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eiji
Guest
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 Posted: Sun Nov 27, 2005 8:39 pm Post subject: sort / vector |
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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()(const 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
elements->push_back(new Element()); // n-times
std::vector<Element*>::const_iterator iterFirst = elements->begin() ;
std::vector<Element*>::const_iterator iterLast = elements->end() ;
std::sort(iterFirst, 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
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roberts.noah@gmail.com
Guest
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| Posted: Sun Nov 27, 2005 9:05 pm Post subject: Re: sort / vector |
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eiji wrote:
| Quote: | 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()(const T& T1, const T& T2) const
std::vector<Element*>* elements = new std::vector<Element*>(0);
std::sort(iterFirst, 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....therefore 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.
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Bob Hairgrove
Guest
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| Posted: Sun Nov 27, 2005 9:13 pm Post subject: Re: sort / vector |
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On 27 Nov 2005 12:39:10 -0800, "eiji" <SaLeuth (AT) gmx (DOT) de> wrote:
| Quote: | 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(){};
|
The destructor doesn't have to be virtual if you do not inherit any
other types from Compare<T>.
| Quote: |
bool operator()(const 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()(const T* T1, const T* T2) const
{return T1->GetNumber() < T2->GetNumber();}
| Quote: | 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.
| Quote: | std::vector<Element*>* elements = new std::vector<Element*>(0);
|
The default constructor will work fine, no need to pass an argument of
zero:
std::vector<Element*>* elements = new std::vector<Element*>;
| Quote: | elements->push_back(new 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.
| Quote: | std::vector<Element*>::const_iterator iterFirst = elements->begin() ;
std::vector<Element*>::const_iterator iterLast = elements->end() ;
std::sort(iterFirst, iterLast, Compare<Element>());
|
This should work if you (a) pass an object and (b) change the
implementation and signature of Compare<>::operator() to accept T*
instead of T&:
Compare<Element> cmp_obj;
std::sort(iterFirst, 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()...
| Quote: | 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
|
FYI -- übrigens gibt es auch eine deutsche NG für C++-Fragen,
de.comp.lang.iso-c++ ... aber hier bist Du mit Deinen Fragen natürlich
genauso willkommen, wie drüben! :)
--
Bob Hairgrove
[email]NoSpamPlease (AT) Home (DOT) com[/email]
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eiji
Guest
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| Posted: Sun Nov 27, 2005 9:14 pm Post subject: Re: sort / vector |
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| Quote: | there are certain rules about vector contents
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Nice to know? Any idea where to find them easily?
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Kai-Uwe Bux
Guest
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| Posted: Sun Nov 27, 2005 9:14 pm Post subject: Re: sort / vector |
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|
eiji wrote:
| Quote: | 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()(const 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
elements->push_back(new Element()); // n-times
std::vector<Element*>::const_iterator iterFirst = elements->begin() ;
std::vector<Element*>::const_iterator iterLast = elements->end() ;
std::sort(iterFirst, 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
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Mike Wahler
Guest
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| Posted: Sun Nov 27, 2005 9:17 pm Post subject: Re: sort / vector |
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"eiji" <SaLeuth (AT) gmx (DOT) de> wrote
| Quote: | 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
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Bob Hairgrove
Guest
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| Posted: Sun Nov 27, 2005 9:17 pm Post subject: Re: sort / vector |
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On 27 Nov 2005 13:05:10 -0800, [email]roberts.noah (AT) gmail (DOT) com[/email] wrote:
| Quote: | 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.
| Quote: | 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_ptr<>):
http://www.boost.org
Beware of std::auto_ptr, however -- that one doesn't work in STL
containers.
--
Bob Hairgrove
[email]NoSpamPlease (AT) Home (DOT) com[/email]
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eiji
Guest
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| Posted: Sun Nov 27, 2005 9:22 pm Post subject: Re: sort / vector |
|
|
Thanks a lot.
bool operator()(const T* T1, const T* T2) const
{return T1->GetNumber() < T2->GetNumber();}
and
std::vector<Element*>::iterator iterFirst = elements->begin() ;
std::vector<Element*>::iterator 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!
| Quote: | (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! |
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Bob Hairgrove
Guest
|
| Posted: Sun Nov 27, 2005 9:27 pm Post subject: Re: sort / vector |
|
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On 27 Nov 2005 13:22:30 -0800, "eiji" <SaLeuth (AT) gmx (DOT) de> wrote:
| Quote: | Thanks a lot.
bool operator()(const T* T1, const T* T2) const
{return T1->GetNumber() < T2->GetNumber();}
and
std::vector<Element*>::iterator iterFirst = elements->begin() ;
std::vector<Element*>::iterator 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).
| Quote: | 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
[email]NoSpamPlease (AT) Home (DOT) com[/email]
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roberts.noah@gmail.com
Guest
|
| Posted: Sun Nov 27, 2005 9:27 pm Post subject: Re: sort / vector |
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Bob Hairgrove wrote:
| Quote: | On 27 Nov 2005 13:05:10 -0800, [email]roberts.noah (AT) gmail (DOT) com[/email] 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
| Quote: | 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.
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Bob Hairgrove
Guest
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| Posted: Sun Nov 27, 2005 10:47 pm Post subject: Re: sort / vector |
|
|
On 27 Nov 2005 13:22:30 -0800, "eiji" <SaLeuth (AT) gmx (DOT) de> wrote:
| Quote: | Thanks a lot.
bool operator()(const T* T1, const T* T2) const
{return T1->GetNumber() < T2->GetNumber();}
and
std::vector<Element*>::iterator iterFirst = elements->begin() ;
std::vector<Element*>::iterator 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!
|
Duh ... I just realized why you cannot use a const_iterator:
std::sort() needs to reassign the vector elements in order to sort
them!
(Guess I shouldn't post so late at night!)
--
Bob Hairgrove
[email]NoSpamPlease (AT) Home (DOT) com[/email]
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