如何在给定 T 和元素个数 n 的情况下构建 hana::tuple_t<T, T, T, ...>

Question

这似乎是一件相当基本的事情，所以我正在寻找一个或多或少简短、内置且易于阅读的解决方案。
我能想到的最短的事情是

hana::unfold_left<hana::tuple_tag>( hana::int_c<n>, [] ( auto count ) {
            return hana::if_( count == hana::int_c<0>, hana::nothing,
                             hana::just( hana::make_pair( count - hana::int_c<1>,
                                                        hana::type_c<T> ) ) );
        } );

远非简短易读...

Answer 1

正如@jv_ 所指出的，hana::replicate 完全可以做到这一点。
参考文档中的示例提供了有关如何实现此目的的足够信息：

// Copyright Louis Dionne 2013-2016
// Distributed under the Boost Software License, Version 1.0.
// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)

#include <boost/hana/equal.hpp>
#include <boost/hana/integral_constant.hpp>
#include <boost/hana/optional.hpp>
#include <boost/hana/replicate.hpp>
#include <boost/hana/tuple.hpp>

namespace hana = boost::hana;
static_assert(hana::replicate<hana::tuple_tag>('x', hana::size_c<2>) == hana::make_tuple('x', 'x'), "");
// Of course, there can't be more than one element in an `optional`.
static_assert(hana::replicate<hana::optional_tag>('x', hana::size_c<2>) == hana::just('x'), "");
int main() { }

如何在给定 T 和元素个数 n 的情况下构建 hana::tuple_t<T, T, T, ...>

How to build a hana::tuple_t<T, T, T, ...> given T and the number of elements n

c++

metaprogramming

template-meta-programming

c++14

boost-hana