Object values altered after return from function - c++

Question

PART OF A HOMEWORK PROBLEM
I have a list of objects, and my goal is to try and find if an object X is present in that list (I am interested only in the first occurrence of that object). My code seems to work fine for the most part, but I have this strange error where the value of only 1 particular object is being modified after it is returned from a function.
I added 10 objects to the list with values 0 through 3. When I search for any number except 0, (./a.out 1 OR ./a.out 2 and so on)I get the right output. But when I search for 0(./a.out 0), the findInList() prints the correct result, but the main() prints the value 18 (which is not even present in the list).
I am attaching the full source code here in case someone wants to compile it and try it out. I am also attaching the gdb step through I did.
SOURCE:
#include <iostream>
#include <string>
#include <functional>
#include <list>
using namespace std;
class Page {
public:
int pgnum; // page number
union {
int lfu_count;
int lru_clock:1; // can only be 0/1
int lru_ref8:8; // we only need 8 bits
};
public:
// Constructors
Page(int num) { pgnum = num; }
Page() {}
// Operator overloading
bool operator== (const Page &p) const {
if(p.pgnum == pgnum)
return true;
else
return false;
}
bool operator!= (const Page &p) const {
return !(p==*this);
}
};
ostream & operator<<(ostream & os, const Page &p) {
os << "Page number: " << p.pgnum;
return os;
}
// Think of this as an equivalent to equals in Java (IT IS NOT, JUST IMAGINE)
struct PageNumber: public binary_function< Page, Page, bool > {
bool operator () ( const Page &p1, const Page &p2 ) const {
return p1 == p2;
}
};
// Function to find an object in any list given an Operation
template <class Operation, class T>
T* findInList( list<T> fullList, T obj, const Operation &op ) {
T* ret = NULL;
typename list<T>::iterator it = fullList.begin();
it = find_if( it, fullList.end(), bind2nd( op, obj ) );
if( it != fullList.end() ) {
cout << "Found obj in list: " << *it << endl;
ret = &(*it); // not the same as it (which is of type iterator)
}
return ret;
}
int main( int argc, char **argv ) {
Page page_to_find;
list<Page> frames;
if( argc != 2 ) {
cout << "Please enter 1 and only 1 argument" << endl;
exit(-1);
}
page_to_find.pgnum = atoi(argv[1]);
Page *p = new Page[10];
for( int i=0; i<10; i++ ) {
p[i].pgnum = i%4;
frames.push_back(p[i]);
}
list<Page>::iterator it_frames = frames.begin();
while( it_frames != frames.end() ) {
cout << "Page in frames: " << *it_frames << endl;
it_frames++;
}
Page* pg = findInList( frames, page_to_find, PageNumber() );
if( pg != NULL )
cout << "Found page: " << *pg << endl;
delete[] p;
return 0;
}

Answer 1

You're returning the address of an object in a list that is pushed into the parameter list by value. Thus it is undefined behavior. Consider changing the parameter of the list in findInList to a reference.
// note reference type change in parameter list.
template <class Operation, class T>
T* findInList( list<T>& fullList, T obj, const Operation &op ) {
T* ret = NULL;
typename list<T>::iterator it = fullList.begin();
it = find_if( it, fullList.end(), bind2nd( op, obj ) );
if( it != fullList.end() ) {
cout << "Found obj in list: " << *it << endl;
ret = &(*it); // not the same as it (which is of type iterator)
}
return ret;
}