Opts: Solve OptionsParser::Exception diamond inheritance problem

smocore/include/opts.h

@@ -29,9 +29,13 @@ public:
     class Exception : public std::exception
     {
     public:
-        Exception() = default;
+        explicit Exception(std::string message_);
         ~Exception() override = default;
-        const char* what() const noexcept override = 0;
+
+        const char* what() const noexcept override;
+
+    protected:
+        std::string message;
     };
 
     std::string argv0;
@@ -45,13 +49,11 @@ public:
 };
 
 class OptionsParserError
-: public std::invalid_argument, public OptionParser::Exception
+: public OptionParser::Exception
 {
 public:
     OptionsParserError(
         const std::string& errorMessage, const OptionParser& parser);
-
-    const char* what() const noexcept override;
 };
 
 class JustPrintUsageNoError
@@ -59,10 +61,6 @@ class JustPrintUsageNoError
 {
 public:
     explicit JustPrintUsageNoError(const OptionParser& parser);
-    const char* what() const noexcept override;
-
-private:
-    std::string message;
 };
 
 #endif // OPTS_H