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From: Rich Sposato (rds_at_[hidden])
Date: 2004-02-03 18:09:17

Stefan Slapeta wrote:
> Rich Sposato wrote:
> > 1. You can search on any type that is comparable to the key_type,
> > than just the key_type. (The STL associative containers only allows
> > searches on the same type as the key.)

> BTW, there is also an extention in STLPort which supports
comparisons like > this.

When I asked about this at STLport, they said they don't have extensions
to the STL. Maybe I did not ask the right question.

> I often wondered why this isn't supported by the standard containers.
> Any idea?

I must caution that flex_set has a minor disadvantage, a problem which
std::set does not have. For primitive types, flex_set will cause
code-bloat, while std::set will not. I don't use flex_set with
primitive types because the compiler will make 2 functions out of this
code for flex_set< long >, while it only makes one function using
std::set< long >.

typedef std::set< long > bunchOfLongs;
long a = 1;
short b = 2;
bunchOfLongs.erase( a );
bunchOfLongs.erase( b );

The compiler will see
flex_set< long >::erase( const long & );
as different from
flex_set< long >::erase( const short & );

But, a compiler will see both of those calls above as:
std:set< long >::erase( const long & );

For that reason, I can't advocate changing std::set's functions to be
templated the way flex_set functions are. I suppose if it had templated
functions like flex_set, there would be no need for flex_set, and
somebody would some day advocate using a less-templated class called
no_bloat_set. :-)

I actually consider the bloat issue as minor because most of the
functions are inline and small. Also, I rarely do a search using more
than one type anyway. My typical usage with flex_set and related
classes is to insert using the key_type, and find, count, and erase
using a different type.

Sometimes, the amount of code actually decreases inspite of a little
bloat because I don't have to construct a temporary anymore.

I think that could answer why the associative containers like map do not
have templated functions. As to the other container types, I don't
think they will need templated search functions. If you use the
sequential containers, you can use the stand-alone find, count,
binary_search, etc... functions and pass in a predicate.


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