Re: sorting stl list with predicate

From:
James Kanze <james.kanze@gmail.com>
Newsgroups:
comp.lang.c++
Date:
Fri, 30 Jan 2009 02:02:29 -0800 (PST)
Message-ID:
<559612a8-3d33-48b9-9f77-e10e45ae8a29@w1g2000prm.googlegroups.com>
On Jan 29, 10:33 am, ManicQin <Manic...@gmail.com> wrote:

On Jan 29, 11:13 am, James Kanze <james.ka...@gmail.com> wrote:

On Jan 28, 6:46 pm, Juha Nieminen <nos...@thanks.invalid> wrote:

ManicQin wrote:

struct greaterScale : public std::greater<PCOperation>

Btw, you don't have to inherit from std::greater (or any
comparator in the STL) in order to write a comparator.
This is template metaprogramming, not object-oriented
programming.


True, but providing additional information in the form of
typedefs is sometimes useful. I'd generally inherit from
std::binary_operator< Operation*, Operation*, bool > for
example. Since std::greater< Operation* > inherits from
this, he's effectively done so, with less characters to
type, but with the result of misleading the reader (since
his object manifestly has nothing to do with std::greater).

If you're doing much of this sort of thing, it might be
worth defining a ComparisonOperator class template:

    template< typename ArgumentType >
    struct ComparisonOperator
        : std::binary_operator< ArgumentType, ArgumentType, bool >
    {
    } ;

and inheriting from it.

(I generally define a compare() member function, then derive
from a ComparisonOperators class template which defines all
of the comparison operators in terms of compare(), and
provides the typedefs.)


Ok I agree with that point as an advice for good programming
I'll take it to my consideration. But I still have the
problem i stated above that when I try to inherit from
binary_function I get
error C2664: 'void __thiscall std::list<class COperation *,class
std::allocator<class COperation *> >::sort(struct std::greater<class
COperation *>)' : cannot convert parameter 1 from 'struct
greaterScale'
 to 'struct std::greater<class COperation *>


It's your typedef's which are causing the confusion, I suspect.
Drop the typedefs, and the problem is obvious. You have a list
of COperation*, and you try to sort it using an operator which
can only be called with COperation const*. Drop the references,
or make them references to COperation*, and everything should be
fine (once you've corrected the predicate so that it is
conform). You could also make the references references to a
const, so that you could initialize them with the results of the
conversion COperation* to COperation const*. But I'd just drop
the reference.

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