变体的 C++ boost::spirit::karma 规则

Question

我的程序中有一个 boost::variant，它采用 double、uint16_t、std::string 等。我正在存储这些，我想使用 boost::karma 生成/打印它们。我是 boost::spirit 的新手，但我知道它适用于变体。我有什么选择？生成其中之一的简单语法/规则是什么样的？任何帮助都会很棒!

Answer 1

1。简单规则

我能想到的绝对最简单的示例展示了 karma 如何甚至可以即时^{[1] 为您的特定变体合成 auto_ 规则]}:

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

int main() {
    typedef boost::variant<double, unsigned int, std::string> V;

    for(auto v : { V{42u}, V{3.1416}, V{"Life Of Pi"} })
        std::cout << karma::format(karma::auto_, v) << "\n";
}

打印:

42
3.142
Life Of Pi

简单易行!

单独语法中的等价物: Live On Coliru

2。更复杂的样本

更复杂的语法(还有 Live On Coliru )，它向您展示了 Spirit 的属性兼容性如何神奇地规定 DoTheRightThing™:

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

namespace karma = boost::spirit::karma;

typedef boost::variant<double, unsigned int, std::string> V;

struct gen : karma::grammar<boost::spirit::ostream_iterator, V()> {
    gen() : gen::base_type(start) 
    {
        using namespace karma;

        start = my_real | my_uint | my_text;

        my_uint = "The value is unsigned integral value (" << uint_ << ")";
        my_real = "The value is double precision floating point value (" << double_ << ")";
        my_text = "The value is an epos: '" << *quoted_char << "'";

        quoted_char = '\\' << char_("'") | graph | "\\x" << uint_generator<uint8_t, 16>();
    }
  private:
    karma::rule<boost::spirit::ostream_iterator, V()>           start;
    karma::rule<boost::spirit::ostream_iterator, double()>      my_real;
    karma::rule<boost::spirit::ostream_iterator, unsigned       int()>   my_uint;
    karma::rule<boost::spirit::ostream_iterator, std::string()> my_text;
    karma::rule<boost::spirit::ostream_iterator, uint8_t()>     quoted_char;
};

int main()
{
    for(auto v : { V{42u}, V{3.1416}, V{"It's a beautiful day!"} })
        std::cout << karma::format(gen(), v) << "\n";
}

这打印:

The value is unsigned integral value (42)
The value is double precision floating point value (3.142)
The value is an epos: 'It\'s\x20a\x20beautiful\x20day!'

main 也可以写成

int main() {
    std::cout << karma::format(gen() % "\n", std::vector<V>{42u,3.1416,"It's a beautiful day!"}) << "\n";
}

这应该让您了解 Spirit Parser/Generator 框架的多功能性。

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^[1] 只要存在自动解析器生成器特征； Spirit 为许多类型提供它们，包括 uint、std::string、double 和 variant(但还有可选值、 vector 、映射、任何可以改编为 fusion 序列的东西等等。)