将属性转换为 boost::variant

Question

在学习如何使用 boost spirit、phoenix 和 fusion 库时，我遇到了这个不能在 msvc（2015，版本 14）和 boost 1.61.0 上编译的最小示例

#include <boost/spirit/include/karma.hpp>
#include <boost/variant/variant.hpp>

namespace ka = boost::spirit::karma;

struct U /* a kind of union (legacy code)*/
    {
        bool kind;
        double foo; /* if kind = true */
        size_t bar; /* if kind = false */
    };

typedef boost::variant<double, size_t> UVariant;

namespace boost { namespace spirit { namespace traits {
    template<>
    struct transform_attribute<U,UVariant,ka::domain>
    {
        typedef UVariant type;
        static type pre(U & u) { 
            switch (u.kind)
            { 
            case true: 
                return type(u.foo); 
            case false: 
                return type(u.bar);
            }
        }
    };
}}}

typedef std::back_insert_iterator<std::string> iterator;

class grm: public ka::grammar<iterator, U()>
{
public:
    grm():grm::base_type(start)
    {
        start = ka::attr_cast<U,UVariant >(foo | bar);
        foo = ka::double_;
        bar = ka::uint_;
        */
    }
private:
    ka::rule<iterator,U()> start;
    ka::rule<iterator,double()> foo;
    ka::rule<iterator,size_t()> bar;
};

int main(int argc, char * argv[])
{
    grm g;
    U u;
    u.kind = true;
    u.foo = 1.0;

    std::string generated;
    std::back_insert_iterator<std::string> sink(generated);
    ka::generate(sink,g,u);


    return 0;
}

然后我收到以下错误消息：

error C2665: 'boost::detail::variant::make_initializer_node::apply::initializer_node::initialize' : none of the 5 overloads could convert all arguments types

报告了一个类似的问题 here 虽然我不明白答案是如何解决这个问题的，以及这是否真的是同一个问题，因为似乎所有类型都已正确提供（不需要类型转换） .

Answer 1

问题似乎是 Spirit 没有选择您的自定义 transform_attribute 自定义点。它使用默认的，它试图从 const U(!!) 构造一个 boost::variant<double,size_t>，但显然失败了。

Karma 始终在内部使用 const 值，因此您需要将 transform_attribute 的专业化更改为：

namespace boost { namespace spirit { namespace traits {
    template<>
    struct transform_attribute<const U,UVariant,ka::domain>
                               ^^^^^^^
    {
        typedef UVariant type;
        static type pre(const U & u) {
                        ^^^^^^^ 
            //same as before
        }
    };
}}}

然后它会被 Karma 拾取，一切都会正常。

完整样本（On rextester）：

#include <boost/spirit/include/karma.hpp>
#include <boost/variant/variant.hpp>

namespace ka = boost::spirit::karma;

struct U /* a kind of union (legacy code)*/
    {
        bool kind;
        double foo; /* if kind = true */
        size_t bar; /* if kind = false */
    };

typedef boost::variant<double, size_t> UVariant;

namespace boost { namespace spirit { namespace traits {
    template<>
    struct transform_attribute<const U,UVariant,ka::domain>
    {
        typedef UVariant type;
        static type pre(const U & u) { 
            if(u.kind)
            { 
                return type(u.foo); 
            }
            else
            {
                return type(u.bar);
            }
        }
    };
}}}

typedef std::back_insert_iterator<std::string> iterator;

class grm: public ka::grammar<iterator, U()>
{
public:
    grm():grm::base_type(start)
    {
        start = ka::attr_cast< UVariant >(foo | bar);
        foo = ka::double_;
        bar = ka::uint_;
    }
private:
    ka::rule<iterator,U()> start;
    ka::rule<iterator,double()> foo;
    ka::rule<iterator,size_t()> bar;
};

int main(int argc, char * argv[])
{
    grm g;
    U u;
    u.kind = false;
    u.foo = 1.0;
    u.bar = 34;

    std::string generated;
    std::back_insert_iterator<std::string> sink(generated);
    ka::generate(sink,g,u);

    std::cout << generated << std::endl;


    return 0;
}

将属性转换为 boost::variant

casting attribute to boost::variant

c++

boost

boost-variant

boost-spirit-karma