passing `' as `this' argument of `' discards qualifiers

[ansetou]

I have a func as below:
class Book
{
...
set<Level> sLevels ;
...
};

void Book::del_order (Order *ord) {
if ( ord && del_oid(ord))
{ // it's in there somewhere

set<Level>::iterator it=sLevels.find(Level(ord)) ;
if ( it!=sLevels.end())
{
it->del_order(ord) ;
}
else
{
DEBUG("ERROR:del_order:can't find the order to delete") ;
}

}
delete ord ;
}

...
}

When compiling, i'm getting:
book.h: In member function `void Book::del_order(Order*)':
book.h:38: passing `const Level' as `this' argument of `void
Level::del_order(Order*)' discards qualifiers

The problem is ord is not const, "it" is not const, sLevels is not const, so what am I missing? Thank you.

[moorecm]

The error message is informing you that your iterator is a const_iterator. This comes as a surprise because you are not explicitly defining it as such, right?

Here's the deal: std::set is an associative container. Associative containers associate a key with each value and they are stored sorted the key. In the case of a set (rather than a map) the key IS the value. If you were to modify the key, the container would have to be notified to re-sort itself (in order to maintain its efficiency guarantees for look ups, which are logarithmic). To prevent that situation the container forces the key to be constant--so you cannot modify an element in place in a set. You would have to remove it, modify it, and insert it.

[Athar]

set iterators are always const_iterators. Since they're generally binary search trees, it wouldn't do for the user to modify the values and thus breaking its order. Updating an element requires an erase + insert.

[ansetou]

you guys are great. this answers my other question about the foundamental different between set and map. I always thought why use map when i can just group the key and value into a new struct and use set.

btw, where did you get such info. I couldn't find anything like this.

[moorecm]

Generic Programming and the STL by Austern. It covers the STL from top to bottom without being too technical. Insight into design goals and implementation is given only when beneficial. I highly recommend it.