Question

我有一个等同于OnActionExecuting()（我们还没有使用C ++ 14）。我还有两个帮助类，删除或添加一个前导号码。

std::integer_sequence

// Sequence type template <typename Type, Type ... Values> struct Sequence { using value_type = Type; }; // Pop a value off of the start of a sequence template <class Class> struct SequencePop; template <typename Type, Type Value, Type ... Values> struct SequencePop<Sequence<Type, Value, Values ...>> { using type = Sequence<Type, Values ...>; }; // Push a value on to the start of a sequence template <class Class, typename Class::value_type Value> struct SequencePush; template <typename Type, Type Value, Type ... Values> struct SequencePush<Sequence<Type, Values ...>, Value> { using type = Sequence<Type, Value, Values ...>; };在我尝试过的所有编译器中都被认为是有效的（g ++ 6.4.1，g ++ 7.1.1，clang ++ 4.0.1）。 SequencePop不能用g ++ 7.1.1编译。错误消息如下。

SequencePush

g ++ 7.1.1是否正确拒绝此代码，如果是，那么如何判断test.cpp:24:8: error: partial specialization ‘struct SequencePush<Sequence<Type, Values ...>, Value>’ is not more specialized than [-fpermissive] struct SequencePush<Sequence<Type, Values ...>, Value> ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ test.cpp:21:8: note: primary template ‘template<class Class, typename Class::value_type Value> struct SequencePush’ struct SequencePush;不是＆＃34;更专业＆＃34;比主要模板？

Answer 1

应该是

template <typename Type,
          Type ... Values,
          typename Sequence<Type, Values ...>::value_type Value>
struct SequencePush<Sequence<Type, Values ...>, Value>
{
    using type = Sequence<Type, Value, Values ...>;
};

Demo

来自Class::value_type的{{1}}和来自Type的{{1}}不是“相关”。

Answer 2

实际上，通用模板SequencePush由一个模板参数（Class）参数化，但您的专业化由两个参数化（Type和Value）。在常规模板中，Type不是参数，因为它是从Class推断出来的。

一个有效但不令人满意的解决方案是：

template <class Class, typename Type, Type Value>                     
struct SequencePush;                                                         

template <typename Type, Type Value, Type ... Values>                        
struct SequencePush<Sequence<Type, Values ...>, Type, Value>                       
{                                                                            
    using type = Sequence<Type, Value, Values ...>;                          
};

demo

变异模板部分特化＆＃34;不是更专业的＆＃34;在g ++ 7.1.1中

2 个答案: