Differentiate a "comparitor" vs a "filter". Both constructs qualify as "predicates", but if you think in terms of "predicates" you can't observe the difference between horse and donkey.

Wrong -- ~~STL predicates are all binary functions including std::less<>~~

Right -- STL filters are Unary; STL comparitors are Binary

P41 [[objectSpace]] lists the stl algorithms accepting Unary predicates. EffSTL P168 has example of custom functor class -- unary

Canonical example 1 -- Most stl algorithms need a .... filter -- Unary predicate operating on ONE element at a time.

* You are given a Binary function or functor

** either a std::less<>

** or a customized function or functor

* so you use bind2nd() to convert binary to unary

Canonical example 2 -- your sort() or associative container needs a .... comparitor -- binary predicate

* If you stop and think, a comparitor must take 2 inputs, so must be a binary predicate

* a comparitor can be a function or functor

* you don't need bind2nd()

Std::less<> or a custom binary functor can be used in filters and comparitors.

$ filter usage needs bind2nd

$ comparitor usage doesn't

Q: What if a STL algo needs a filter when you only have a binary predicate (including a comparitor)?

A: use bind1st and bind2nd. I guess the binary predicate needs to extend binary_function. See effSTL.

Note filter and comparitor are logical constructs or patterns. Physically they are implemented as function or functor returning **bool** (therefore known as predicate). Each STL algorithm or container requires either a real filter or a real comparitor, but physical implementation is not focus of this blog.

## Monday, April 20, 2009

### filter^comparitor -- 2 "genders" among STL predicates

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