add function to display the exact backend as a string

Number_types/include/CGAL/Number_types/internal/Exact_type_selector.h

@@ -317,6 +317,25 @@ template < typename ET >
 struct Exact_type_selector : Exact_field_selector< ET > {};
 #endif
 
+constexpr const char* exact_nt_backend_string()
+{
+  switch(Default_exact_nt_backend)
+  {
+    case GMP_BACKEND:
+      return "GMP_BACKEND";
+    case GMPXX_BACKEND:
+      return "GMPXX_BACKEND";
+    case BOOST_GMP_BACKEND:
+      return "BOOST_GMP_BACKEND";
+    case BOOST_BACKEND:
+      return "BOOST_BACKEND";
+    case LEDA_BACKEND:
+      return "LEDA_BACKEND";
+    case MP_FLOAT_BACKEND:
+      return "MP_FLOAT_BACKEND";
+  }
+}
+
 } } // namespace CGAL::internal
 
 #undef CGAL_EXACT_SELECTORS_SPECS

Number_types/test/Number_types/CMakeLists.txt

@@ -61,6 +61,7 @@ create_single_source_cgal_program("unsigned.cpp")
 create_single_source_cgal_program("utilities.cpp")
 create_single_source_cgal_program("Exact_rational.cpp")
 create_single_source_cgal_program("Mpzf_new.cpp")
+create_single_source_cgal_program("check_exact_backend.cpp")
 
 if( CGAL_Core_FOUND )
   create_single_source_cgal_program( "CORE_Expr_ticket_4296.cpp" )

Number_types/test/Number_types/check_exact_backend.cpp

@@ -26,4 +26,6 @@ int main()
 #endif
     ( Default_exact_nt_backend!=MP_FLOAT_BACKEND || (std::is_same_v<CGAL::Exact_integer,CGAL::MP_Float> && std::is_same_v<CGAL::Exact_rational,CGAL::Quotient<CGAL::MP_Float>>) )
   );
+
+  std::cout << "Exact backend is " << exact_nt_backend_string() << "\n";
 }