On Tue, Sep 28, 2010 at 1:48 PM, joel falcou <joel.falcou@lri.fr> wrote:
On 28/09/10 19:37, David Abrahams wrote:
This BehaveAsPair thing doesn't identify any real abstraction. I promise you're not going to come across any uses for BehaveAsPair other than in solving the exact implementation problem you're attacking.
I see
In short, it lacks "truthiness." :-)
Agreed
Was there something unsatisfying about the solution I posted in http://permalink.gmane.org/gmane.comp.lib.boost.devel/209012 ?
Nope, I just missed it.
Oh wait! I think I have another example that's more truthy. :) I'll use Boost.ConceptCheck to flesh it out. Let's define the concept of an AdditivePair to represent the arguments to binary addition. To model the concept a type needs to support first(x), second(x) and add(x) for an object x where all three functions have integer values. So, the (simplest) concept checking class would be: template <class T> struct AdditivePair { BOOST_CONCEPT_USAGE(AdditivePair) { int a = first(x); int b = second(x); int c = add(x); } T x; }; We can now write Eric's function using concept requirements. template<class T> BOOST_CONCEPT_REQUIRES( ((AdditivePair<T>)), (int) ) sum_ints( T const & t ) { return add( t ); } We can now adapt std::pair<int,int> to model the AdditivePair concept. int first(std::pair<int,int> x) { return x.first; } int second(std::pair<int,int> x) { return x.second; } int add(std::pair<int,int> x) { return first(x) + second(x); } And we can also adapt int to model the AdditivePair concept; i.e. int is an additive pair where the second member is 0. int first(int x) { return x; } int second(int x) { return 0; } int add(int x) { return x; } That seems pretty natural to me. Daniel Walker