如何写一个for循环的花序列？ [英] How to write a for loop for a Hana sequence?

问题描述

我有一个Boos.Hana序列和我想打印屏幕用逗号分隔。然而，逗号分隔的元素而已，所以我要检查，如果我在最后一个元素。

I have a Boos.Hana sequence and I would like to print it to screen separated by commas. However the commas separate elements only, so I have to check if I am at the last element.

目前我劈为pretty坏（看着指针，铸造无效* 。

Currently my hack is pretty bad (looking at the pointer and casting to void*.

template<class P, class... Ts>
decltype(auto) operator<<(
    std::ostream& os, 
    boost::hana::tuple<Ts...> const& tpl
){  
    os << "{";
    boost::hana::for_each(
        tpl, [&](auto& x){
            os << x;
            if((void*)&boost::hana::back(tpl) != (void*)&x) os << ", ";
        }
    );
    return os << "}";
}

在Boost.Fusion的情况下，它是更为复杂，因为我使用的融合迭代器（的boost ::融合::开始和的boost ::融合::结束），但至少我可以比较的迭代器。 （布尔最后=的result_of :: equal_to＆LT;类型名的result_of ::接下来的＆lt;第一＆GT; ::类型，最后＆GT; ::值）。

In the case of Boost.Fusion it was more complicated because I use fusion iterators (boost::fusion::begin and boost::fusion::end) but at least I could compare the iterators. (bool last = result_of::equal_to<typename result_of::next<First>::type, Last>::value).

要问这个问题的另一种方式是，如果有哈纳（元）迭代器。

Another way to ask this question is if there are (meta) iterators in Hana.

推荐答案

首先，要回答你的评论， drop_back 确实让副本。哈纳所有算法进行复印，并渴望，因为这里记录。

First, to answer your comment, drop_back does make a copy. All algorithms in Hana make copies and are eager, as documented here.

其次，你可以使用<一个href=\"http://boostorg.github.io/hana/structboost_1_1hana_1_1Sequence.html#ab6e88c5dd3f638a60c0a451ad6db95ec\"相对=nofollow> 花::点缀 增加各元素之间用逗号，导致类似

Secondly, you could use hana::intersperse to add a comma between each element, resulting in something like

template<class P, class... Ts>
decltype(auto) operator<<(
    std::ostream& os, 
    boost::hana::tuple<Ts...> const& tpl
){  
    os << "{";
    boost::hana::for_each(boost::hana::intersperse(tpl, ", "), 
        [&](auto const& x){
            os << x;
        });
    return os << "}";
}

不过，最好的解决办法可能是使用实验::打印，这不正是你想要什么：

However, the best solution would probably be to use experimental::print, which does exactly what you want:

#include <boost/hana/experimental/printable.hpp>
#include <boost/hana/tuple.hpp>
#include <iostream>

int main() {
    auto ts = hana::make_tuple(1, 2, 3);
    std::cout << hana::experimental::print(ts);
}

---

修改

如果你想使用点缀的解决方案，但不想让序列的拷贝，你可以做到以下几点：

If you want to use the intersperse solution, but do not want to make a copy of the sequence, you can do the following:

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

template <class... Ts>
decltype(auto) operator<<(std::ostream& os, hana::tuple<Ts...> const& tpl) {
    os << "{";
    char const* sep = ", ";
    auto refs = hana::transform(tpl, [](auto const& t) { return std::ref(t); });
    hana::for_each(hana::intersperse(refs, std::ref(sep)),
        [&](auto const& x){
            os << x.get();
        });
    return os << "}";
}

不过说真的，你或许应该使用花::实验::打印。如果你的用例是性能的关键，你想避免创建一个的std ::字符串，我会质疑中的std :: ostream的<用法/ code>摆在首位。

But really, you should probably be using hana::experimental::print. And if your use case is performance critical and you want to avoid creating a std::string, I would question the usage of std::ostream in the first place.

修改结束

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