );
    bool operator == ( const MAP_DTP<TKey, TValue> & rhs );
    MAP_DTP<TKey, TValue> & operator = ( const MAP_DTP<TKey, TValue> & rhs );

};

#endif /* __MAP_DTP_H */

Do you guys see anything that might be wrong in my syntax or logic? Thanks for any help.

here i will go ahead an upload all the files.

my driver program is really simple:

Code:

#include <iostream>
#include <string>
#include "mapdtp.h"

using namespace std;

int main ( void )
{
    MAP_DTP <string, string> myMap;
    
    return 0;    
}

I didn't upload it, because you can only upload 5 files. :-)

But the 5 files that I did upload are all that are needed.

sort240 - contains simpleVector (used in the program) and MATRIX_DTP (not used)
bst.h - contains the Binary Search Tree class declaration
bst.cpp - contains the class definition
mapdtp.h - contains the MAP_DTP class declaration
mapdtp.cpp - contains the class definition

what compiler do you use?

and what happens if you put the member functions in the class declaration?
I remember you have to do:

template<class T>
void X<T>::Print(const T &t)
{
}

if you put the body inside a cpp file

i am using devc++ as my IDE, which uses g++ as the compiler.

In BST_Node and BST, you have the declaration
friend class BST;

This makes the compiler think BST is a non-template class, confusing it when encountering the template.

I don't know how the syntax goes to make all template instantiations a friend, but IMO the design is screwed anyway - the generic underlying tree should not even know about the map implemented on top of it, much less make it a friend.
Then there's the problem that, the way you have it, your map can only use keys and values implicitely convertible to strings. Pretty useless, if you ask me.
Oh, and expect linker errors when you finally get it to work. Template implementation in source files and all that stuff.

A more typical way to implement this is to write a BST template that has only a value-type and a comparison-predicate template argument. It uses the comparison predicate to sort the values.
Then, the map is implemented by using the BST with a tuple of the key and the value as the value type and supplying a comparison predicate that only compares the keys, ideally according to the predicate supplied to the map by the user.

>your map can only use keys and values implicitely convertible to strings. Pretty useless, if you ask me.

Yeah, I know. That is one change I hadn't made yet. The BST class was originally written earlier as a non-template class for an assignment. Then I wanted to build a key-value map, and thought that a BST would be a good way to store the data, so I started modifying the code to support it...but that part wasn't finished yet.

Needless to say, the fact that at the present moment in time BST only supports strings shouldn't be the cause of compiler errors.

>This makes the compiler think BST is a non-template class

BST is a non-template class as of right now.

Maybe you meant to say MAP_DTP instead of BST? If that is so, then you are correct, and that might be the problem. Maybe I should finish the conversion of BSTNode and BST to templates. I made MAP_DTP a friend to both, wanting it to be able to access the private variables and functions of both.

Yeah, I meant MAP_DTP, not BST. Remove the friends and the errors go away. (Of course, you have many, many more.)

yeah...so in order to really get everything working right I need to make both BSTNode and BST templated...and it would certainly be a good thing to do, but as I think about it, for the scope of the project that I am working on...I really don't need templates...so I just decided to "de-template-ize" MAP_DTP and only use strings as key-value pairs... Now the MAP_DTP class is working perfectly... In the future I will templatize all of it when needed...

Wait a moment, you're not just using this as an exercise for templates and BSTs?

Why don't you use std::map?

im using it both as an exercise to refresh myself a bit, and also as part of a project I am working on... can't use std::map because the professor wont let us for this specific project, otherwise i would have done that a long time ago.