Re: Type Functors

=?ISO-8859-1?Q?Erik_Wikstr=F6m?= <>
Fri, 08 Jun 2007 22:50:10 GMT
On 2007-06-08 23:40, desktop wrote:

V.R. Marinov wrote:

On Jun 8, 8:10 pm, desktop <> wrote:

The problem seems to occur when I do an insert:

test t1;

as long as I don't insert I can make a set with or without specifying
std::less<test> and without implementing the '<' in test.

When doing an insert I must implement the '<' operator in test and it
makes no difference if I specify std::less<test> or not.

OK. So std::set<> sorts the elements when they are inserted. In order
to sort them it needs
a criteria. The default criteria used by std::sort is the template
function (or functor/function object) called std::less<>. Here is how
std::less<> might look like:

 template <class _Tp>
    struct less : public binary_function<_Tp, _Tp, bool>
      operator()(const _Tp& __x, const _Tp& __y) const
      { return __x < __y; }

So as you can see the std::less<test> uses the < operator.
So *you* need to provide < operator in way possible for std::less to
use it.

But I am not sure what to put in the body of '<'. Currently I have:

     bool operator <(test const& t) const {
                return (*this).getpp() < t.getpp();

but that was just to fill in the body.

The thing is that if you decide to std::greater<test> instead of
you would have to provide a > operator.
Plus you might need to provide some more operators,
so I would suggest you create a separate header
file (e.g. test_op.h) and provide all operators needed.
This is how the < operator might look like
if you decide your class not have friend operators ;)

bool operator<(const test& lhs, const test& rhs)
    return lhs.get() < rhs.get();

I get an error that the operator must take exactly one argument. But I
still don't understand how each of my test objects get a unique key that
the std::less function can use to insert the object the correct place.

There is no unique key, the tree does not need any key. All the tree/
algorithm is concerned with is nodes and in connecting them in a
specific order, this order is determined by comparing the value of each
node with the value of all the other nodes. and This is where the <
operator comes in, it compares a value with another and tells the tree
if it is greater or less than the other, and from that information a
decision is made of where to place the node.

This is why you can store anything in a set/map since the container does
not care what it is, just it's relation (as defined by a comparison
operation) to the other stuff you put in. From this we can see that the
difference between a set and a map is quite small, the only difference
is that in a map you insert a key-value pair and perform all comparisons
on the key, whereas in the set you insert only the value and perform all
the comparisons on that (you can think of it as a map with a key-key pair).

Erik Wikstr?m

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