Class FilterGenerator

tutorials/external/catch.hpp:3734–3765

    template <typename T, typename Predicate>
    class FilterGenerator : public IGenerator<T> {
        GeneratorWrapper<T> m_generator;
        Predicate m_predicate;
    public:
        template <typename P = Predicate>
        FilterGenerator(P&& pred, GeneratorWrapper<T>&& generator):
            m_generator(std::move(generator)),
            m_predicate(std::forward<P>(pred))
        {
            if (!m_predicate(m_generator.get())) {
                // It might happen that there are no values that pass the
                // filter. In that case we throw an exception.
                auto has_initial_value = next();
                if (!has_initial_value) {
                    Catch::throw_exception(GeneratorException("No valid value found in filtered generator"));
                }
            }
        }

        T const& get() const override {
            return m_generator.get();
        }

        bool next() override {
            bool success = m_generator.next();
            if (!success) {
                return false;
            }
            while (!m_predicate(m_generator.get()) && (success = m_generator.next()) == true);
            return success;
        }
    };

    template <typename T, typename Predicate>
    GeneratorWrapper<T> filter(Predicate&& pred, GeneratorWrapper<T>&& generator) {