gistfile1.cpp

    class OpDebug : public Decorable<OpDebug> {
    public:

        //
        // Removed existing members for brevity.
        //

        class Extension {
            virtual void appendToString(StringBuilder*) = 0;
            virtual void appendToBson(BSONObjBuilder*) = 0;
        protected:
            // Can't delete through base pointer.
            ~Extension() = default;
        };

        template <typename T>
        static Decoration<T> declareExtension() {
            BOOST_STATIC_ASSERT(std::is_base_of<Extension, T>());
            auto decoration = declareDecoration<T>();
            _extensions.push_back([decoration](CurOp* curOp) {
                return decoration(curOp);
            });
            return decoration;
        }

    private:
        static std::vector<stdx::function<Extension&(CurOp*)>> _extensions;
    };