std::list.sort( compFunc ) error

[Tim Partridge]

I have a std list of student objects and I would like to sort them by
providing a comparison function. I have gotten it to work if my function
is global, but I would like to encapsulate it in a class. For example, my
attempt below fails, but if I move sortFunctor out of the class, it will
compile.

#include <list>
using namespace std;

class foo {
public:
foo(int i) : i(i) {};
~foo() {};
int i;

// compiles if this in the global namespace
bool sortFunctor(con st foo &f1, const foo &f2) {
return (f1.i < f2.i);
}
};

int main() {
list< foo > l;
foo f1(8);
foo f2(3);

l.push_back(f1) ;
l.push_back(f2) ;

// compiles if foo:: is removed
l.sort(foo::sor tFunctor);

return 0;
}

Regards,
Tim Partridge

[Karl Heinz Buchegger]

Re: std::list.sort( compFunc ) error

Tim Partridge wrote:[color=blue]
>
> I have a std list of student objects and I would like to sort them by
> providing a comparison function. I have gotten it to work if my function
> is global, but I would like to encapsulate it in a class. For example, my
> attempt below fails, but if I move sortFunctor out of the class, it will
> compile.[/color]

The trick is, that this class needs an operator()
[color=blue]
>
> #include <list>
> using namespace std;
>
> class foo {
> public:
> foo(int i) : i(i) {};
> ~foo() {};
> int i;
>
> // compiles if this in the global namespace
> bool sortFunctor(con st foo &f1, const foo &f2) {[/color]

bool operator() ( const foo& f1, const foo& f2 ) {
[color=blue]
> return (f1.i < f2.i);
> }
> };
>
> int main() {
> list< foo > l;
> foo f1(8);
> foo f2(3);
>
> l.push_back(f1) ;
> l.push_back(f2) ;
>
> // compiles if foo:: is removed
> l.sort(foo::sor tFunctor);[/color]

l.sort( foo() );
[color=blue]
>
> return 0;
> }
>[/color]

--
Karl Heinz Buchegger
kbuchegg@gascad .at

[Jeff Flinn]

Re: std::list.sort( compFunc ) error


"Karl Heinz Buchegger" <kbuchegg@gasca d.at> wrote in message
news:4011528D.D 7F8B30A@gascad. at...[color=blue]
> Tim Partridge wrote:[color=green]
> >
> > I have a std list of student objects and I would like to sort them by
> > providing a comparison function. I have gotten it to work if my function
> > is global, but I would like to encapsulate it in a class. For example,[/color][/color]
my[color=blue][color=green]
> > attempt below fails, but if I move sortFunctor out of the class, it will
> > compile.[/color]
>
> The trick is, that this class needs an operator()[/color]

Why?
[color=blue]
>[color=green]
> >
> > #include <list>
> > using namespace std;
> >
> > class foo {
> > public:
> > foo(int i) : i(i) {};
> > ~foo() {};
> > int i;
> >
> > // compiles if this in the global namespace
> > bool sortFunctor(con st foo &f1, const foo &f2) {[/color]
>
> bool operator() ( const foo& f1, const foo& f2 ) {
>[color=green]
> > return (f1.i < f2.i);
> > }[/color][/color]

What state from (*this) is being used here?

The only thing missing from the OP's class declaration/definition is that
sortFunctor should be static. I would rename sortFunctor to Compare, as it
is not a Functor, and it really just compares to foo's.

Better yet would be to make i private, and provide operator<. That way you
could just call l.sort().

Jeff F
[color=blue][color=green]
> > };
> >
> > int main() {
> > list< foo > l;
> > foo f1(8);
> > foo f2(3);
> >
> > l.push_back(f1) ;
> > l.push_back(f2) ;
> >
> > // compiles if foo:: is removed
> > l.sort(foo::sor tFunctor);[/color]
>
> l.sort( foo() );
>[color=green]
> >
> > return 0;
> > }
> >[/color]
>
> --
> Karl Heinz Buchegger
> kbuchegg@gascad .at[/color]

[Karl Heinz Buchegger]

Re: std::list.sort( compFunc ) error

Jeff Flinn wrote:[color=blue]
>
> "Karl Heinz Buchegger" <kbuchegg@gasca d.at> wrote in message
> news:4011528D.D 7F8B30A@gascad. at...[color=green]
> > Tim Partridge wrote:[color=darkred]
> > >
> > > I have a std list of student objects and I would like to sort them by
> > > providing a comparison function. I have gotten it to work if my function
> > > is global, but I would like to encapsulate it in a class. For example,[/color][/color]
> my[color=green][color=darkred]
> > > attempt below fails, but if I move sortFunctor out of the class, it will
> > > compile.[/color]
> >
> > The trick is, that this class needs an operator()[/color]
>
> Why?[/color]

Because the OP wanted to create a Functor. And a functor
is a class, which has such an operator.
[color=blue]
>[color=green]
> >[color=darkred]
> > >
> > > #include <list>
> > > using namespace std;
> > >
> > > class foo {
> > > public:
> > > foo(int i) : i(i) {};
> > > ~foo() {};
> > > int i;
> > >
> > > // compiles if this in the global namespace
> > > bool sortFunctor(con st foo &f1, const foo &f2) {[/color]
> >
> > bool operator() ( const foo& f1, const foo& f2 ) {
> >[color=darkred]
> > > return (f1.i < f2.i);
> > > }[/color][/color]
>
> What state from (*this) is being used here?[/color]

None.
A Functor is an object that can be used in templates when a function
is required.
[color=blue]
>
> The only thing missing from the OP's class declaration/definition is that
> sortFunctor should be static.[/color]

Thats a bad idea in the general case although it works in this
specific case.

--
Karl Heinz Buchegger
kbuchegg@gascad .at

[Karl Heinz Buchegger]

Re: std::list.sort( compFunc ) error

Jeff Flinn wrote:[color=blue]
>[/color]

Dear Jeff. Please disregard my previous post.

After reareading the OP post, I noticed that I completely
read a different program then was written down by the OP.

Somehow I got the expression, that the OP wanted to create
a functor class named foo.
Only after rereading I noticed that foo is the class the OP
builds his list with.


Sorry for the confusion.

--
Karl Heinz Buchegger
kbuchegg@gascad .at

[Karl Heinz Buchegger]

Re: std::list.sort( compFunc ) error

Karl Heinz Buchegger wrote:[color=blue]
>
> Jeff Flinn wrote:[color=green]
> >[/color]
>
> Dear Jeff. Please disregard my previous post.
>
> After reareading the OP post, I noticed that I completely
> read a different program then was written down by the OP.
>
> Somehow I got the expression, that the OP wanted to create[/color]

typed to quick: ... got the impression, ...


--
Karl Heinz Buchegger
kbuchegg@gascad .at

[Jeff Flinn]

Re: std::list.sort( compFunc ) error


"Karl Heinz Buchegger" <kbuchegg@gasca d.at> wrote in message
news:40117624.A B09FF86@gascad. at...[color=blue]
> Jeff Flinn wrote:[color=green]
> >[/color]
>
> Dear Jeff. Please disregard my previous post.
>
> After reareading the OP post, I noticed that I completely
> read a different program then was written down by the OP.
>
> Somehow I got the expression, that the OP wanted to create
> a functor class named foo.
> Only after rereading I noticed that foo is the class the OP
> builds his list with.[/color]


Keine Probleme.

The OP's choice of name "sortFuncto r" didn't help.

Jeff F