Operater < overloading for struct problem

[Frank]

Hello everyone,

I am having trouble overloading the < operator for an assignment. I
use a struct that contains information and I would like to sort this
structure using STL sort with my own criteria of sorting. Basically, I
would like to sort on visitor count of the Attraction structure.
However, it never uses the < overloaded operator with my code.

Handler.h:

#ifndef H_HANDLER //Guard
#define H_HANDLER

#include <iostream>
#include <vector>
using namespace std;

struct Attraction {
string name;
int visitors;
};

class Handler { //Define the class Handler

private:
vector<Attracti on*attractions ;
vector<Attracti on*>::iterator p ;

public: //Public functions
~Handler();
void addAttraction(s tring name, int visitors);
void printAttraction s();
};
#endif

and in the Handler.cpp I have:

Handler.cpp - snippet:

bool operator<(const Attraction& a,const Attraction& b){
return a.visitors < b.visitors;
}

Here is the function that performs the sort after adding a value:

void Handler::addAtt raction(string name, int visitors){

Attraction *attr;
attr=new Attraction();

attr->name=name;
attr->visitors=visit ors;
attractions.pus h_back(attr);
sort(attraction s.begin(),attra ctions.end());

}

However, whatever I do, it will never use the overloaded < operator
for sorting. What am I doing wrong? If I add the overloaded function
in the header it starts complaining because it will also be inserted
into the main program which is confusing since I have a guard around
the header file.

Regards,
Frank

[Victor Bazarov]

Frank wrote:

I am having trouble overloading the < operator for an assignment. I
use a struct that contains information and I would like to sort this
structure using STL sort with my own criteria of sorting. Basically, I
would like to sort on visitor count of the Attraction structure.
However, it never uses the < overloaded operator with my code.

Handler.h:

#ifndef H_HANDLER //Guard
#define H_HANDLER

#include <iostream>
#include <vector>
using namespace std;

struct Attraction {
string name;
int visitors;
};

class Handler { //Define the class Handler

private:
vector<Attracti on*attractions ;
vector<Attracti on*>::iterator p ;

public: //Public functions
~Handler();
void addAttraction(s tring name, int visitors);
void printAttraction s();
};
#endif

and in the Handler.cpp I have:

Handler.cpp - snippet:

bool operator<(const Attraction& a,const Attraction& b){
return a.visitors < b.visitors;
}

Here is the function that performs the sort after adding a value:

void Handler::addAtt raction(string name, int visitors){

Attraction *attr;
attr=new Attraction();

attr->name=name;
attr->visitors=visit ors;
attractions.pus h_back(attr);
sort(attraction s.begin(),attra ctions.end());

}

However, whatever I do, it will never use the overloaded < operator
for sorting. What am I doing wrong?

Your operator< is defined for _objects_, but your vector is storing
_pointers_. Drop the 'new', drop the pointers, keep objects, and
everything will be fine.

You can declare/define a local Attraction object in 'addAttraction'
and then push_back it, the vector will make a copy.

[..]

V
--
Please remove capital 'A's when replying by e-mail
I do not respond to top-posted replies, please don't ask

[mlimber]

On Mar 1, 10:45 am, "Frank" <f.f.nee...@gma il.comwrote:

Hello everyone,

I am having trouble overloading the < operator for an assignment. I
use a struct that contains information and I would like to sort this
structure using STL sort with my own criteria of sorting. Basically, I
would like to sort on visitor count of the Attraction structure.
However, it never uses the < overloaded operator with my code.

Handler.h:

#ifndef H_HANDLER //Guard
#define H_HANDLER

#include <iostream>
#include <vector>
using namespace std;

struct Attraction {
string name;
int visitors;

};

class Handler { //Define the class Handler

private:
vector<Attracti on*attractions ;
vector<Attracti on*>::iterator p ;

public: //Public functions
~Handler();
void addAttraction(s tring name, int visitors);
void printAttraction s();

};

#endif

and in the Handler.cpp I have:

Handler.cpp - snippet:

bool operator<(const Attraction& a,const Attraction& b){
return a.visitors < b.visitors;
}

Here is the function that performs the sort after adding a value:

void Handler::addAtt raction(string name, int visitors){

Attraction *attr;
attr=new Attraction();

attr->name=name;
attr->visitors=visit ors;

attractions.pus h_back(attr);
sort(attraction s.begin(),attra ctions.end());

}

However, whatever I do, it will never use the overloaded < operator
for sorting. What am I doing wrong? If I add the overloaded function
in the header it starts complaining because it will also be inserted
into the main program which is confusing since I have a guard around
the header file.

I suspect the fundamental problem is that you are sorting pointers
rather than objects. In other words, std::sort works on the value_type
of the container, which in your case is Attraction*, not Attraction.

So, do you need to operator on the Attraction object polymorphically ?
If not, don't use pointers; just allocate it on the stack, and let
vector make a copy (assuming attractions is now of type
vector<Attracti on>:

void Handler::addAtt raction( const string& name, const int visitors )
{
const Attraction attr = { name, visitors };
attractions.pus h_back(attr);
sort(attraction s.begin(),attra ctions.end());
}

Note also that I changed the function parameters to const (see
http://www.parashift.com/c++-faq-lit...rrectness.html) and the
string to a reference so it doesn't make an additional copy (see
http://www.parashift.com/c++-faq-lite/references.html).

If you do need to use it polymorphically and to store it in a vector,
prefer smart pointers such as std::tr1::share d_ptr (aka
boost::shared_p tr) or one of the smart pointers found in this FAQ and
those following:

http://www.parashift.com/c++-faq-lit...html#faq-16.22

BTW, you could put just the function prototype for your operator< in
the header or you could declare it "inline" to get rid of the
duplication problem.

Cheers! --M

[red floyd]

Frank wrote:

Victor and mlimber have addressed your main issue. I'd like to address
one other item.

Handler.h:

#ifndef H_HANDLER //Guard
#define H_HANDLER

#include <iostream>
#include <vector>
using namespace std;

Do *NOT* put "using namespace std;" into a header file. It can
seriously mess up other code. It should only go into non-header files,
after all include directives.

[Frank]

On 1 mrt, 17:35, red floyd <no.s...@here.d udewrote:

Frank wrote:

Victor and mlimber have addressed your main issue. I'd like to address
one other item.

Handler.h:

#ifndef H_HANDLER //Guard
#define H_HANDLER

#include <iostream>
#include <vector>
using namespace std;

Do *NOT* put "using namespace std;" into a header file. It can
seriously mess up other code. It should only go into non-header files,
after all include directives.

Thanks everyone for the quick response!
It works like a charm now,
Regards,
Frank

[Roland Pibinger]

On Thu, 1 Mar 2007 10:56:53 -0500, "Victor Bazarov" wrote:

>Your operator< is defined for _objects_, but your vector is storing
_pointers_. Drop the 'new', drop the pointers, keep objects, and
everything will be fine.

Except that objects in the OO sense like Attraction, Handler, ...
usually are not copyable.

>You can declare/define a local Attraction object in 'addAttraction'
and then push_back it, the vector will make a copy.

That's the problem.

Best regards,
Roland Pibinger

[Victor Bazarov]

Roland Pibinger wrote:

On Thu, 1 Mar 2007 10:56:53 -0500, "Victor Bazarov" wrote:

>Your operator< is defined for _objects_, but your vector is storing
_pointers_. Drop the 'new', drop the pointers, keep objects, and
everything will be fine.

Except that objects in the OO sense like Attraction, Handler, ...
usually are not copyable.

"Usually"? What in the world do you mean? See to original post for
clarification.

>You can declare/define a local Attraction object in 'addAttraction'
and then push_back it, the vector will make a copy.

That's the problem.

REALLY? Do you mean that the OP's Attraction object is non-copiable?
Could you please elaborate?

V
--
Please remove capital 'A's when replying by e-mail
I do not respond to top-posted replies, please don't ask

[mlimber]

On Mar 1, 2:31 pm, "Victor Bazarov" <v.Abaza...@com Acast.netwrote:

Roland Pibinger wrote:

On Thu, 1 Mar 2007 10:56:53 -0500, "Victor Bazarov" wrote:

Your operator< is defined for _objects_, but your vector is storing
_pointers_. Drop the 'new', drop the pointers, keep objects, and
everything will be fine.

Except that objects in the OO sense like Attraction, Handler, ...
usually are not copyable.

"Usually"? What in the world do you mean? See to original post for
clarification.

You can declare/define a local Attraction object in 'addAttraction'
and then push_back it, the vector will make a copy.

That's the problem.

REALLY? Do you mean that the OP's Attraction object is non-copiable?
Could you please elaborate?

Roland's just being an OO purist. Objects in his world are always
polymorphic, but in C++'s multiple paradigms, they are not. In this
case, I don't think the OP was intending to use pure OO anyway, so our
answers sufficed.

Cheers! --M

[Roland Pibinger]

On Thu, 1 Mar 2007 14:31:38 -0500, "Victor Bazarov" wrote:

>Roland Pibinger wrote:

>On Thu, 1 Mar 2007 10:56:53 -0500, "Victor Bazarov" wrote:

>>You can declare/define a local Attraction object in 'addAttraction'
and then push_back it, the vector will make a copy.

That's the problem.

REALLY? Do you mean that the OP's Attraction object is non-copiable?
Could you please elaborate?

Objects (in the OO sense) are characterized by identity, state and
behavior. When you duplicate (copy) objects you get into trouble with
identity and state, e.g.

Account a1 (12345);
Account a2 = a1;
a1.deposit (100);
a2.withdraw (200);

What does it mean to copy the account with account number 12345? What
is the balance of this account now? To answer your question,
Attraction (whatever that exactly means in the example) is an object
(in the sense of OO) and should not be copied.

Best regards,
Roland Pibinger

[Roland Pibinger]

On 1 Mar 2007 12:12:00 -0800, "mlimber" <ml*****@gmail. comwrote:

>Roland's just being an OO purist. Objects in his world are always
polymorphic, but in C++'s multiple paradigms, they are not.

So 'C++'s multiple paradigms' have changed the paradigms? C++ has
changed the meaning of object (OO) and polymorphism?