Undefined reference c++, unusual case

Question

I'm creating a queue class in c++ and am having trouble compiling with a makefile. My queue.cpp class is here

#include "queue.h"
#include <stdlib.h>

queue::queue()
{
   front_p = NULL;
   back_p = NULL;
   current_size = 0;
}

void queue::enqueue(int item)
{
    node newnode = node(item, NULL);
   if (front_p == NULL) //queue is empty
    {
       front_p = &newnode;
       back_p = &newnode;
    }
   else 
   {
       back_p->next = &newnode;
       back_p = &newnode;
   }
   current_size ++;
}

My header file (queue.h) is here

class queue
{
  public:
    queue(); // constructor - constructs a new empty queue.
    void enqueue( int item ); // enqueues item.
    int dequeue();  // dequeues the front item.
    int front();   // returns the front item without dequeuing it.
    bool empty();  // true iff the queue contains no items.
    int size();  // the current number of items in the queue.
    int remove(int item); // removes all occurrances of item 
      // from the queue, returning the number removed.

  private:
    class node  // node type for the linked list 
    {
       public:
           node(int new_data, node * next_node ){
              data = new_data ;
              next = next_node ;
           }
           int data ;
           node * next ;
    };

    node* front_p ;
    node* back_p ;
    int current_size ; // current number of elements in the queue.
};

test program (tester.cpp)

#include <iostream>
#include "queue.h"
#include <stdlib.h>
using namespace std;

int main(int argc, char * const argv[])
{
    cout << "Lalalalala" << endl;
    queue q1;
    q1.enqueue(5);
}

makefile

all: tester

tester: queue.o
    g++ -o tester tester.cpp

queue.o: queue.cpp queue.h
    g++ -c queue.cpp

clean:
    rm -f tester *.o

when I type "make" or "make all" I get this error:

g++ -o tester tester.cpp
/tmp/ccTOKLWU.o: In function `main':
tester.cpp:(.text+0x33): undefined reference to `queue::queue()'
tester.cpp:(.text+0x44): undefined reference to `queue::enqueue(int)'
collect2: ld returned 1 exit status
make: *** [tester] Error 1

The unusual thing about it, is that when compiled in visual studio on a windows machine, there are no errors. I haven't the faintest idea why it shouldn't compile on a linux machine in the way I am doing so. Would anyone kindly explain?

Answer 1

Your makefile is incorrect - it compiles tester.cpp with a dependency on queue.o, but it does not link queue.o at all. That is why the compilation of tester.cpp results in an unresolved reference.

You should change your make file as follows:

all: tester

tester: queue.o tester.o
    g++ queue.o tester.o -o tester

tester.o: tester.cpp tester.h
    g++ -c tester.cpp

queue.o: queue.cpp queue.h
    g++ -c queue.cpp

clean:
    rm -f tester *.o