templated class in multiple files

[linucksrox]

I am writing the functions for a class that is templated using 3 files: qpqueue.h, qpqueue.cpp, and main.cpp
here's qpqueue.h:

#include <queue>
const int MAXPRIORITY = 10;

template <typename T>
class qpqueue
{
  public:
    qpqueue();

    void push (const T & item, int p);
    // insert item with priority p, 
    // 0 <= p <= MAXPRIORITY

    void pop();
     // find the non-empty queue with largest index and remove its
     // front element
     // Precondition: the queue is not empty, the function throws
     //      underflowError exception if the queue is empty

     T& top(); 
     // find the non-empty queue with largest index and return 
     // its front element.
     // Precondition: the queue is not empty, the function 
     // throws the underflowErr exception if the queue is empty

     const T& top() const;
     // constant version of top();

     bool empty() const;
     // is the queue empty?

     int size() const;
     // return the number of elements in the queue

  private:
     queue<T> priority[MAXPRIORITY +1];
      // priority[i] contains all elements with priority i 
      //   in their order of insertion
     int pqsize;
      // number of elements in the priority queue
};

When I try to compile my project, I get an error saying, "ISO C++ forbids declaration of 'queue' with no type."
I'm not sure if there's a problem with the implementation of my qpqueue.cpp file, or if I just need to have code in main() that actually implements the qpqueue class with a specified type. Thanks.

[Daved]

What's the line with the error?

Generally, you have to implement templated classes all within the header file, or #include the cpp file at the bottom of the header file. You probably won't see that error until you get the current one fixed, though. My guess is you put queue:: without the <T> somewhere in the cpp file.

[linucksrox]

the line with the error is in private when i declare queue<T> priority[MAXPRIORITY + 1];
I'm not sure how to do the .cpp file... here's what i have:

#include "qpqueue.h"

qpqueue<T>::qpqueue()
{
    pqsize = 0;
}

void qpqueue<T>::push (const T & item, int p)
{
    if (0 <= p && p <= MAXPRIORITY)
    {
        priority[p].push(item);
        pqsize++;
    }
}

is that right?

[Daved]

Change it to std::queue to fix the error.

For your cpp file, you need template <typename T> before each function definition.

And finally, you'll need to #include "qpqueue.cpp" at the bottom of qpqueue.h inside the include guard (if you have an include guard-- if you don't, add one). This is the only time it makes sense to #include a cpp file.

[linucksrox]

in my .cpp file, my constructor is this:

template <typename T>
qpqueue<T>::qpqueue()
{
    pqsize = 0;
}

obviously that's wrong... what did I do wrong?

EDIT:
i figured out my problem: i did not have the include guard in the .cpp file, it was only in the .h file, and since i included the .cpp file in the .h file, i figured i didn't need to include the .h file in the .cpp file. circular includes and stuff. but now it's all compiling. thanks for the help Daved

[Daved]

I don't know if I understand your edit, but the general way to do it is to put include guards around everything in your header file only. Then add the #include "qpqueue.cpp" inside the include guard at the bottom of the header. In the cpp file, you should #include "qpqueue.h" and you should not have include guards. It will work because of the include guards inside the header file.

File.h:

#ifndef FILE_H
#define FILE_H

// template class declaration

#include "File.cpp"

#endif // FILE_H

File.cpp:

#include "File.h"

// class function definitions