Significantly expand compile time rounding example

Author: mborland

examples/rounding_mode_compile_time.cpp

@@ -1,18 +1,63 @@
 // Copyright 2024 Matt Borland
 // Distributed under the Boost Software License, Version 1.0.
 // https://www.boost.org/LICENSE_1_0.txt
-
+//
+// To define a global compile-time rounding mode
+// you must define the macro before inclusion of *ANY* decimal header
 #define BOOST_DECIMAL_FE_DEC_DOWNWARD
-#include <boost/decimal.hpp>
+
+#include <boost/decimal/decimal32_t.hpp>    // For type decimal32_t
+#include <boost/decimal/literals.hpp>       // For decimal literals
+#include <boost/decimal/iostream.hpp>       // For decimal <iostream> support
+#include <boost/decimal/cfenv.hpp>          // For rounding mode access
+#include <iostream>
+
+void print_rounding_mode(const boost::decimal::rounding_mode current_mode)
+{
+    // All 5 rounding modes are defined by the enum rounding_mode
+    using boost::decimal::rounding_mode;
+
+    switch (current_mode)
+    {
+        case rounding_mode::fe_dec_downward:
+            std::cout << "fe_dec_downward\n";
+            break;
+        case rounding_mode::fe_dec_to_nearest:
+            std::cout << "fe_dec_to_nearest\n";
+            break;
+        case rounding_mode::fe_dec_to_nearest_from_zero:
+            std::cout << "fe_dec_to_nearest_from_zero\n";
+            break;
+        case rounding_mode::fe_dec_toward_zero:
+            std::cout << "fe_dec_toward_zero\n";
+            break;
+        case rounding_mode::fe_dec_upward:
+            std::cout << "fe_dec_upward\n";
+            break;
+    }
+}
 
 int main()
 {
     using namespace boost::decimal::literals;
+    using boost::decimal::decimal32_t;
 
-    constexpr auto lhs {"5e+50"_DF};
-    constexpr auto rhs {"4e+40"_DF};
-    constexpr auto downward_res {lhs - rhs};
+    // This uses one of the same examples from our runtime rounding mode example
+    // Now we can see the effects on the generation of constants,
+    // since we can static_assert the result
+
+    constexpr decimal32_t lhs {"5e+50"_DF};
+    constexpr decimal32_t rhs {"4e+40"_DF};
+    constexpr decimal32_t downward_res {lhs - rhs};
     static_assert(downward_res == "4.999999e+50"_DF, "Incorrectly rounded result");
 
-    return 0;
+    std::cout << "The default rounding mode is: ";
+    print_rounding_mode(boost::decimal::rounding_mode::fe_dec_default);
+
+    // Here we can see that the rounding mode has been set to something besides default
+    // without having had to call fesetround
+    //
+    // This works with all compilers unlike changing the rounding mode at run-time
+    std::cout << "The current rounding mode is: ";
+    print_rounding_mode(boost::decimal::fegetround());
 }