Re: Why can't I insert or delete the inside element of set<set<string>>?

"kanze" <>
7 Jun 2006 16:54:55 -0400
Rain Ma wrote:

Hi, I was using the STL set as follows and what I want to do
here is to change mySet from {{"Hi", "Hello world to"}, {"A",
"B"}} to {{"Hello world to"},{"A","B"}}, but it fails to
compile. Why?

Because it is illegal.

Think of it for a moment. An std::set is an ordered set;
internally, the elements are stored in order (in a balanced tree
of some sort -- typically a red-black tree). Changing an
element can change its place in the ordering. Suppose the set
also contained an element { "Help" }: before the change, this
element would be in front of the element you change; after, it
should be behind. But of course, changing the element does NOT
notify the set, so in the internal structure, the elements are
no longer in the correct order.

Because of this, all of the accesses to elements in the set are
through const references or pointers. In the case of std::set,
iterator and const_iterator both act in a similar manner.

And how can I manage to achieve my goal?

By extracting the element, modifying it, and reinserting it:

    std::set< string > elem = *iter ;
    mySet.erase( iter ) ;
    elem.erase( "Hi" ) ;
    mySet.insert( elem ) ;

Although not really in the philosophy of the STL, one could
imagine a function modifyElement, which returns a smart
non-const pointer to the element. Basically, the function would
remove the node from the tree structure, and the final
destructor of the smart pointer would reinsert it.

Alternatively, if you want to maintain the identity of the
element, and avoid the copies, without modifying the STL (which
I definitly don't recommend) or implementing your own compatible
container, you need to use a std::set< std::set<std::string>* >.
This means managing the memory yourself, but you can easily
create a smart pointer to modify the pointed to element.

James Kanze GABI Software
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