Re: using less<string>...
Martin JHrgensen wrote:
Victor Bazarov wrote:
Martin JHrgensen wrote:
-snip-
I tried to google around a bit and found that both lines call the
map constructor to create a map<string, int> and the less<string>
is a comparison function - a 'weak ordering'. Can I see how
less<string> is defined somewhere?
It's defined in <functional> header.
I don't understand that. I didn't include any <functional> header.
So? You did include <map>, didn't you? Look there, it's very likely
if they have to use 'less<>', they included <functional> (or some other
header that defines 'less<>' template. The Standard says that if *you*
want to use 'less<>', you should include <functional>. That's good
enough reason for me to start looking there.
What's the definition of less<some type>?
The Standard says that it's
template <class T> struct less : binary_function<T,T,bool> {
bool operator()(const T& x, const T& y) const;
};
and that the operator() returns x < y. What's beyond that? Well, here
is what I know: <string> header defines operator< for 'std::basic_string'
which in turn calls the 'compare' member of 'std::basic_string'. How
*it* does the comparison is not defined, only what the result should be.
So, you're still have to look at the library source if it's availabe to
you. Or in the book.
Why don't I see any difference between both methods? Is less<string>
default?
Yes.
Is there also a more<string>-function?
No, not 'more<string>'. It's called greater<string>.
Ok.
So I also tried to look for it at www.cppreference.com which has
some nice descriptions but I couldn't find it...
Can anyone clarify this issue to me?
A good book on Standard Library can. Nicolai Josuttis published a
very good one.
I don't think I need a third book for such a simple question.
Oh, you already have two books? That should be plenty. There was an old
Russian joke when two policemen were trying to decide what to buy one of
their colleagues as a birthday present. One says, "Let's buy him a book".
The other one shakes his head and says, "Nah, he's already got one".
I would
guess that the definition of less is something like:
bool less(char *first, char *second)
{
int i;
bool lesser_than = 0;
for(i=0; i<strlen(...); i++)
{
// compare all letters
if(something)
lesser_than = 1;
}
return lesser_than;
}
But I guess I can see it somewhere, can't I?
If your standard library comes with the source code, you could look
there. If it doesn't, we can't help. Every implemntation is
different and there is no guarantee that if you find one somewhere,
the one your compiler uses would be the same.
Perhaps it's slightly
off-topic if it depends on my operating system but suppose I use both
g++ on mac/linux and visual studio 2005. Isn't those definitions
inside some \include\-directory ?
Possible. Again, it's only so if your library comes in source code.
Or should I ask in another group?
If you need some information for a particular compiler you should
ask in the newsgroup dedicated to that compiler. Here we can only
give you general C++ answers, information on how to do things portably
and what the Standard says about things in the language or the library
(but usually not how they are implemented, that's up to the compiler
and/or library vendors).
V
--
Please remove capital As from my address when replying by mail