Question

Isn’t a pointer just a reference when you don’t de-reference it?

#include "stdafx.h"
#define BOOST_TEST_MODULE example
#include <boost/test/included/unit_test.hpp>


std::list<int>* user_defined_func( ) {
    std::cout << "BEGIN: user_defined_func" << std::endl;
    std::list<int>* l = new std::list<int>;

    l->push_back(8);
    l->push_back(0);

    std::cout << "END: user_defined_func" << std::endl;

    return l;
}


bool validate_list(std::list<int> &L1)
{

   std::cout << "BEGIN: validate_list" << std::endl;

   std::list<int>::iterator it1 = L1.begin();

   for(; it1 != L1.end(); ++it1)
   {
      if(*it1<= 1){

         std::cout << "Validation failed because an item in the list was less than or equal to 1." << std::endl;
         std::cout << "END: validate_list" << std::endl;
         return false;
       }
   }

   std::cout << "Test passed because all of the items in the list were greater than or equal to 1" << std::endl;
   std::cout << "END: validate_list" << std::endl;
  return true;
}

BOOST_AUTO_TEST_SUITE( test )
BOOST_AUTO_TEST_CASE( test )
{
   std::list<int>* list1 = user_defined_func();
   BOOST_CHECK_PREDICATE( validate_list, (list1) );
}
BOOST_AUTO_TEST_SUITE_END()

In the line,

BOOST_CHECK_PREDICATE( validate_list, (list1) ); 

above, I was told that I can’t pass pointer to the function expecting reference. I thought that a pointer (that hasn’t been de-referenced) was just an address (i.e. a reference). What am I missing here?

Answer 1

Isn’t a pointer just a reference when
you don’t de-reference it?

No, a pointer contains a value that is interpreted as a memory address. (Whether it contains a value that is actually a valid memory address is another question)

A reference is an alias, another way of referring to an existing value.

int i = 5;
int* p = &i; // The value in p is the address that i is stored at.
int& r = i;  // The value in r is 5, because r is an alias of i.
             //   Whenever you use r, it's as if you typed i, and vice versa.
             //   (In this scope, anyway).

int sum = i + r; // Identical to i + i or r + i or r + r.

Edit:

since list1 is a pointer, how do I access the reference…?

You have two choices. You can derefence the pointer to get at the list it points to:

std::list<int>* list1 = user_defined_func();
std::list<int>& list1ref = *list1;
BOOST_CHECK_PREDICATE( validate_list, list1ref );
delete list1;

Of course, this could be shortened to:

std::list<int>* list1 = user_defined_func();
BOOST_CHECK_PREDICATE( validate_list, *list1 );
delete list1;

Your validation function could take a pointer instead of a reference (remember to change L1.{something} to L1->{something}):

bool validate_list(std::list<int>* L1) { ... }