Re: Idea for more delegations in <algorithm>

[Gabriel Dos Reis <gdr at integrable-solutions dot net>]

Chris Jefferson <caj@cs.york.ac.uk> writes:

| I've had a look at enable_if, and I think we can make use of it
| still. Unless someone says what is below is pointless / broken, I'll
| make a clean demo patch sometime tonight / tomorrow.
| 
| First a little background. If you have a function where the return
| type is enable_if<a,b> then if a::value=true then the return type is
| simply b. If however a::value=false, then due to magic of templates
| this function is simply ignored by the compiler! So if you have a
| bunch of identical functions except for the return value (which are
| all enable_ifed), then as long as at most one is ever true, you get
| dispatch!
| So consider a template
| 
| typename<typename It, typename tag, typename tag>
|    struct Between
|    { static const bool value = ?;};
| 
| Where value is true if It's tag type is between the two given
| iterator_tags (inclusive).
| 
| Then do
| 
| template<typename It, typename val>
| enable_if<Between<It, input_iterator_tag, forward_iterator_tag>, It>
| __find(It,It,val) {..}
| 
| typename<typename It, typename val>
| enable_if<Between<It, bidirectional_iterator_tag,
|                       random_access_iterator_tag, It>
| __find(It,It,val) {..}
| 
| typename<typename It, typename val>
| It find(It,It,val)
| { return __find(..); }
| 
| Of course, we could just use classes, but this seems much shorter :) The

Really? :-)

| only tiny problem is you have to remember to make sure you cover all the
| cases with your "Betweens" (that the algorithm accepts) and you don't
| cover any twice.. not too hard however!

-- Gaby