使用decltype作爲模板參數

Question

我使用了一些模板技巧來推導出一些函數的返回類型，正如您所看到的，但由於某些原因，此代碼無法編譯。我希望你的幫助能夠理解錯誤的原因，如果可能的話，如何解決錯誤。

變體類可以找到here。

我有以下代碼：

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; 
} 

編譯器錯誤

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

這個問題可以被關閉，這個問題是由於我的一個愚蠢的錯誤，INT實際上不是一個有效的類型，因此模板扣除失敗。