How to declare a typedef that references itself?

问题

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.

How do I create a "recursive" templated type?

Something like:

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

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

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;
};

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

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

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

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

回答1:

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

来源：https://stackoverflow.com/questions/37219718/how-to-declare-a-typedef-that-references-itself

标签

c++

templates

c++11

variadic-templates

generic-programming