std::invoke\u result\u t编译时语法错误

我正在创建一个函数,它接受一个iterable(容器),它的begin和end方法返回迭代器,迭代器的解引用可以通过传递的lambda修改。这听起来很复杂,但我想做一些类似Python的超级整洁

modified_iterator = (fn(x) for x in my_iterator)

代码:

    template<typename Container, typename Fn>
    class IterableWrapper {
    public:
        template<typename Iterator>
        class IteratorWrapper : public Iterator {
        public:
            template<typename ...Args>
            explicit IteratorWrapper(Fn fn, Args ... args) : Iterator(args ...), fn(fn) {}

            //typename std::invoke_result_t<Fn, typename std::invoke_result_t<typename Iterator::operator*>> not working
            typename std::invoke_result_t<Fn,uint64_t> operator* () const {
                return fn(Iterator::operator*());
            }
        private:
            Fn fn;
        };

        IterableWrapper(const Container&& c, Fn&& fn) : c(std::move(c)), fn(std::forward<Fn>(fn)) {}

        auto begin() const {
            return IteratorWrapper<decltype(c.begin())>(fn, c.begin());
        };

        auto end() const {
            return IteratorWrapper<decltype(c.end())>(fn, c.end());
        };

    private:
        Container c;
        Fn fn;
    };

    template<typename C, typename Fn>
    auto wrap_iterable(C& c, Fn&& fn) = delete;

    template<typename C, typename Fn>
    auto wrap_iterable(C&& c, Fn&& fn) {
        return IterableWrapper<C, Fn>(std::move(c), std::forward<Fn>(fn));
    }

预期用途:

    auto new_iterable = wrap_iterable(std::vector<uint64_t>{1,2,3,4}, [](auto&& item) { return std::pow(item, 2); });

我不想硬编码 uint64_t 在invoke\u结果中 IteratorWrapper::operator* 。它应该是的返回类型 operator* 在基类中(即模板化类型 Iterator )。

但是用注释掉的返回类型替换硬编码的头会导致编译错误。新标题:

typename std::invoke_result_t<Fn, typename std::invoke_result_t<typename Iterator::operator*>> operator* () const

错误:

In file included from /Users/adam/school/cpp/invertedindex/main.cpp:203:0:
/Users/adam/school/cpp/invertedindex/inverted_index.hpp:61:105: error: template argument 1 is invalid
             typename std::invoke_result_t<Fn, typename std::invoke_result_t<typename Iterator::operator*>> operator* () const {  // typename std::invoke_result_t<Fn, typename std::invoke_result_t<typename Iterator::operator*>> not compiling??
                                                                                                         ^~
/Users/adam/school/cpp/invertedindex/inverted_index.hpp:61:121: error: template argument 2 is invalid
             typename std::invoke_result_t<Fn, typename std::invoke_result_t<typename Iterator::operator*>> operator* () const {  // typename std::invoke_result_t<Fn, typename std::invoke_result_t<typename Iterator::operator*>> not compiling??
                                                                                                                         ^~~~~
/Users/adam/school/cpp/invertedindex/inverted_index.hpp:61:127: error: expected identifier before '{' token
             typename std::invoke_result_t<Fn, typename std::invoke_result_t<typename Iterator::operator*>> operator* () const {  // typename std::invoke_result_t<Fn, typename std::invoke_result_t<typename Iterator::operator*>> not compiling??
                                                                                                                               ^
/Users/adam/school/cpp/invertedindex/inverted_index.hpp:61:127: error: expected unqualified-id before '{' token
In file included from /Users/adam/school/cpp/invertedindex/main.cpp:203:0: