Find out the parameter is constexpr

Question

Any type_traits or method could find out the parameters is a constexpr or not?

Example

size_t fibo_runtime(size_t num)
{
  //implementation
}

constexpr          


        

Answer 1

I think this may get you what you want

#include 

template 
class is_constexpr
{
   typedef char true_type ;
   struct false_type { true_type _[2] ; } ;

   template 
   static true_type has_constexpr( U & ) ;

   template 
   static false_type has_constexpr(...) ;

   public:
      enum { value = ( sizeof(has_constexpr(0)) == sizeof(true_type)) } ;
} ;

int main()
{
   constexpr int i = 10 ;
   int k = 20 ;

   std::cout << is_constexpr::value << std::endl ;
   std::cout << is_constexpr::value << std::endl ;   
}

I used Understanding SFINAE as a reference.

Doing some more research I think I the answer to the other part of the question is yes, since it looks a constexpr function template is not always usable in a constant expression. So this leads to a solution like so, with this somewhat contrived example:

template 
T f2( T num )
{ 
   return num + 1;
}

template 
constexpr T f1( T num )
{
   return num ;
}

template 
constexpr T f(T num)
{
   return  is_constexpr::value ? f1(num) : f2(num) ;
}