C++ check if statement can be evaluated constexpr

Question

Is there a method to decide whether something can be constexpr evaluated, and use the result as a constexpr boolean? My simplified use case is as follows:

te         


        

Answer 1

Not exactly what you asked (I've developer a custom type trait specific for a get_value() static method... maybe it's possible to generalize it but, at the moment, I don't know how) but I suppose you can use SFINAE and make something as follows

#include 
#include 

template 
constexpr auto icee_helper (int)
   -> decltype( std::integral_constant{},
                std::true_type{} );

template 
constexpr auto icee_helper (long)
   -> std::false_type;

template 
using isConstExprEval = decltype(icee_helper(0));

template 
struct derived
 {
   template 
   void do_stuff()
    { std::cout << "constexpr case (" << I << ')' << std::endl; }

   void do_stuff (std::size_t i)
    { std::cout << "not constexpr case (" << i << ')' << std::endl; }

   void execute ()
    {
      if constexpr ( isConstExprEval::value )
         do_stuff();
      else
         do_stuff(base::get_data());
    }
 };

struct foo
 { static constexpr std::size_t get_data () { return 1u; } };

struct bar
 { static std::size_t get_data () { return 2u; } };

int main ()
 { 
   derived{}.execute(); // print "constexpr case (1)"
   derived{}.execute(); // print "not constexpr case (2)"
 }