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Math Preregistration Syst
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 Posted: Fri Jun 27, 2003 3:27 pm Post subject: template <class C> void list::sort( C fn ) question |
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I'm using a std::list as a container for some pointers to objects, for
example
list< C* > lst;
I would like to sort them using two different criteria, say first by
C.first and then by C.second. Is it possible to use template <class C>
void list::sort( C fn ) and define two different fns, and use them
something like
lst.sort< C >( sortOnFirst() );
lst.sort< C >( sortOnSecond() ):
or am I not on the right track? I don't understand exactly how this
works. Alternatively, I can use a vector and std::sort() if you can
convince me it would be a better idea, but the main point is that I need
to sort according to two different criteria.
Thanks for you input!
Tim Partridge
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Rolf Magnus
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| Posted: Fri Jun 27, 2003 4:40 pm Post subject: Re: template <class C> void list::sort( C fn ) question |
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Ron Natalie wrote:
| Quote: |
"Math Preregistration System" <preregd1 (AT) beta (DOT) math.uwaterloo.ca> wrote
in message news:bdhno8$pon$1 (AT) tabloid (DOT) uwaterloo.ca...
lst.sort< C >( sortOnFirst() );
lst.sort< C >( sortOnSecond() ):
or am I not on the right track? I don't understand exactly how this
works.
You need to define sortOnFirst, you don't want to put the parens
there.
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You do if you make them function objects.
| Quote: | You haven't give any clue as to the nature of C, but your functions
should look something like:
bool sortOnFirst(const C& left, const C& right) {
// return true if left is logically less than right in this
sorting order
// ...
}
lst.sort<C>(sortOnFirst)
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Or:
struct sortOnFirst : public std::binary_function<C, C, bool>
{
bool operator()(const C& left, const C& right) const
{
// return true if left is logically less than right in this
sorting order
// ...
}
};
lst.sort(sortOnFirst());
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Rolf Magnus
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| Posted: Fri Jun 27, 2003 4:42 pm Post subject: Re: template <class C> void list::sort( C fn ) question |
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Klaus Eichner wrote:
| Quote: | inline bool sortOnFirst_then_Second(C* a, C* b)
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Note that the inline keyword here doesn't provide any benefit, since the
function will be called through a pointer, which excludes the
possibility to expand the function inline. If you want that, you need a
function object.
| Quote: | {
if (sortOnFirst(a,b)) return true;
else if (sortOnFirst(b,a)) return false;
else {// ...here 'a' and 'b' are considered
// to be "equal" according to
// sortOnFirst, therefore we have
// to consult sortOnSecond...
return sortOnSecond(a,b);
}
...and executing std::list::sort only once ?
lst.sort(sortOnFirst_then_Second);
Thanks for you input!
Tim Partridge
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Ron Natalie
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| Posted: Fri Jun 27, 2003 5:45 pm Post subject: Re: template <class C> void list::sort( C fn ) question |
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"Rolf Magnus" <ramagnus (AT) t-online (DOT) de> wrote
| Quote: | Klaus Eichner wrote:
inline bool sortOnFirst_then_Second(C* a, C* b)
Note that the inline keyword here doesn't provide any benefit, since the
function will be called through a pointer, which excludes the
possibility to expand the function inline. If you want that, you need a
function object.
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That and the objects it wants to pass to it are C's not C*'s.
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