Exception std based option (#1232)

* Add ability to derive etl::exception from std::exception

* Only add test for exception std base when using STL

* Use ETL_NOEXCEPT macro to define a function as noexcept

* Rename macro to keep in line with common style

* Add using_std_exception to etl::traits

---------

Co-authored-by: Béla Iványi <bela.ivanyi@idata.hu>

include/etl/exception.h

@@ -33,6 +33,14 @@ SOFTWARE.
 
 #include "platform.h"
 
+#if ETL_USING_STD_EXCEPTION
+#include <exception>
+
+  #define ETL_EXCEPTION_CONSTEXPR
+#else
+  #define ETL_EXCEPTION_CONSTEXPR ETL_CONSTEXPR
+#endif
+
 ///\defgroup exception exception
 /// The base class for all ETL exceptions.
 ///\ingroup utilities
@@ -44,6 +52,9 @@ namespace etl
   /// A low overhead exception base class.
   //***************************************************************************
   class exception
+#if ETL_USING_STD_EXCEPTION
+    : public std::exception
+#endif
   {
   public:
 
@@ -54,7 +65,7 @@ namespace etl
     //*************************************************************************
     /// Constructor.
     //*************************************************************************
-    ETL_CONSTEXPR
+    ETL_EXCEPTION_CONSTEXPR
     exception(string_type reason_, string_type file_, numeric_type line_)
       : reason_text(reason_),
         file_text(file_),
@@ -65,7 +76,7 @@ namespace etl
     //*************************************************************************
     /// Constructor.
     //*************************************************************************
-    ETL_CONSTEXPR
+    ETL_EXCEPTION_CONSTEXPR
     exception(string_type reason_, string_type /*file_*/, numeric_type line_)
       : reason_text(reason_),
         line(line_)
@@ -77,8 +88,11 @@ namespace etl
     /// Gets the reason for the exception.
     /// \return const char* to the reason.
     //***************************************************************************
-    ETL_CONSTEXPR
-    string_type what() const
+    ETL_EXCEPTION_CONSTEXPR
+    string_type what() const ETL_NOEXCEPT
+#if ETL_USING_STD_EXCEPTION
+      override
+#endif
     {
       return reason_text;
     }

include/etl/platform.h

@@ -293,6 +293,17 @@ SOFTWARE.
   #define ETL_HAS_VIRTUAL_MESSAGES 1
 #endif
 
+//*************************************
+// Indicate if etl::exception is to be derived from std::exception.
+#if defined(ETL_USE_STD_EXCEPTION)
+#if ETL_NOT_USING_STL
+  #error "Requested std base for etl::exception, but STL is not used"
+#endif
+  #define ETL_USING_STD_EXCEPTION 1
+#else
+  #define ETL_USING_STD_EXCEPTION 0
+#endif
+
 //*************************************
 // Indicate if etl::literals::chrono_literals uses ETL verbose style.
 #if defined(ETL_USE_VERBOSE_CHRONO_LITERALS) && ETL_USING_CPP11
@@ -631,6 +642,7 @@ namespace etl
     static ETL_CONSTANT bool using_legacy_bitset              = (ETL_USING_LEGACY_BITSET == 1);
     static ETL_CONSTANT bool using_exceptions                 = (ETL_USING_EXCEPTIONS == 1);
     static ETL_CONSTANT bool using_libc_wchar_h               = (ETL_USING_LIBC_WCHAR_H == 1);
+    static ETL_CONSTANT bool using_std_exception              = (ETL_USING_STD_EXCEPTION == 1);
     
     // Has...
     static ETL_CONSTANT bool has_initializer_list             = (ETL_HAS_INITIALIZER_LIST == 1);

test/etl_profile.h

@@ -44,6 +44,9 @@ SOFTWARE.
 #define ETL_IN_UNIT_TEST
 #define ETL_DEBUG_COUNT
 #define ETL_ARRAY_VIEW_IS_MUTABLE
+#if !defined(ETL_NO_STL)
+#define ETL_USE_STD_EXCEPTION
+#endif
 
 #define ETL_MESSAGE_TIMER_USE_ATOMIC_LOCK
 #define ETL_CALLBACK_TIMER_USE_ATOMIC_LOCK

test/test_exception.cpp

@@ -61,5 +61,21 @@ namespace
         CHECK_EQUAL(123, c.line_number());
       }     
     }
+
+#if ETL_USING_STD_EXCEPTION
+    TEST(test_exception_std_base)
+    {
+      etl::exception e("An exception", "Some file", 123);
+
+      try
+      {
+        throw e;
+      }
+      catch (std::exception& c)
+      {
+        CHECK_EQUAL(std::string("An exception"), std::string(c.what()));
+      }
+    }
+#endif
   };
 }