c++ - if constexpr instead of tag dispatch -


i want use if constexpr instead of tag dispatching, not sure how use it. example code below.

template<typename t> struct mytag {    static const int supported = 0; };  template<> struct mytag<std::uint64_t> {   static const int supported = 1; };  template<> struct mytag<std::uint32_t> {   static const int supported = 1; };  class mytest { public:    template<typename t>    void do_something(t value)    {       // instead of doing       bool supported = mytag<t>::supported;        // want       if constexpr (t == std::uint64_t)           supported = true;    } };

one way define constexpr predicate checks type of argument, constexpr switch on result of predicate.

i think way nice because separates functional logic precondition logic.

#include <iostream> #include <cstddef> #include <type_traits>  class mytest { public:     template<typename t>     void do_something(t value)     {         // define our predicate         // lambdas constexpr-if-possible in c++17         constexpr auto is_supported = [](auto&& x) {             if constexpr (std::is_same<std::decay_t<decltype(x)>, std::uint64_t>())                 return true;             else                 return false;         };          // use result of predicate                 if constexpr (is_supported(value))         {             std::cout << "supported\n";         }         else          {             std::cout << "not supported\n";         }     } };  int main() {     auto t = mytest();      t.do_something(int(0));     t.do_something(std::uint64_t(0));     t.do_something(double(0));     t.do_something(static_cast<unsigned long>(0));  // careful std::uint_xx aliases  }

example results:

not supported supported not supported supported

another way express might be:

class mytest { public:      template<class t>     static constexpr bool something_possible(t&&)     {         return std::is_same<std::decay_t<t>, std::uint64_t>();     }      template<typename t>     void do_something(t value)     {         // switch behaviour on result of constexpr predicate             if constexpr (something_possible(value))         {             std::cout << "supported\n";         }         else          {             std::cout << "not supported\n";         }     } };

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