how can i make this function return null?

Question

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C++ return a “NULL” object if search result not found

I'm trying to return NULL on a certain condition but it won't let me, why not and how can i make it return a null value or 0?

struct Entity
{
    USHORT X;
    USHORT Y;
    UINT Serial;
    USHORT SpriteID;
    EntityType Type;
    Direction FacingDirection;
};

the function is:

Entity& GetEntityAt(int index)
                {
                    if (!GameObjects.empty())
                    {
                        lock_guard<mutex> lock(PadLock);
                        Entity& result = GameObjects[index];
                        return result;
                    }
                    return NULL; // <- this won't compile
                }

See here: http://stackoverflow.com/questions/10371094/returning-a-null-reference-in-c — jogojapan, Jan 11 '13 at 03:58
Change `Entity&` to `Entity*` and other according changes if you want to return NULL. — iammilind, Jan 11 '13 at 03:58

score 4 · Answer 1 · answered Jan 11 '13 at 03:58

4

There is no such thing as a null reference in C++. Your choices include:

change your function to return a (smart) pointer.
create a dummy sentinel object (const Entity null_entity), and return a reference to that.

answered Jan 11 '13 at 03:58

Oliver Charlesworth

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What about `boost::optional`? :o – chris Jan 11 '13 at 04:01
@chris: I'm not familiar with that. But that's why I hedged my bets and said "your choices include..." ;) – Oliver Charlesworth Jan 11 '13 at 04:01
Or throw an exception. Depends what the calling code expects. – Potatoswatter Jan 11 '13 at 04:02
@OliCharlesworth, I see, that part sounded a bit limiting to me. Anyway, `boost::optional` is like a nullable type. Either it's valid and has a value, or it's invalid. Though maybe more expressive, it allows for some of the same syntax as a smart pointer. – chris Jan 11 '13 at 04:03

score 3 · Answer 2 · answered Jan 11 '13 at 03:57

References can't be NULL. You have to use pointers instead:

Entity* GetEntityAt(int index)
                {
                    if (!GameObjects.empty())
                    {
                        lock_guard<mutex> lock(PadLock);
                        return &GameObjects[index];
                    }
                    return NULL;
                }

DWright · Answer 3 · 2013-01-11T04:05:23.693

1

A reference IS the object it references. Since NULL is not an object, you cannot return NULL if the function returns an reference.

edited Jan 11 '13 at 04:05

answered Jan 11 '13 at 03:58

DWright

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how can i make this function return null?

3 Answers3