如何声明引用自身的typedef？ [英] How to declare a typedef that references itself?

问题描述

我有一个Variant类型，我想在像JSON解析器的东西使用。 JSON中的类型可以包括对象和数组。这些对象和数组可以包含自己类型的成员：

I have a Variant type which I want to use in something like a JSON parser. A type in JSON can include objects and arrays. These objects and arrays can contain members of their own type:

typedef variant<int,float,bool,std::string> baseType;
typedef std::vector<baseType> arrayType; // problem, can't have arrays of arrays
typedef std::unordered_map<std::string,baseType> objType; // problem, can't have objects or arrays within objects.

如何创建一个递归模板类型？

How do I create a "recursive" templated type?

类似：

typedef variant<int,float,bool,std::string,std::vector<type>,std::unordered_map<std::string,type> type;

我知道boost;我对使用它的答案不感兴趣。我试图找出它如何做它与 recursive_wrapper 但是，自由使用预处理器宏是使我非常困难。

I'm aware of boost; I'm not interested in answers that use it. I've tried to work out how it does it with recursive_wrapper but the liberal use of pre-processor macros is making it very hard for me.

EDIT

在Yakk的帮助下，我已经走得很远：

With the help of Yakk below, I've gotten this far:

template<typename... Ts>
class Variant;

template<class T>
struct tag
{
    using type=T;
};

template<class X, class A, class B>
struct subst : tag<X>
{};

template<class X, class A, class B>
using subst_t = typename subst<X,A,B>::type;

template<class A, class B>
struct subst<A,A,B> : tag<B>
{};

template<class X, class A, class B>
struct subst<X&,A,B> : tag<subst_t<X,A,B>&>
{};

template<class X, class A, class B>
struct subst<X&&,A,B> : tag<subst_t<X,A,B>&&>
{};

template<class X, class A, class B>
struct subst<X const,A,B> : tag<subst_t<X,A,B>const>
{};

template<class X, class A, class B>
struct subst<X volatile,A,B> : tag<subst_t<X,A,B>volatile>
{};

template<class X, class A, class B>
struct subst<X const volatile,A,B> : tag<subst_t<X,A,B>const volatile>
{};

template<template<class...> class Z, class... Xs, class A, class B>
struct subst<Z<Xs...>,A,B> : tag<Z<subst_t<Xs,A,B>...>>
{};

template<template<class,size_t> class Z, class X, size_t n, class A, class B>
struct subst<Z<X,n>,A,B> : tag<Z<subst_t<X,A,B>,n>>
{};

template<class R, class...Xs, class A, class B>
struct subst<R(Xs...),A,B> : tag<subst_t<R,A,B>(subst_t<Xs,A,B>...)>
{};

struct RecursiveType {};

template<typename Sig>
struct RecursiveVariant
{
    using VariantType = Variant<subst_t<Sig,RecursiveType,RecursiveVariant>>;

    template<typename V,
        typename std::enable_if<
            !std::is_same<RecursiveVariant,typename std::decay<V>::type>::value
            && std::is_convertible<V,VariantType>::value
        >
    ::type>
    RecursiveVariant(V&& vIn)
    :
        m_variant(vIn)
    {}

    RecursiveVariant(){};


    template<typename T, typename... Args>
    void Set(Args&&... args)
    {
        m_variant.Set<T,Args...>(std::forward<Args>(args)...);
    }

    template<typename T>
    const T& Get() const
    {
        return m_variant.Get<T>();
    }

    template<typename T>
    T& Get()
    {
        return m_variant.Get<T>();
    }

    VariantType m_variant;
};

实际的Variant类型是这里 http://codereview.stackexchange.com/questions/127372/variant-class-that-i-dont-think -is-missing-anything

The actual Variant type I have is here http://codereview.stackexchange.com/questions/127372/variant-class-that-i-dont-think-is-missing-anything

我认为上述 RecursiveWrapper 是关闭的。根据我的理解，我应该使用它作为实际类型，即

I think the above RecursiveWrapper is way off. As I understand it, I should use it as the actual type, i.e.

RecursiveWrapper<RecursiveType> Variant

但是，这不能正确，因为我没有指定 Variant 。关于代码的问题我不明白是

However, this can't be correct because nowhere have I specified the types allowed in Variant. Questions about the code I don't understand are

1. 什么是 RecursiveType ？


2. 如何将可变参数模板类型转移到底层 Variant ？

1. What is RecursiveType?
2. How can I forward variadic template types to the underlying Variant?

推荐答案

我将使用boost变量，特别是 http://www.boost.org/doc/libs/1_61_0/doc/html/boost/make_recursive_variant.html 以及 http://www.boost.org /doc/libs/1_61_0/doc/html/variant/tutorial.html#variant.tutorial.recursive.recursive-variant

I would use boost variant, in particular see http://www.boost.org/doc/libs/1_61_0/doc/html/boost/make_recursive_variant.html and http://www.boost.org/doc/libs/1_61_0/doc/html/variant/tutorial.html#variant.tutorial.recursive.recursive-variant

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