Re: How are objects inserted into a set?

"Victor Bazarov" <>
Thu, 7 Jun 2007 15:03:01 -0400
desktop wrote:

Victor Bazarov wrote:

desktop wrote:

I have implemented a red-black tree which is used for std::set.

In the C++ standard 3 different insert methods are specified for the
associative container. But as i see it insert adds an object to the
set based on a key. But where does the key come from?

It's the same as the value. Doesn't the book you're reading about
'std::set' tell you that?

If I make my own object I don't specify any unique key that insert
uses to place the object.

You do. By supplying an object with a value.

Does the insert call generate some unique key that it uses each time
an object is inserted?



It seems that I don't understand set.

Hmm... Yes. Do you understand std::map? std::set is very similar to
std::map. Essentially 'std::set<T>' is just like 'std::map<const T,T>'.

Inserting into a vector works
class test {
int getpp(){return pp;}
void setpp(int i){pp = i;}
int pp;

int main() {
std::vector<test> hh;
test t1;
hh.push_back(t1); // Works fine

std::set<test> my_set;
const test& tref = t1; // see *
my_set.insert(tref); // fails with error: no match for
                   ?operator<? in ?__x < __y?


Can I only insert into std::set if my class 'test' define '<' and
properly some of the other operators?

"properly"? Yes, to use the default sorting mechanism your class
needs to have operator< defined for it. You can make it a member or
you can make it a stand-alone function.

You don't have to have operator< defined if you use custom sorting
functor in your set.

I still don't see how insert gets the key from 'test' so it can put it
the right place in the tree.

What book are you reading that doesn't explain how sorting of
objects works?

* According to

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