使用 decltype 作为模板参数

Question

如您所见，我正在使用一些模板技巧来推断某些函数的返回类型，但由于某种原因，此代码无法编译。我希望您帮助了解错误的原因以及如果可能的话如何解决它。

变体类可以在这里找到。

我有以下代码：

class Value {
....
    using values = variant<std::shared_ptr<const std::string> ,  unsigned int, float, bool>;
    values _holder;
    template<typename To> using ResolveReturnConvertType = decltype(mapbox::util::apply_visitor(typename ConverterAdaptor<To>::ConverterType(), _holder));

    template<typename To>
    auto convertTo() const noexcept ->  ResolveReturnConvertType<To>
    {
        static_assert(std::is_constructible<Value, To>::value, "Type requested to convert is not valid");
        using ReturnType =  ResolveReturnConvertType<To>; //deduces ReturnType = int in this case, that type is valid
        _holder.get<ReturnType>() // fails to compile, error below
        std::cout << typeid(ReturnType).name() << std::endl; //works why?
        return  mapbox::util::apply_visitor(typename ConverterAdaptor<To>::ConverterType(), _holder);
    }
}//end of Value
//example main
int main() {
    Value v("232");
    std::cout << v.convertTo<int>() << std::endl;
}

编译器错误

g++  -std=c++11 -Ideps -I.   -c -o Value/Value.o Value/Value.cpp
In file included from Value/Value.cpp:1:0:
Value/Value.hpp: In instantiation of ‘Value::ResolveReturnConvertType<To>             Value::convertTo() const [with To = int; Value::ResolveReturnConvertType<To> =     int; typename ConverterAdaptor<To>::ConverterType = NumericConverter<int>]’:
Value/Value.cpp:27:33:   required from here
Value/Value.hpp:88:3: error: no matching function for call to   ‘mapbox::util::variant<std::shared_ptr<const std::basic_string<char,     std::char_traits<char>, std::allocator<char> > >, unsigned int, float, bool,     std::shared_ptr<const std::vector<std::shared_ptr<const Value>,     std::allocator<std::shared_ptr<const Value> > > >, std::shared_ptr<const     std::unordered_map<std::shared_ptr<const std::basic_string<char,     std::char_traits<char>, std::allocator<char> > >, std::shared_ptr<const Value>,     std::hash<std::shared_ptr<const std::basic_string<char, std::char_traits<char>,     std::allocator<char> > > >, std::equal_to<std::shared_ptr<const     std::basic_string<char, std::char_traits<char>, std::allocator<char> > > >,     std::allocator<std::pair<const std::shared_ptr<const std::basic_string<char,     std::char_traits<char>, std::allocator<char> > >, std::shared_ptr<const Value> >     > > > >::get() const’
   _holder.get<ReturnType>();
           ^
In file included from Value/Value.hpp:8:0,
             from Value/Value.cpp:1:
deps/variant/variant.hpp:680:23: note: candidate: template<class T, typename     std::enable_if<(mapbox::util::detail::direct_type<T, std::shared_ptr<const     std::basic_string<char, std::char_traits<char>, std::allocator<char> > >,     unsigned int, float, bool, std::shared_ptr<const     std::vector<std::shared_ptr<const Value>, std::allocator<std::shared_ptr<const     Value> > > >, std::shared_ptr<const std::unordered_map<std::shared_ptr<const     std::basic_string<char, std::char_traits<char>, std::allocator<char> > >,     std::shared_ptr<const Value>, std::hash<std::shared_ptr<const     std::basic_string<char, std::char_traits<char>, std::allocator<char> > > >,     std::equal_to<std::shared_ptr<const std::basic_string<char,     std::char_traits<char>, std::allocator<char> > > >,     std::allocator<std::pair<const std::shared_ptr<const std::basic_string<char,     std::char_traits<char>, std::allocator<char> > >, std::shared_ptr<const Value> >     > > > >::index != mapbox::util::detail::invalid_value), void>::type* <anonymous>     > T& mapbox::util::variant<Types>::get() [with T = T; typename     std::enable_if<(mapbox::util::detail::direct_type<T, Types ...>::index !=     mapbox::util::detail::invalid_value)>::type* <anonymous> = <enumerator>; Types =     {std::shared_ptr<const std::basic_string<char, std::char_traits<char>,     std::allocator<char> > >, unsigned int, float, bool, std::shared_ptr<const     std::vector<std::shared_ptr<const Value>, std::allocator<std::shared_ptr<const     Value> > > >, std::shared_ptr<const std::unordered_map<std::shared_ptr<const     std::basic_string<char, std::char_traits<char>, std::allocator<char> > >,     std::shared_ptr<const Value>, std::hash<std::shared_ptr<const     std::basic_string<char, std::char_traits<char>, std::allocator<char> > > >,     std::equal_to<std::shared_ptr<const std::basic_string<char,     std::char_traits<char>, std::allocator<char> > > >,     std::allocator<std::pair<const std::shared_ptr<const std::basic_string<char,     std::char_traits<char>, std::allocator<char> > >, std::shared_ptr<const Value> >     > > >}]
     VARIANT_INLINE T& get()

Answer 1

这个问题

可以关闭，这个问题是由于我的一个愚蠢的错误，int实际上不是一个有效的类型，因此模板推导失败了。