On Nov 22, 2007 12:35 AM, Giovanni Piero Deretta <gpderetta@gmail.com> wrote:
On 11/20/07, Dean Michael Berris <mikhailberis@gmail.com> wrote:
On Nov 20, 2007 12:37 AM, Giovanni Piero Deretta <gpderetta@gmail.com>
[...] This doesn't return the right thing with non const parameters. A very simple result_of compatible wrapper around fusion::at_c will do. In fact I think that fusion should provide function objects for every one of its algorithm.
You mean:
std::pair<int, int> pair(1, 2); std::cout << boost::select<1>()(pair) << std::endl; // should output 2
?
I think I'm missing something here... Can you elaborate?
In fact I was wrong. Now I do not think it return the right thing even with const parameters :). The problem is that you always return a copy of the Nth element (as a temporary). This work fine if you only need read access *and* the object is small. In this example 'select' copy the vector at every extraction!
std::map<int, std::vector<int> > map = ....;
void foo(std::vector<int> const&);
std::for_each(map.begin(), map.end(), bind(foo, bind(boost::select<1>(), _1)));
If you remove the const from the signature of foo, the example won't compile at all. It is a bit elaborate to deduce the return value of a generic tuple accessor in the most general case possible, but it can be done. Anyways, fusion at_c already takes care of all the work.
Ah, you're referring to const-correctness and not returning a reference instead. In this case, I agree that this isn't acceptable in a lot of cases, thanks for pointing it out. Would it help if the current version of select<> would be called select_copy<> and select<> would return a correctly const-qualified reference instead? I've also tried going with fusion::at_c in my initial attempt to get the correct result type, only that I run into problems with const correctness. I'll try my hand at it some other time though. Thanks for the elaboration! -- Dean Michael C. Berris Software Engineer, Friendster, Inc. [http://cplusplus-soup.blogspot.com/] [mikhailberis@gmail.com] [+63 928 7291459] [+1 408 4049523]