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还伤脑筋？

我正在研究tuple类型转换助手，但遇到了问题。要么我复制代码，要么我包装一个template template. 不过，我很难让它工作。

这是我需要处理的：

template<template<typename> class trans> struct no_index_transformer
{
    template<size_t, typename transform_t> struct transformer
    {
        using t = typename trans<transform_t>::t;
    };
};

no_index_transformer接受没有(index) 传递给它的template templatea 。内部是我需要通过的。但是，当我这样做时会出现错误，因此我不确定正确的通过方式是什么。transformersize_ttransformer templateno_index_transformer<>::transformer

我在以下行收到错误：

template<template<typename> class transformer, typename tuple> using transform_tuple_t                  
    = transform_tuple_index<no_index_transformer<transformer>::transformer, 0, tuple>::t;

“模板模板参数的模板参数必须是类模板或类型别名模板。”

这是变压器：

template<typename, typename> struct tuple_cat;
template<typename... types_one, typename... types_two> struct tuple_cat<std::tuple<types_one...>, std::tuple<types_two...>>
{
public:
    using t = std::tuple<types_one..., types_two...>;
};

template<template<size_t, typename> class transformer, size_t index, typename tuple> class transform_tuple_index;
template<template<size_t, typename> class transformer, size_t index, typename current_t, typename... types> class transform_tuple_index<transformer, index, std::tuple<current_t, types...>>
{
    using current = std::tuple<typename transformer<index, current_t>::t>;
    using next = typename transform_tuple_index<transformer, index + 1, std::tuple<types...>>::t;
    using combined = typename tuple_cat<current, next>::t;
    static constexpr bool test_{ std::is_same<std::tuple<void>, next>::value };

public:
    using t = typename std::conditional<test_, current, combined>::type;
};
template<template<size_t, typename> class transformer, size_t index> class transform_tuple_index<transformer, index, std::tuple<>>
{
public:
    using t = std::tuple<void>;
};

transformer我宁愿不必编写和template维护size_t.

有没有办法让我正确地通过这个template？

作为参考，在我要传递的名称之前添加template<>和/或无效。typename

4

1 回答 1

6

The correct syntax is

template<template<typename> class transformer, typename tuple> 
using transform_tuple_t                  
    = typename transform_tuple_index<no_index_transformer<transformer>::template transformer, 0, tuple>::t;
//    ^^^^^^^^                                                          ^^^^^^^^

Since no_index_transformer<transformer> is dependent, you need to tell the compiler that no_index_transformer<transformer>::transformer is a template with template (not template<>). Since transform_tuple_index<...> is dependent, you need to tell the compiler that transform_tuple_index<...>::t is a type using typename.

于 2015-01-01T18:12:39.187 回答