访问tuple_t的类型 [英] Accessing the types of a tuple_t

问题描述

我有此代码:

auto myTuple = hana::tuple_t<int, char*, long>;
std::cout << boost::typeindex::type_id<decltype(myTuple[1_c])>().pretty_name() << std::endl;

这将输出:

boost::hana::type_impl<char*>::_

我想访问'char *'类型，但是如果我这样做:

I want to access the 'char*' type, but if I do:

std::cout << boost::typeindex::type_id<decltype(myTuple[1_c])::type>().pretty_name() << std::endl;

它输出:

error: 'decltype(myTuple[1_c])' (aka 'boost::hana::type_impl<char *>::_ &') is not a class, namespace, or scoped enumeration
std::cout << boost::typeindex::type_id<decltype(myTuple[1_c])::type>().pretty_name() << std::endl

这是因为它是参考，如果我这样做:

It's because it is a reference, if I do:

std::cout << boost::typeindex::type_id<decltype(boost::hana::traits::remove_reference(myTuple[1_c]))::type>().pretty_name() << std::endl;

然后它输出'char *'.

Then it outputs 'char*'.

这是访问tuple_t类型的方法吗?一定没有那么麻烦的方法.

Is this the way to access the types of a tuple_t? There must be less cumbersome way.

推荐答案

这确实很棘手. Hana在hana::type上提供了一元加号运算符，该运算符会将所有经过ref限定的hana::type衰减为右值.所以基本上，

This is indeed tricky. Hana provides a unary plus operator on hana::type that decays any ref-qualified hana::type to a rvalue. So basically,

#include <boost/hana.hpp>
namespace hana = boost::hana;
using namespace hana::literals;

auto myTuple = hana::tuple_t<int, char*, long>;
using T = decltype(+myTuple[1_c])::type;
//                 ^~~~~ notice unary plus here

还请注意，您可能对使用<boost/hana/experimental/printable.hpp>中的hana::experimental::print感兴趣.这是一项实验性功能(因此不稳定)，但是我可以向您保证，它最终应以一种或另一种形式进入库中:

Also note that you might be interested in using hana::experimental::print from <boost/hana/experimental/printable.hpp>. This is an experimental (hence unstable) feature, but I can assure you it should eventually make its way into the library, in one form or the other:

#include <boost/hana.hpp>
#include <boost/hana/experimental/printable.hpp>
#include <iostream>
namespace hana = boost::hana;

int main() {
    auto myTuple = hana::tuple_t<int, char*, long>;
    std::cout << hana::experimental::print(myTuple) << std::endl;
}

输出:

(type<int>, type<char*>, type<long>)

编辑:一元加号运算符记录在 hana::type .

Edit: The unary plus operator is documented in the reference of hana::type.

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