C++ return a "NULL" object if search result not found

Question

I'm pretty new to C++ so I tend to design with a lot of Java-isms while I'm learning. Anyway, in Java, if I had class with a 'search' method that would return an object T from a Collection< T > that matched a specific parameter, I would return that object and if the object was not found in the collection, I would return a NULL. Then in my calling function I would just check if(T != NULL) { ... }

In C++, I'm finding out that I can't return a NULL if the object doesn't exist. I just want to return an 'indicator' of type T that notifies the calling function that no object has been found. I don't want to throw an exception because it's not really an exceptional circumstance.

class Node {
....

Attr& getAttribute(const string& attribute_name) const {
   //search collection
   //if found at i
        return attributes[i];
   //if not found
        return NULL;
}

private:
   vector<Attr> attributes;
}

Answer 1

If you want a NULL return value you need to use pointers instead of references.

References can't themselves be NULL.

(Note to the future comment posters: Yes you can have the address of a reference be NULL if you really really try to).

See my answer here for a list of differences between references and pointers.

Answer 2

In C++, references can't be null. If you want to optionally return null if nothing is found, you need to return a pointer, not a reference:

Attr *getAttribute(const string& attribute_name) const {
   //search collection
   //if found at i
        return &attributes[i];
   //if not found
        return NULL;
}

Otherwise, if you insist on returning by reference, then you should throw an exception if the attribute isn't found.

(By the way, I'm a little worried about your method being const and returning a non-const attribute. For philosophical reasons, I'd suggest returning const Attr *. If you also may want to modify this attribute, you can overload with a non-const method returning a non-const attribute as well.)

Answer 3

You are unable to return NULL because the return type of the function is an object reference and not a pointer.

Answer 4

The reason that you can't return NULL here is because you've declared your return type as Attr&. The trailing & makes the return value a "reference", which is basically a guaranteed-not-to-be-null pointer to an existing object. If you want to be able to return null, change Attr& to Attr*.

Answer 5

You can easily create a static object that represents a NULL return.

class Attr;
extern Attr AttrNull;

class Node { 
.... 

Attr& getAttribute(const string& attribute_name) const { 
   //search collection 
   //if found at i 
        return attributes[i]; 
   //if not found 
        return AttrNull; 
} 

bool IsNull(const Attr& test) const {
    return &test == &AttrNull;
}

 private: 
   vector<Attr> attributes; 
};

And somewhere in a source file:

static Attr AttrNull;