c++ - Checking for a nested, templated class -

-

i'm trying build trait checks existence of nested, templated class. take checking whether class o has nested class inner template parameter t:

template <typename o, typename t> struct has_inner {     static const bool value = std::is_class<typename o::template inner<t> >::value; };

however, not work properly. given 2 example classes dummy , ok

struct dummy {}; struct ok {     template <typename t>     struct inner {     }; };

the check on ok

std::cout << std::boolalpha << has_inner<ok, float>::value << std::endl;

will work, whereas check on dummy

std::cout << std::boolalpha << has_inner<dummy, int>::value << std::endl;

will fail compile on clang 3.2 error

error: 'inner' following 'template' keyword not refer template     static const bool value = std::is_class<typename o::template inner<t> >::value;                                                                  ^~~~~ note: in instantiation of template class 'has_inner<dummy, int>' requested here     std::cout << std::boolalpha << has_inner<dummy, int>::value << std::endl;

it appears compiler tries form templated expression prior passing on std::is_class. consequently see 2 solutions:

  1. tell compiler delay template expansion, or
  2. use different approach altogether.

however, don't know how perform either, can help?

the problem

you implement traits using, , relying on, sfinae, implementation doesn't take advantage of.

as stated compiler try instantiate typename o::template inner<t>, no matter if it's possible or not; , if isn't possible compiler throw error diagnostic in face.

what need conditional check see if t has template-class inside it, without instantiating if hasn't.

the solution - sfinae rescue!

an implementation might below snippet, explanation follow.

namespace impl {   template<class t, class... args>   struct has_inner {     template<class u, typename = typename u::template inner<args...>> // (a)     static std::true_type  test (int);      template<class u>     static std::false_type test (...);                                // (b)      using result_type = decltype (test<t> (0));                       // (c)   }; }  template<class... ts> using has_inner = typename impl::has_inner<ts...>::result_type;

note: using decltype(test<t>(0)) have either std::true_type, or std::false_type both standard behavior when dealing results type-traits.

the rules of sfinae states if function template yield invalid function declaration upon instantiation, if function didn't exist, compiler try searching match, instead of giving up.

this happens @ (c), try call (a) if fails (ie. invalid expression yield inside template<class u, typename ...> end calling (b).

(b) isn't match successful instantiation of (a), if (a) can't instantiated.. (b) do.


Comments

Post a Comment

Popular posts from this blog

C# random value from dictionary and tuple -

-
i have following code, , need random item contains string "region_1" dictionary. public static dictionary<int, tuple<string, int, currencytype>> itemarray = new dictionary<int, tuple<string, int, currencytype>>() { { 0xb3e, new tuple<string, int, currencytype>("region_1", 1500, currencytype.fame) } }; public static int generateitemid(string shopid) { var generateditem = itemarray.elementat(new random().next(0, itemarray.count)).key; } how select this? it's not possible efficiently... first create static random @ class level... prevent non-random behaviour if run query on short period of time... (it's seeded discrete clock) static random rnd = new random(); then: var item = itemarray.values .where(t => t.item1 == shopid) .orderby(_ => rnd.next()) .firstordefault()
Read more

cgi - How do I interpret URLs without extension as files rather than missing directories in nginx? -

-
so i'm trying install zoneminder , have run under nginx, has few compiled files need run using fast-cgi. these files lack extension. if file has extension, there no issue , nginx interprets file , return 404 if can't find it. if has no extension, assume it's directory , return 404 when can't find sort of index page. here have now: # security camera domain. server { listen 888; server_name mydomain.com; root /srv/http/zm; # enable php support. include php.conf; location / { index index.html index.htm index.php; } # enable cgi support. location ~ ^/cgi-bin/(.+)$ { alias /srv/cgi-bin/zm/; fastcgi_pass unix:/run/fcgiwrap.sock; fastcgi_param script_filename $document_root/$1; include fastcgi.conf; } } the idea is, under cgi-bin directory goes through fastcgi pipe. any idea on howi can fix this? upon closer inspection, realized zoneminder has 2 cgi sc...
Read more

.htaccess - htaccess convert request to clean url and add slash at the end of the url -

-
ok , searched more 4 days , more 20 posts here in stach or in websites without kind of success first here htaccess file rewriteengine on rewriterule ^([a-za-z0-9_]+)/?$ index.php?page=$1 [nc] rewriterule ^([a-za-z0-9_]+)\/([a-za-z0-9_]+)\/?$ index.php?page=$1&action=$2 [nc] i tried many codes 1 worked me can call request , works me www.example.com/pagename => www.example.com/index.php?page=pagename here questions , please provide me information "in detail" understand code well 1- how convert request automatically www.example.com/index.php?page=pagename www.example.com/pagename 2- if tried add slash @ end shows me page without images or styling mean www.example.com/pagename => works www.example.com/pagename/ => doesn't work that's :) for first part need 301 rule this: rewritecond %{the_request} /index\.php\?page=([^\s&]+) [nc] rewriterule ^ /%1? [r=301,l] for second part use absolute path in css, js, images files r...
Read more