How can I get the innermost template parameter type?

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Q

In a dummy example of a class

typedef myStruct>> mv;

int is the innermost

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2条回答

时光说笑

2020-12-21 01:56

Building on 0x499602D2's answer, we get the following to only consider the first template parameter if ever there are several. And yes it compiles. It's also slightly simpler.

#include <tuple>
#include <type_traits>
#include <vector>

template<typename T>
struct innermost_impl
{
    using type = T;
};

template<template<typename...> class E, typename Head, typename... Tail>
struct innermost_impl<E<Head, Tail...>>
{
    using type = typename innermost_impl<Head>::type;
};

template<typename T>
using innermost = typename innermost_impl<T>::type;

template<class>
struct X;

static_assert(std::is_same<innermost<X<X<X<int>>>>, int>::value, "");

static_assert(
    std::is_same<innermost<std::vector<X<std::vector<int>>>>, int>::value,
    ""
);

int main()
{
}

Note that no attempt is made to validate that the same template is used over and over.

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太阳男子

2020-12-21 01:58

Try the following. It also returns a tuple if the template has more than one element:

#include <tuple>
#include <type_traits>

template<typename T>
struct innermost_impl
{
    using type = T;
};

template<template<typename> class E, typename T>
struct innermost_impl<E<T>>
{
    using type = typename innermost_impl<T>::type;
};

template<template<typename...> class E, typename... Ts>
struct innermost_impl<E<Ts...>>
{
    using type = std::tuple<typename innermost_impl<Ts>::type...>;
};

template<typename T>
using innermost = typename innermost_impl<T>::type;

template<class>
struct X;

static_assert(std::is_same<innermost<X<X<X<int>>>>, int>::value, "");

int main()
{
}

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