CODEWITHSUNDEEP
c++boostboost-hana
chila
chila

Reputation: 2442

Accessing the types of a tuple_t

I have this code:

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

This outputs:

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

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;

It outputs: 

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

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;

Then it outputs 'char*'.

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

Upvotes: 1

Views: 156

Answers (1)

Louis Dionne
Louis Dionne

Reputation: 3093

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

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;
}

Output:

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

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

Upvotes: 4

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