c++ - Using SFINAE to select function based on whether a particular overload of a function exists -


this question has answer here:

i have been trying choose between 2 templated functions based on whether overload operator<<(std::ostream&, const t&) exists.

example:

template <typename t, typename std::enable_if</* ? */, int>::type = 0> std::string stringify(const t& t) {     std::stringstream ss;     ss << t;     return ss.str(); }  template <typename t, typename std::enable_if</* ? */, int>::type = 0> std::string stringify(const t& t) {     return "no overload of operator<<"; }  struct foo { };  int main() {     std::cout << stringify(11) << std::endl;     std::cout << stringify(foo{}) << std::endl; }

is possible? , if so, how solve problem?

there's no need enable_if, use expression sfinae select correct overload when operator<< present.

namespace detail {     template<typename t>     auto stringify(std::stringstream& ss, t const& t, bool)         -> decltype(ss << t, void(), std::string{})     {         ss << t;         return ss.str();     }      template<typename t>     auto stringify(std::stringstream&, t const&, int)         -> std::string     {         return "no overload of operator<<";     } }  template <typename t> std::string stringify(const t& t) {     std::stringstream ss;     return detail::stringify(ss, t, true); }

live demo

the stringify function template delegates 1 of detail::stringify function templates. then, first 1 selected if expression ss << t well-formed. unnamed bool parameter being used disambiguation between 2 detail::stringify implementations. since primary stringify function passes true argument detail::stringify, first 1 better match when operator<< overload present. otherwise second 1 selected.

this expression decltype(ss << t, void(), std::string{}) in trailing return type of first stringify template merits more detailed explanation. here have single expression consisting of 3 sub-expressions separated comma operator.

the first one, ss << t determines whether function template passes template parameter substitution , added overload resolution set. occur if expression well-formed, i.e. if type in question overloads operator<<.

the middle sub-expression, void() doesn't other ensure user-defined operator, not selected (because cannot overload operator, void parameter type).

the third, , rightmost, sub-expression, std::string{} determines return type of detail::stringify function.


Comments

Post a Comment

Popular posts from this blog

inversion of control - Autofac named registration constructor injection -

does autofac support specifying registration name in components' constructors? example: ninject's namedattribute . you need use autofac.extras.attributed package on top achieve this so let's have 1 interface , 2 classes: public interface ihello { string sayhello(); } public class englishhello : ihello { public string sayhello() { return "hello"; } } public class frenchhello : ihello { public string sayhello() { return "bonjour"; } } then have consumer class, in want select instance injected: public class helloconsumer { private readonly ihello helloservice; public helloconsumer([withkey("en")] ihello helloservice) { if (helloservice == null) { throw new argumentnullexception("helloservice"); } this.helloservice = helloservice; } public string sayhello() { return this.helloservice.sayhello()...
Read more

ios - Change Storyboard View using Seague -

i have following code checks connection server, , depending on want change views if no connection found. nsstring *connect = [nsstring stringwithcontentsofurl:[nsurl urlwithstring:@"http://myserver.com"] encoding:nsutf8stringencoding error:nil]; if (connect == null) { [self performseguewithidentifier:@"network_failure" sender:self]; } else { // other code here } my problem not change views. have added storyboard segue between 2 views identifier of network_failure . not have navigation controller or that, problem? thanks help! no not problem. first of all, in objectivec exists nil object oriented null . must use that. so: if(connect == nil) or better: if(!connect) so due fact nil not same of null , probably code does't enter in if scope . you have been able discover breakpoint. anyway: be sure set trigger segue (line) 2 view controller , , name network_failure (that not best name. should instead view1toview2 ) as, m...
Read more

verilog - Systemverilog dynamic casting issues -

i've code snippet following in testbench function void write_to_port( my_data_type_base data ); my_data_type_extended data_ext; if(!$cast(data_ext, data)); `uvm_error(get_type_name(), "failed cast"); `uvm_info(get_name(), $psprintf("data_ext :\n%s", data_ext.sprint()), uvm_medium) // write data_ext out port.... endfunction when run it, i'm getting uvm_error "failed cast." i'm not quire sure why $cast not returning 1. can see, i'm printing out extended class data item after casting uvm_info. can see it's being cast properly. if don't use $cast if condition, don't runtime error. isn't coding practice use if dynamic cast check if $cast returning 1 ? what might reason behind cast not returning 1 in above case? i think semicolon on line 'if' not belong? i think consumes if statement, , uvm_error executes regardless of how if evaluates: if(!$cast(data_ext, data)); <- no semico...
Read more