boost::combine，基于范围的和结构化绑定

问题描述

有没有办法让 boost::combine 与结构化绑定和基于范围的 for 一起工作(这样结构绑定中的标识符实际上指向容器的元素而不是嵌套的任何 boost::combine 在幕后使用)?以下(live example)无法编译:

Is there a way to make boost::combine work with structured bindings and range-based for (so that identifiers in the structure binding actually point to containers' elements instead of nested tuples of whatever boost::combine uses under the hood)? The following (live example) fails to compile:

#include <boost/range/combine.hpp>
#include <iostream>

int main()
{
    std::vector<int> a{1,2,3};
    std::vector<int> b{2,3,4};

    for (auto [f, s] : boost::combine(a, b))
    {
        std::cout << f << ' ' << s << std::endl   
    }
}

推荐答案

真正的答案是使用 boost::tie 或抓取 range-v3 zip() 这实际上产生了一个 std::tuple.

The real answer is to use either boost::tie or grab the range-v3 zip() which actually yields a std::tuple.

仅用于教育目的的答案只是调整 boost::tuples::cons 的结构化绑定机制.该类型已经有一个 get() 与 ADL 一起工作并做正确的事情，所以我们需要做的就是提供 tuple_size 和 tuple_element(这最终真的很容易做到，因为 Boost 中已经存在这些确切的特征):

The for educational purposes only answer is just to adapt the structured bindings machinery for boost::tuples::cons. That type already has a get() which works with ADL and does the right thing, so all we need to do is provide tuple_size and tuple_element (which ends up being really easy to do since these exact traits already exist in Boost):

namespace std {
    template <typename T, typename U>
    struct tuple_size<boost::tuples::cons<T, U>>
        : boost::tuples::length<boost::tuples::cons<T, U>>
    { };

    template <size_t I, typename T, typename U>
    struct tuple_element<I, boost::tuples::cons<T, U>>
        : boost::tuples::element<I, boost::tuples::cons<T, U>>
    { };
}

但实际上不要在实际代码中这样做，因为实际上只有类型作者应该选择加入这种事情.

But don't actually do that in real code, since really only the type author should opt-in to this kind of thing.

这将使结构化绑定正常工作.

That'll make the structured binding just work.

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