What support for STL sets?

[feraudyh]

Hello,
I need to store ordered sets of various types so I thought STL sets would be just the thing, so I wrote a test programme that began like this:

#include <QtCore/QCoreApplication>
#include <set>
#include <iterator>
#include <iostream>
using std::cout;
using std::endl;
 
int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);
    set<int> intSet1;
// more follows

#include <QtCore/QCoreApplication>
#include <set>
#include <iterator>
#include <iostream>
using std::cout;
using std::endl;

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);
    set<int> intSet1;
// more follows

To copy to clipboard, switch view to plain text mode 

Unfortunately the compiler tells me that "set was not declared in this scope".

Is STL not supported, or have I made a mistake?
I'm using QTCreator++ 1.3.0 with MinGW in Windows XP

[SixDegrees]

set is part of the std namespace; you need to refer to it as std::set, or work around it with a 'using' statement as you have for cout and cerr. I'd recommend explicitly qualifying it with std::set, given the commonality of the name.

[feraudyh]

Thankyou,
My programme works now, and I appreciate the advice concerning the systematic use of std::set.

[feraudyh]

OK my programme with set<int> was just a warm-up exercise.
What I'd like to do now is create a set whose comparison criterion is based on a Function object, (aka a Functor).
The aim is to compare two dimensional points by comparing their distance from a base point, The base point is something that is going to be moving in the program, so this is implemented as a private field m_basepoint of a class.
The comparison is implemented by overloading the () operator something like this:

bool operator()(QPointF p1,QPointF p2){return distance(m_basepoint,p1) < distance(m_basepoint,p2);}

bool operator()(QPointF p1,QPointF p2){return distance(m_basepoint,p1) < distance(m_basepoint,p2);}

To copy to clipboard, switch view to plain text mode 

(where distance is an obvious distance function) and putting this in a class CompareToBase
Unfortunately
it seems that

set<Type, Functor>

set<Type, Functor>

To copy to clipboard, switch view to plain text mode 

is not foreseen in STL.
also

set<QPointF,CompareToBase>

set<QPointF,CompareToBase>

To copy to clipboard, switch view to plain text mode 

has a problem: how do you set the base_point?

[Zlatomir]

Your operator() should return bool.

[feraudyh]

Yes, that was a typo. I fixed it, thanks.

[Zlatomir]

What is the issue then? You don't know how to initialize the set?

set<QPointF,CompareToBase> S(a, a+N); //where a is  OPointF array of N elements // or for other containers use iterators .begin(), .end()

set<QPointF,CompareToBase> S(a, a+N); //where a is  OPointF array of N elements // or for other containers use iterators .begin(), .end()

To copy to clipboard, switch view to plain text mode 

LE: Documentation for set

[feraudyh]

Well, the issue is how to set the "memory" or internal state of the comparison, i.e. the value of m_basepoint.
It would be nice if I could set this though set initialisation, but I didnt know i could, I will have another look at the documentation for set.
I suppose you mean the SGI documentation.

[Zlatomir]

m_basepoint has nothing to do with the set, isn't it in the implementation of the class that will be contained in the set? And i recommend to initialize all objects that you create, especially if you want to put them in a std::set (witch must compare objects, so: you can't add uninitialized objects to a set)

And, be careful with set, because it can contain an object only once, if this is a problem then you should look for std::multiset. Choose your containers with care.

The SGI is an implementation of STL, STL itself is only a paper, just like C++ standard, but the container interface should be the same in SGI implementation, GCC, QT, or Microsoft (i think this is done be Dinkumware) or anybody-else who implement the STL...

[feraudyh]

Yes, m_compare has nothing to do with the set, if we are thinking of mathematical object but it does have an impact on the ordering. You see these sets are not just mathematical sets but they are ordered using a binary tree.
For me the order is very important.

[Zlatomir]

For me the order is very important.

That is why you need to have the elements initialized before you can add them to std::set, or std::multiset.

The set can't order your objects if they don't have a value, so the data member must have a value so that set can sort your objects.
The set doesn't know anything about your data, it just calls the CompareToBase functor, witch must be able to tell if one object is smaller (or not) then the other objects that are in set.

So: you need to have the objects created and initialized (read them form keyboard or from file etc) and then you put them in set/multiset (witch will sort them).

Or, if you are not comfortable yet with set/multiset, use an c-style array of your data type and sort it with std::sort like this:

std::sort(ar, ar+N, CompareToBase); // ar is the array, N is array size, and your CompareToBase functor

std::sort(ar, ar+N, CompareToBase); // ar is the array, N is array size, and your CompareToBase functor

To copy to clipboard, switch view to plain text mode 

or an std::vector, like this:

std::sort(vec.begin(),  vec.end(), CompareToBase); // vec is std::vector<your_data_type>

std::sort(vec.begin(),  vec.end(), CompareToBase); // vec is std::vector<your_data_type>

To copy to clipboard, switch view to plain text mode 

You need: #include <algorithm> if you use std::sort, anyway you need your data to be initialized, and vector and array don't sort automatically as you add elements.

[feraudyh]

Ok, let's suppose my operator returns bool as I should have written.
Somehow going through the STL documentation, I have this feeliing I am going to have to pull out tricks like the use of
bind2nd or something concerning adaptable functors....
Am I looking in the right direction?
All of this brings to mind the Lambda Calculus in disguise.