Question

让我们考虑以下代码：

#include <boost/phoenix.hpp>
#include <algorithm>
#include <iostream>
#include <vector>

int main()
{
    std::vector<int> const small_ints {1, 2, 3, 4, 5};
    std::vector<int> const big_ints {11, 12, 13, 14, 15};

    namespace phoenix = boost::phoenix;
    using namespace boost::phoenix;
    using namespace boost::phoenix::placeholders;

    std::vector<int>::const_iterator big_ints_it;
    std::for_each(small_ints.cbegin(), small_ints.cend(),
        for_(phoenix::ref(big_ints_it) = begin(phoenix::cref(big_ints)), phoenix::ref(big_ints_it) != end(phoenix::cref(big_ints)), ++phoenix::ref(big_ints_it))
        [
            std::cout << val('(') << arg1 << ',' << *phoenix::ref(big_ints_it) << "), "
        ]
    );
}

通过打印预期的对序列

可以正常工作

(1,11), (1,12), (1,13), (1,14), (1,15), (2,11), (2,12), …, (5,14), (5,15),

这只是双重迭代产生的笛卡尔积。

然而for_很难看！查看所有ref，cref（我不得不明确地从phoenix中选择，否则会与std版本发生冲突）。更不用说我必须保持无用的big_ints_it变量！

另请注意，这遵循example provided by Phoenix documentation本身。

现在我尝试使用for_each mentioned along iteration algorithms，期待更简单的版本：

std::for_each(small_ints.cbegin(), small_ints.cend(),
    for_each(phoenix::cref(big_ints),
        std::cout << val('(') << arg1 << ", " << arg2 << "), "
    )
);

但它甚至没有编译！或者至少使用Visual Studio 2013和Boost 1.61。

我收到多个错误。首先是

1>c:\programming\boost_1_61_0\boost\proto\detail\poly_function.hpp(205): error C2039: 'type' : is not a member of 'boost::proto::functional::at::result<Sig>'
1>          with
1>          [
1>              Sig=boost::proto::functional::at (boost::phoenix::vector2<const boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::phoenix::detail::tag::function_eval,boost::proto::argsns_::list3<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::terminal,boost::proto::argsns_::term<boost::phoenix::impl::for_each>,0>,boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::terminal,boost::proto::argsns_::term<boost::reference_wrapper<const std::vector<int,std::allocator<int>>>>,0>>,boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::shift_left,boost::proto::argsns_::list2<boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::shift_left,boost::proto::argsns_::list2<boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::shift_left,boost::proto::argsns_::list2<boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::shift_left,boost::proto::argsns_::list2<boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::shift_left,boost::proto::argsns_::list2<boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::terminal,boost::proto::argsns_::term<std::basic_ostream<char,std::char_traits<char>> &>,0>>,boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::terminal,boost::proto::argsns_::term<char>,0>>>,2>>,boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::terminal,boost::proto::argsns_::term<boost::phoenix::argument<1>>,0>>>,2>>,boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::terminal,boost::proto::argsns_::term<char [3]>,0>>>,2>>,boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::terminal,boost::proto::argsns_::term<boost::phoenix::argument<2>>,0>>>,2>>,boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::terminal,boost::proto::argsns_::term<char [4]>,0>>>,2>>>,3>> *,const int &> &,boost::mpl::int_<2>)
1>          ]
…

我甚至无法输入整个第一个错误，因为StackOverflow说，问题正文限制为30,000个字符，我输入了135,034个字符......

Answer 1

问题是你在内在表达中使用arg2。让我们来看看情况。

你基本上有：

- for_each(begin,end,outer_expr);
- outer_expr=for_each(begin,end,inner_expr);

outer_expr和inner_expr都是lambda表达式，它接受一个参数（其各自向量的元素）。

您需要的是将参数从outer_expr传递到inner_expr的方法。幸运的是，Boost.Phoenix提供了一种简单的方法：boost::phoenix::lambda：

std::for_each(small_ints.cbegin(), small_ints.cend(),
            phx::for_each(phx::cref(big_ints),
            phx::lambda(_a=arg1)[phx::ref(std::cout) << '(' << _a << ", " << arg1 << "), "]
        )
    );

或者变得有点傻，但也许有助于提高可读性：

boost::phoenix::placeholders::arg1_type current_small_int, current_big_int;
boost::phoenix::local_names::_a_type passed_small_int;

std::for_each(small_ints.cbegin(), small_ints.cend(),
            phx::for_each(phx::cref(big_ints),
            phx::lambda(passed_small_int=current_small_int)[phx::ref(std::cout) << '(' << passed_small_int << ", " << current_big_int << "), "]
        )
    );

完整示例(Running on WandBox)

#include <iostream>
#include <algorithm>
#include <vector>
#include <boost/phoenix.hpp>


int main()
{
    std::vector<int> const small_ints {1, 2, 3, 4, 5};
    std::vector<int> const big_ints {11, 12, 13, 14, 15};

    namespace phx = boost::phoenix;
    using boost::phoenix::placeholders::arg1;
    using boost::phoenix::local_names::_a;

    std::vector<int>::const_iterator big_ints_it;
    std::for_each(small_ints.cbegin(), small_ints.cend(),
        phx::for_each(phx::cref(big_ints),
        phx::lambda(_a=arg1)[phx::ref(std::cout) << '(' << _a << ", " << arg1 << "), "]
    )
    );
}

PS：如果您的编译器可以初始化这样的向量，它应该可以使用实际的lambdas，这可能会减少头痛。

如何在Boost.Phoenix中使用基于范围的for_each？

1 个答案: