Return type of list front (C++)

Question

So I want to use a list for a part of my program. I'm trying to get acquainted to the library list, so I wrote a quick little program to help myself understand what's going on. It all works properly, but there's one thing I don't understand.

According to this: http://www.cplusplus.com/reference/list/list/front/ The return type for the front function should be a reference of the type (in this case, room) of the first element (the only element in this case).

But I was able to access the values without having to reference, because it seems that all the values were passed directly (not by reference). Is this expected? Is the website wrong? Is my compiler wrong (CodeBlocks 13.12)?

Here's my code:

#include <iostream>
#include <list>
using namespace std;

struct room {
    int content;
    struct room * north;
    struct room * south;
    struct room * east;
    struct room * west;
} ;

int main ()
{
    list<room> mylist;
    cout << ( mylist.empty() ? "List is empty.\n" : "List is not empty.\n" ) << endl;

    room * room1 = new room;
    room1->content = 15;
    cout
    << room1->content << "\n"
    << room1->north << "\n"
    << room1->south << "\n"
    << room1->east << "\n"
    << room1->west << "\n";

    cout << "\n" << room1 << endl;

    mylist.push_front(*room1);
    cout << ( mylist.empty() ? "\nList is empty.\n" : "\nList is not empty.\n" ) << endl;

    delete room1;

    room test_room = mylist.front();

    cout
    << test_room.content << "\n"
    << test_room.north << "\n"
    << test_room.south << "\n"
    << test_room.east << "\n"
    << test_room.west << "\n";

    cout << "\n" << &test_room << endl;

    return 0;
}

Answer 1

There are two types of constructors added automatically to any class you declare: The default constructor, which default-initializes all members of that class §12.1.4 and the copy constructor §12.8.7.

In your case, we have to take a look at the copy constructor:

The declaration of the copy constructor looks like this (if you would write it down specifically §12.8.8):

struct room{
    room(const room&); // Copy-Ctor, implicitly added by the compiler
};

As you can see, it takes a const reference to another room and initilizes all values of itself by copying from the passed in room.

---

The first thing that happens is that the list adds a copy of the room you passed into mylist.push_front(*room1) (you can see that push_front() takes elements by const-ref §23.3.5.4 but internally copies them. To do this, the copy-constructor is called the first time.

When you later access the element with mylist.front() it returns a reference but because you initialize a value of room, instead of a reference - room test_room = mylist.front(); - the copy-constructor is invoked a second time. To correctly capture the front of the list by reference you need to do room& test_room = mylist.front();.

Note: All §nbr refer to the according section in the C++ standard.