Mark bad_optional_access as [[nodiscard]] (default), control via optional_CONFIG_NO_NODISCARD (nonstd-lite-project issue 74)

Add configuration option `optional_CONFIG_NO_NODISCARD`.

nonstd-lite/nonstd-lite#74

Author: martinmoene

README.md

@@ -247,6 +247,11 @@ Define this to 1 if you want to compile without extensions. Default is undefined
 -D<b>optional\_CONFIG\_NO\_EXCEPTIONS</b>=0  
 Define this to 1 if you want to compile without exceptions. If not defined, the header tries and detect if exceptions have been disabled (e.g. via `-fno-exceptions`). Default is undefined.
 
+#### Disable \[\[nodiscard\]\]
+
+-D<b>optional\_CONFIG\_NO\_NODISCARD</b>=0
+Define this to 1 if you want to compile without \[\[nodiscard\]\]. Note that the default of marking functions and `class bad_optional_access` with \[\[nodiscard\]\] is not part of the C++17 standard. The rationale to use \[\[nodiscard\]\] is that unnoticed discarded (error) values may break the error handling flow.
+
 #### Macros to control alignment
 
 If *optional lite* is compiled as C++11 or later, C++11 alignment facilities are used for storage of the underlying object. When compiled as pre-C++11, *optional lite* tries to determine proper alignment itself. If this doesn't work out, you can control alignment via the following macros. See also section [Implementation notes](#implementation-notes).

include/nonstd/optional.hpp

@@ -50,6 +50,14 @@
 #define optional_CONFIG_NO_EXTENSIONS  0
 #endif
 
+// Control marking class bad_optional_access and several methods with [[nodiscard]]]:
+
+#if !defined(optional_CONFIG_NO_NODISCARD)
+# define optional_CONFIG_NO_NODISCARD  0
+#else
+# define optional_CONFIG_NO_NODISCARD  1
+#endif
+
 // Control presence of exception handling (try and auto discover):
 
 #ifndef optional_CONFIG_NO_EXCEPTIONS
@@ -386,7 +394,7 @@ namespace nonstd {
 # define optional_constexpr14  /*constexpr*/
 #endif
 
-#if optional_HAVE( NODISCARD )
+#if optional_HAVE( NODISCARD ) && !optional_CONFIG_NO_NODISCARD
 # define optional_nodiscard  [[nodiscard]]
 #else
 # define optional_nodiscard  /*[[nodiscard]]*/
@@ -917,7 +925,7 @@ const nullopt_t nullopt(( nullopt_t::init() ));
 
 #if ! optional_CONFIG_NO_EXCEPTIONS
 
-class bad_optional_access : public std::logic_error
+class optional_nodiscard bad_optional_access : public std::logic_error
 {
 public:
   explicit bad_optional_access()