sort / vector

[eiji]

Hi folks,
I have a problem compiling this under VC6!
Maybe someone has some minutes to look at this:

template<class T> class Compare
{
public:
Compare(){};
virtual ~Compare(){};

bool operator()(cons t T& T1, const T& T2) const
{return T1.GetNumber() < T2.GetNumber(); }
};

class Element
{
public:
Element(Model* model, unsigned int number);
virtual ~Element();

unsigned int GetNumber() const { return _number;}
}

std::vector<Ele ment*>* elements = new std::vector<Ele ment*>(0);

elements->push_back(ne w Element()); // n-times
std::vector<Ele ment*>::const_i terator iterFirst = elements->begin() ;
std::vector<Ele ment*>::const_i terator iterLast = elements->end() ;

std::sort(iterF irst, iterLast, Compare<Element >());

My Compiler tells me(sorry my compiler speaks german, but it is
readable):
error C2664: '()' : Konvertierung des Parameters 1 von 'class
fesolv::Element *' in 'const class fesolv::Element &' nicht moeglich
Ursache: Konvertierung von 'class fesolv::Element *' in 'const
class fesolv::Element ' nicht moeglich

I don't know where to start.
Do sort also works for vector<T*>* or only for vector<T*> ????

Thanks,
eiji

[roberts.noah]

eiji wrote:

Hi folks,
I have a problem compiling this under VC6!
Maybe someone has some minutes to look at this:

template<class T> class Compare
{
bool operator()(cons t T& T1, const T& T2) const std::vector<Ele ment*>* elements = new std::vector<Ele ment*>(0); std::sort(iterF irst, iterLast, Compare<Element >());

Notice that your compare function expects references but your vector
contains pointers. The "T" for your compare would want to be Element*
and then of course the body of that operator would need
changing....the refore a seperate ComparePtr functor may be in order.

Keeping pointers in a vector is *generally* a bad move. There are of
course always exceptions to any rule but when you are going to be
keeping pointers in a vector try to find a different way. One way to
do so without all the pita that goes with it is to create a smart
pointer that can be contained in a vector (there are certain rules
about vector contents). I'm not even sure pointers meet those reqs.

[Bob Hairgrove]

On 27 Nov 2005 12:39:10 -0800, "eiji" <Sa*****@gmx.de > wrote:

Hi folks,
I have a problem compiling this under VC6!
Maybe someone has some minutes to look at this:

template<cla ss T> class Compare
{
public:
Compare(){};
virtual ~Compare(){};
The destructor doesn't have to be virtual if you do not inherit any
other types from Compare<T>.
bool operator()(cons t T& T1, const T& T2) const
{return T1.GetNumber() < T2.GetNumber(); }
};
This won't work for pointers; either you'll need to change this to
accommodate T*, or you will need a second type to deal with them,
e.g.:

bool operator()(cons t T* T1, const T* T2) const
{return T1->GetNumber() < T2->GetNumber(); }
class Element
{
public:
Element(Model* model, unsigned int number);
virtual ~Element();

unsigned int GetNumber() const { return _number;}
}
Some questions/remarks:

(1) Where is _number declared?
(2) You should never use names beginning with an underscore for
anything (see section 17.4.3.1.2 of the C++ for the reason).
(3) If you never want to construct an Element with no arguments, you
should declare a default constructor, but make it private and do not
implement it. This will prevent the compiler from giving you a default
constructor that you don't want.
std::vector<El ement*>* elements = new std::vector<Ele ment*>(0);
The default constructor will work fine, no need to pass an argument of
zero:

std::vector<Ele ment*>* elements = new std::vector<Ele ment*>;
elements->push_back(ne w Element()); // n-times
Hmmm ... I see we have a problem here if you want to hide the default
constructor ... but if you provide initializers/arguments, it's OK.
std::vector<El ement*>::const_ iterator iterFirst = elements->begin() ;
std::vector<El ement*>::const_ iterator iterLast = elements->end() ;

std::sort(iter First, iterLast, Compare<Element >());
This should work if you (a) pass an object and (b) change the
implementation and signature of Compare<>::oper ator() to accept T*
instead of T&:

Compare<Element > cmp_obj;
std::sort(iterF irst, iterLast, cmp_obj);

It might also work with Compare<Element >(), but you are creating a
temporary. I don't know why, but I always seem to have trouble when
passing a temporary object to std::sort()...
My Compiler tells me(sorry my compiler speaks german, but it is
readable):
error C2664: '()' : Konvertierung des Parameters 1 von 'class
fesolv::Elemen t *' in 'const class fesolv::Element &' nicht moeglich
Ursache: Konvertierung von 'class fesolv::Element *' in 'const
class fesolv::Element ' nicht moeglich

I don't know where to start.
Do sort also works for vector<T*>* or only for vector<T*> ????

Thanks,
eiji

FYI -- übrigens gibt es auch eine deutsche NG für C++-Fragen,
de.comp.lang.is o-c++ ... aber hier bist Du mit Deinen Fragen natürlich
genauso willkommen, wie drüben! :)

--
Bob Hairgrove
No**********@Ho me.com

[eiji]

>there are certain rules about vector contents

Nice to know? Any idea where to find them easily?

[Kai-Uwe Bux]

eiji wrote:

Hi folks,
I have a problem compiling this under VC6!
Maybe someone has some minutes to look at this:

template<class T> class Compare
{
public:
Compare(){};
virtual ~Compare(){};

bool operator()(cons t T& T1, const T& T2) const
{return T1.GetNumber() < T2.GetNumber(); }
};

class Element
{
public:
Element(Model* model, unsigned int number);
virtual ~Element();

unsigned int GetNumber() const { return _number;}
}

std::vector<Ele ment*>* elements = new std::vector<Ele ment*>(0);

elements->push_back(ne w Element()); // n-times
std::vector<Ele ment*>::const_i terator iterFirst = elements->begin() ;
std::vector<Ele ment*>::const_i terator iterLast = elements->end() ;

std::sort(iterF irst, iterLast, Compare<Element >());

My Compiler tells me(sorry my compiler speaks german, but it is
readable):
error C2664: '()' : Konvertierung des Parameters 1 von 'class
fesolv::Element *' in 'const class fesolv::Element &' nicht moeglich
Ursache: Konvertierung von 'class fesolv::Element *' in 'const
class fesolv::Element ' nicht moeglich

I don't know where to start.
Do sort also works for vector<T*>* or only for vector<T*> ????

Since your vector has T* elements, your comparison function needs to take T*
as arguments.
Best

Kai-Uwe Bux

[Mike Wahler]

"eiji" <Sa*****@gmx.de > wrote in message
news:11******** **************@ g47g2000cwa.goo glegroups.com.. .

there are certain rules about vector contents

Nice to know? Any idea where to find them easily?

Objects to be stored in standard library containers
(e.g. vectors) must be copyable and assignable.

-Mike

[Bob Hairgrove]

On 27 Nov 2005 13:05:10 -0800, ro**********@gm ail.com wrote:

Keeping pointers in a vector is *generally* a bad move.
Why? People do it all the time. As long as the lifetime of the objects
pointed to is well-managed, there shouldn't be a problem, and it is
the only way to have a container of polymorphic types.
There are of
course always exceptions to any rule but when you are going to be
keeping pointers in a vector try to find a different way. One way to
do so without all the pita that goes with it is to create a smart
pointer that can be contained in a vector (there are certain rules
about vector contents). I'm not even sure pointers meet those reqs.

No need to create a smart pointer that works in containers, it's
already been done by the Boost folks (look for boost::shared_p tr<>):

http://www.boost.org

Beware of std::auto_ptr, however -- that one doesn't work in STL
containers.

--
Bob Hairgrove
No**********@Ho me.com

[eiji]

Thanks a lot.

bool operator()(cons t T* T1, const T* T2) const
{return T1->GetNumber() < T2->GetNumber(); }
and
std::vector<Ele ment*>::iterato r iterFirst = elements->begin() ;
std::vector<Ele ment*>::iterato r iterLast = elements->end() ;
(no const iterators)
and
Compare<Element > cmp_obj;

works fine!

I didn't put in all code, because the classes are quit big!

(2) You should never use names beginning with an underscore for
anything (see section 17.4.3.1.2 of the C++ for the reason).

Nice to know!

[Bob Hairgrove]

On 27 Nov 2005 13:22:30 -0800, "eiji" <Sa*****@gmx.de > wrote:

Thanks a lot.

bool operator()(cons t T* T1, const T* T2) const
{return T1->GetNumber() < T2->GetNumber(); }
and
std::vector<El ement*>::iterat or iterFirst = elements->begin() ;
std::vector<El ement*>::iterat or iterLast = elements->end() ;
(no const iterators)
It should also work with const_iterator, I believe, although Scott
Meyers (among others) recommends to "prefer iterator over
const_iterator" (from an old "C++ Users Journal" article).
and
Compare<Elemen t> cmp_obj;

works fine!

I didn't put in all code, because the classes are quit big!

(2) You should never use names beginning with an underscore for
anything (see section 17.4.3.1.2 of the C++ for the reason).

Nice to know!

--
Bob Hairgrove
No**********@Ho me.com

[roberts.noah]

Bob Hairgrove wrote:

On 27 Nov 2005 13:05:10 -0800, ro**********@gm ail.com wrote:

Keeping pointers in a vector is *generally* a bad move.
Why? People do it all the time. As long as the lifetime of the objects
pointed to is well-managed, there shouldn't be a problem,

That IS the problem.

and it is the only way to have a container of polymorphic types.

Better to wrap in a smart pointer. There are exceptions when you don't
need/want a smart pointer but imo most of the time you should.
Overhead is minimal compared to the time it can save in ownership
issues.