References

[Nosferatu]

Hello!

I'm quite new in C++ and even if i have some solid C experience, if find some things confusing.

Here's my problem:
1. I have a list<column> container in my program.
2. I want to insert something.
3. The STL list.insert requires an iterator parameter to be passed to it.
4. I want to insert an element at a specified index.
5. I don't assume that the index is within the range of the list, so when i try to get the iterator at the specified position, i would like to return some NULL pointer if the index is out of range. However, you cannot assign a NULL pointer to a reference (i guess that's because the refs are automatically dereferenced).
The problem is that i dont' want to set any flags telling me that something went wrong, i just want to know if the reference returned is valid or not.

So, i have 2 questions:
Q1: How do i use references in order to simulate some NULL pointer?
Q2: How do i use an iterator to return a POINTER to the object, instead a reference to it?

I hope i'm not deeeeeply confuzed here... But if i am, i would appreciate greatly any help!

Thanks

 

[greatkingrat]

There are varios way to solve your problem and
I append an example. It's not my suggestion because
basically I don't like STL data structure.

class node {
int a;
public:
node( int value ) : a(value) {}
void print() { cout << &quot;value=&quot; << a << '\n'; };
};


int main( void )
{
/* part 1 : node itself for template argument */
list<node> l;
list<node>::iterator i;

l.insert( l.begin(), node(1) );
for( i=l.begin(); i!=l.end(); i++ ) {
node& n = *i;
n.print();
}

/* above loop says that 'i' is invalid when it
is l.end() so you can return l.end() if invalid
index was supplied. */

/* part 2 : node pointer for template argument */
list <node*> lp;
list<node*>::iterator ip;

lp.insert( lp.begin(), new node(1) );
for( ip=lp.begin(); ip!=lp.end(); ip++ ) {
node* n = *ip;
n->print();
}

/* currently ip==lp.end() and *ip is NULL.
You can inspect *ip to know the iterator is valid
And I think it can be answer for your 2nd question. */
cout << *ip << '\n';

for( ip=lp.begin(); ip!=lp.end(); ip++ )
delete *ip;

return 0;
}
Hee S. Chung
heesc@netian.com

http://www.netian.com/~heesc

 

[Nosferatu]

Thanks for the post Hee.

The thing i am using right now is the list<node> thing, and there is the problem. The thing is that i should return a reference to the object in the list somewhere where the list itself is not known, for example in a derived class (the list would be private in the base class). So, i don't want to test against list.end(), not even in the base class (just a style consideration).

Isn't any way to check if a reference is valid, something simple enough as the testing against a NULL pointer it is to pointers?

Anyway, i think i will change the approach and build a list of pointers instead that building a list of objects. As far as i know, building a list of objects involves recreating the objects via CC, so i think this could be time consuming at some point.
What dou you think about that? Nosferatu
We are what we eat...

 

[greatkingrat]

1. According to C++ grammar, it is guaranteed that
reference is always a valid object itself. NULL pointer
value can be viewed as an exception of pointer values
but it is different for reference.

I think error can be reported by return value but
you cannot return the iterator in this case.

bool insert( list<node>::iterator& i, int index );

to receive an iterator, a reference is supplied
as an argument.

I hope you can find your best way.

2. Your last thought is exact. It is one of the reason
why I don't like STL.
Hee S. Chung
heesc@netian.com

http://www.netian.com/~heesc

 

[Nosferatu]

Thank you for your guidance.
I think the problem therefore lies in my strongly C preformatted head. I had the feeling that i won't be able to do this with references, but i wanted to be sure of that.

Thank you again. Nosferatu
We are what we eat...
There's no such thing as free meal...