How do I declare an object using this provided header file? (missing template arguments error)

Question

My instructor provided us with a header file that defines the class AvlTree, but for some reason I can't declare objects normally in main(). What am I doing wrong? Here is the relevant portion of the header file:

#ifndef AVL_TREE_H
#define AVL_TREE_H

#include <iostream> // NULL, cin, cout
using namespace std;

template <typename Comparable>
class AvlTree
{
  public:
    AvlTree( ) : root( NULL )
      { }
    AvlTree( const AvlTree & rhs ) : root( NULL )
    {
        *this = rhs;
    }

private:
  struct AvlNode
  {
    Comparable element;
    AvlNode    *left;
    AvlNode    *right;
    int        height;

    AvlNode( const Comparable & theElement, AvlNode *lt,
                                            AvlNode *rt, int h = 0 )
      : element( theElement ), left( lt ), right( rt ), height( h ) { }
  };

  AvlNode *root;
};

#endif

Here's what I'm trying to do in main:

#include "AvlTree.h"

void readFile(AvlTree &t1, AvlTree &t2)
{
    // do some stuff
    return;
}

void splayAccess(AvlTree &t1, AvlTree &t2)
{
    // do some stuff
    return;
}

int main (void)
{
    // object declarations
    AvlTree tree1;
    AvlTree tree2;

    // function calls
    readFile(tree1, tree2);
    splayAccess(tree1, tree2);

    return 0;
}

And here are the errors (GNU compiler):

cwd0042@cse04:~/3110/hw4$ g++ header.h mcve.cpp
mcve.cpp:3:15: error: variable or field ‘readFile’ declared void
mcve.cpp:3:23: error: missing template arguments before ‘&’ token
mcve.cpp:3:24: error: ‘t1’ was not declared in this scope
mcve.cpp:3:36: error: missing template arguments before ‘&’ token
mcve.cpp:3:37: error: ‘t2’ was not declared in this scope
mcve.cpp:9:18: error: variable or field ‘splayAccess’ declared void
mcve.cpp:9:26: error: missing template arguments before ‘&’ token
mcve.cpp:9:27: error: ‘t1’ was not declared in this scope
mcve.cpp:9:39: error: missing template arguments before ‘&’ token
mcve.cpp:9:40: error: ‘t2’ was not declared in this scope
mcve.cpp: In function ‘int main()’:
mcve.cpp:18:10: error: missing template arguments before ‘tree1’
mcve.cpp:18:10: error: expected ‘;’ before ‘tree1’
mcve.cpp:19:10: error: missing template arguments before ‘tree2’
mcve.cpp:19:10: error: expected ‘;’ before ‘tree2’
mcve.cpp:22:11: error: ‘tree1’ was not declared in this scope
mcve.cpp:22:18: error: ‘tree2’ was not declared in this scope
mcve.cpp:22:23: error: ‘readFile’ was not declared in this scope
mcve.cpp:23:26: error: ‘splayAccess’ was not declared in this scope

If your instructor provided you with a header file that contains "using namespace std;" you have bigger problems than just this assignment. You have an incompetent instructor. You are doing yourself a disservice, to your future career prospects, by staying in this class. Find another instructor. "using namespace std;". In a header file. Good grief. Run away. As fast as you can. — Sam Varshavchik
– Sam Varshavchik, Commented Jul 23, 2016 at 16:28
Thanks for the sentiment, but this doesn't exactly help me solve my problem. — Cole Dapprich
– Cole Dapprich, Commented Jul 23, 2016 at 16:29
@Sam Varshavchik Was about to post more or less exactely that. You beat me to it. — Jesper Juhl
– Jesper Juhl, Commented Jul 23, 2016 at 16:30
please post a minimal reproducible example, Comparable is nowhere define and hence your code cannot be compiled at all — m.s.
– m.s., Commented Jul 23, 2016 at 16:30
Fixed the Comparable issue. Just a defined template that I forgot to change when copying and pasting. Sam, if you're not going to help me with the issue I asked about, please take your bad vibes elsewhere. — Cole Dapprich
– Cole Dapprich, Commented Jul 23, 2016 at 16:33

Sam Varshavchik · Accepted Answer · 2016-07-23 16:44:42Z

2

template <typename Comparable>
class AvlTree

// ...

This declares a template class called AvlTree. This template takes one parameter.

void readFile(AvlTree &t1, AvlTree &t2)

The syntax for declaring a function is:

{return type} function-name( {parameter list} )

{parameter list} is an optional list of comma-separated parameters the function. Loosely speaking, each parameter is specified as

{type} {name}

The type of the parameter, followed by its name (again, loosely speaking).

"AvlTree" is not a type. It is a template. To make it a type, you need to provide appropriate template parameters.

void readFile(AvlTree<int> &t1, AvlTree<int> &t2)

Now you declared a function that takes two parameters, each one is a reference to an instance of a AvlTree<int>, which is a type. "AvlTree" by itself is not a type. It's a name of a template.

The same problem is causing all the other compilation errors, here.

Whether ReadFile() should take AvlTree<int> parameters, or AvlTree<char> parameters, or maybe it, itself, should be a template function, is something for you to figure out.

answered Jul 23, 2016 at 16:44

Sam Varshavchik

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1 Comment

Cole Dapprich Over a year ago

Thank you, that was the issue. I apologize for my lack of knowledge, I've only been coding for 2 years, so this is the first time I've encountered a template. I assumed he was just using it to mask the typename of the element so that I could place whatever I wanted inside; I didn't know I had to explicitly define what I wanted it to take. As for the header file and my instructor's credentials, all I can say is that this is a class on data structures and algorithms, not strictly coding, and I know he primarily works with python.

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