except: add string_error mixin

except.hpp

@@ -8,7 +8,9 @@
 
 #pragma once
 
+#include <utility>
 #include <iosfwd>
+#include <string>
 
 namespace cruft {
     /// A base exception class for all cruft libraries.
@@ -24,6 +26,38 @@ namespace cruft {
     };
 
 
+    /// A mixin error class that can be used when inheriting from
+    /// `cruft::error`.
+    ///
+    /// It accepts a single std::string, saving it so that it can be passed
+    /// through to a std::ostream later on.
+    ///
+    /// By setting `BaseT` to a locally useful error class we can still
+    /// maintain an appropriate exception hierarchy for a given module.
+    /// eg, pass `foo::error` (which derives eventually from `cruft::error`)
+    /// in the module `foo`.
+    ///
+    /// \tparam BaseT   The desired base class.
+    template <typename BaseT>
+    class string_error : public BaseT {
+    public:
+        template <typename... Args>
+        string_error (std::string const &_message, Args &&...args)
+            : BaseT (std::forward<Args> (args)...)
+            , m_message (_message)
+        { ; }
+
+        std::ostream&
+        describe (std::ostream &os) const noexcept override
+        {
+            return os << m_message;
+        }
+
+    private:
+        std::string m_message;
+    };
+
+
     /// Use `error::describe` to render the supplied error object to a
     /// std::ostream.
     std::ostream&