c++ - 如何在使用 boost 进行 QI 解析期间舍入一个数字？

Question

用一个简单的 Boost qi 语法，我怎样才能让它四舍五入我的数字？
这是解析器:

factor =
            float_                          [_val = _1]
            |   ('-' >> factor              [_val = -_1])
            |   ('+' >> factor              [_val = _1])
            ;

这可以解析一个浮点数，也可以是负数。
我想对浮点数进行四舍五入，所以我会在语法中添加这样的内容:

|   ('~' >> factor              [_val = round(_1)])

但这会导致编译时错误:

no type named ‘__type’ in ‘struct __gnu_cxx::__enable_if<false, double>’

这个错误对我来说信息量不大，你能帮忙吗？我希望能够舍入一个数字，即:

~1.8 -> 2
~1.2 -> 1

注意:我正在解析 phrase_parse .

Answer 1

语义 Action 需要 Phoenix Actors，这是延迟函数。
选项:

适配宏 https://www.boost.org/doc/libs/1_73_0/libs/phoenix/doc/html/phoenix/modules/function/adapting_functions.html

phoenix::function<>

phoenix::bind

自己写，详情见 https://www.boost.org/doc/libs/1_68_0/libs/spirit/doc/html/spirit/qi/tutorials/semantic_actions.html#spirit.qi.tutorials.semantic_actions.examples_of_semantic_actions

简化试验台
只解析一个数字:

#include <boost/spirit/include/qi.hpp>
#include <boost/spirit/include/phoenix.hpp>
namespace qi = boost::spirit::qi;

int main() {
    std::string s = "1.75";

    double v;
    if (qi::parse(begin(s), end(s), qi::double_, v)) {
        std::cout << "Parsed: " << v << "\n";
    }
}

打印 Live On Coliru :

Parsed: 1.75

适应
使用宏:

#include <boost/spirit/include/qi.hpp>
#include <boost/spirit/include/phoenix.hpp>
#include <cmath>
namespace qi = boost::spirit::qi;

int main() {
    std::string s = "1.75";

    double v;
    if (qi::parse(begin(s), end(s), qi::double_, v)) {
        std::cout << "Parsed: " << v << "\n";
        std::cout << "Rounded: " << round(v) << "\n";
    }
}

打印 Live On Coliru :

Parsed: 2

function<>您可以在此处对签名进行硬编码:

boost::phoenix::function<double(*)(double)> round_(::round);

然而，真正的力量来自多态可调用对象:

struct round_f {
    template <typename T> auto operator()(T const& v) const {
        using std::round; // activate ADL
        return round(v);
    }
};
boost::phoenix::function<round_f> round_{};

现在您可以使用 round_具有自由函数重载的任何类型的actor round兼容的过载。如果明天你决定解析 long double 会很方便, float或 boost::multiprecision::cpp_dec_float .
看到它 Live On Coliru

#include <boost/spirit/include/qi.hpp>
#include <boost/spirit/include/phoenix.hpp>
#include <cmath>
namespace qi = boost::spirit::qi;

struct round_f {
    template <typename T> auto operator()(T const& v) const {
        using std::round; // activate ADL
        return round(v);
    }
};
boost::phoenix::function<round_f> round_{};

int main() {
    std::string s = "1.75";

    double v;
    using namespace qi::labels;
    if (qi::parse(begin(s), end(s), qi::double_ [ _val = round_(_1) ], v)) {
        std::cout << "Parsed: " << v << "\n";
    }
}

打印

Parsed: 2

使用 phoenix::bind作为较低级别的构建 block ，您可以绑定(bind)未包装的可调用对象:

if (qi::parse(begin(s), end(s), qi::double_ 
            [ _val = phoenix::bind(round_f{}, _1) ], v))
{
    std::cout << "Parsed: " << v << "\n";
}

如果你不介意丑:

if (qi::parse(begin(s), end(s), qi::double_ 
    [ _val = phoenix::bind(static_cast<double(&)(double)>(std::round), _1) ], v))
{
    std::cout << "Parsed: " << v << "\n";
}

参见 Live On Coliru