2008/11/6 Steven Watanabe <watanabesj@gmail.com>:
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template<class Domain, class Codomain, ...> class interval_map{ //(3) Adaptable functor template<template<class>class Combinator> interval_map& add(const value_type& value_pair, const Combinator<Codomain>& combine) { /*combine functor passed or called somewhere*/ }
Can't you use:
template<class Combinator> interval_map& add(const value_type& value_pair, const Combinator& combine); ? I could
In other words, does Combinator really have to be a template?
No it doesn't. But any Combinator has to combine Codomain values. I feel that this is expressed more precisely in definition (3).
Now, there seems to be a convention from the STL that functors shall be passed BY VALUE.
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I do not really understand this convention and feel some resistance to follow it. In addition the call by value implementation can (and will) lead to heavy inefficiency by unaware usage of fat functors.
The assumption is that function objects are not fat. If you have a large function object, it is always possible to create another function object that just stores a reference to the original function object.
Unfortunately the assumption can not be that all programmers are always aware of the many c++ intricacies. There's a rule stated by Scott Meyers in one of his books: "Make your classes easy to use correctly and difficult to use incorrectly." In common, I would expect, that class objects are passed by reference. So in the struggle to meet the next deadline one could unknowingly code m.add(some, add_if_in(big_set)); where add_if_in contains the a big set which then is copied and again copied with every subsequent functor passing inside add. The perfect programmer won't do that, but this library code is IMO easy to use incorrectly. Joachim