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Consider the following definition of
a polymorphic queue, and the class PrintJobs:
template <class T>
class Queue {
private:
int size;
T *first, *last;
public:
void Add(T); // Add item to the back of the queue
T Remove(); // Remove and return the item at the front
of the queue
int Size(); // Return the number of elementes in the queue
}
class PrintJobs {
char* JobName;
void* Job;
PrintJobs* next;
}
(a) Explain what is meant by a template class in C++, and give an
example of where it may be useful to employ a template class. [2 marks]
(a) • A template class is a parameterizable
component of a class definition; (1 mark)
• It should be used where a particular class can be used with
a number of different component types. (1 mark)
(b) Construct an object MyQueue, which
is an instantiation of the Queue class with template type PrintJobs.
[2 marks]
(b) Queue MyQueue<PrintJobs>;
• Instantiating MyQueue; (1 mark)
• With the correct template; (1 mark)
(c) Implement the function Add, such
that it takes an object of template type T and adds it to the back
of the queue. [4 marks]
(c) A sample definition of Add follows:
void Queue::Add(<T>& datum) {
last.next = datum;
datum.next=
null;
last=
datum;
}
And the following marking scheme should be used:
• The function signature, with the correct type and reference;
(1 mark)
• Updating the next pointer in the last element in the queue;
(1 mark)
• Ensuring the next pointer in the new element doesn’t point
anywhere inappropriate; (1 mark)
• Updating the last pointer at the end of the queue; (1 mark)
[4 marks]
(d) Implement the function SplitQueue,
such that it takes in a reference to a queue and separates it into
two queues a and b, such that any PrintJob with the name "priority"
is put into queue a, and the others into queue b. [5 marks]
(d) A sample definition of SplitQueue follows:
void SplitQueue(Queue& Q, Queue& a, Queue& b) {
PrintJobs
temp;
while(Q.Size()!=
0) {
temp=
Q.Remove();
if(strcmp(
temp.JobName, "priority")== 0) {
a.Add(temp);
}
else {
b.Add(temp);
}
}
}
And the following marking scheme should be used:
• Correct function signature; (1 mark)
• Declaring a local variable to hold the next element in the
queue (as first and last are
not accessible); (1 mark)
• Looping over the non empty queue; (1 mark)
• Sensibly comparing the JobName in the current element; (1 mark)
• And adding it to the appropriate queue depending on the results
of this comparison; (1 mark)
[5 marks]
(e) Give the name for the form of parameter
passing used in the above SplitQueue function, and explain what effect
this type of parameter passing has. [2 marks]
(e) • Reference parameters; (1 mark)
• It means that the actual object itself is operated on, not
a copy; (1 mark)
[2 marks]
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