这是一个基于此处解释的自定义指令。
不幸的是，由于您需要的信息包含在迭代器的私有成员中，这仅适用于您发布的非常简单的示例。如果您在所有内容未对齐之前输出任何其他内容。如果您愿意稍微修改detail/output_iterator.hpp中的代码，您可以解决此问题。您可以在中注释“private:”，position_policy也可以简单地添加一个get_out_column与get_out_count.
为了使用它，您需要更改：
karma::int_ % karma::eol;
至：
custom_generator::align_list_to_current_position[karma::int_];
如您所见，自定义指令需要大量样板文件，但该代码的很大一部分对每个指令都是通用的。其实，除了改名字，我只需要改三件事：
确保它tracking在一组必需的属性中：
typedef typename boost::mpl::int_<
Subject::properties::value | karma::generator_properties::tracking
> properties;
使指令的属性与 list(%) 的属性相同（通过查看此处）：
template <typename Context, typename Iterator>
struct attribute
: boost::spirit::traits::build_std_vector<
typename boost::spirit::traits::attribute_of<Subject, Context, Iterator>::type
>
{};
最后改变generate功能。在这个函数中，我只是构建了一个列表，它的左成员是你传递给指令的任何成员，右成员是 karma::eol 的串联和需要对齐的多个空格。
#include <iostream>
#include <string>
#include <vector>
#include <boost/spirit/include/karma.hpp>
//START OF ALIGN_LIST_TO_CURRENT_POSITION.HPP
#include <boost/spirit/include/karma_generate.hpp>
///////////////////////////////////////////////////////////////////////////////
// definition the place holder
namespace custom_generator
{
BOOST_SPIRIT_TERMINAL(align_list_to_current_position);
}
///////////////////////////////////////////////////////////////////////////////
// implementation the enabler
namespace boost { namespace spirit
{
// We want custom_generator::align_list_to_current_position to be usable as a directive only,
// and only for generator expressions (karma::domain).
template <>
struct use_directive<karma::domain, custom_generator::tag::align_list_to_current_position>
: mpl::true_ {};
}}
///////////////////////////////////////////////////////////////////////////////
// implementation of the generator
namespace custom_generator
{
// That's the actual columns generator
template <typename Subject>
struct align_list_to_current_position_generator
: boost::spirit::karma::unary_generator<
align_list_to_current_position_generator<Subject> >
{
// Define required output iterator properties: take the properties needed by the subject and add tracking
typedef typename boost::mpl::int_<Subject::properties::value | boost::spirit::karma::generator_properties::tracking> properties;
// Define the attribute type exposed by this parser component
template <typename Context, typename Iterator>
struct attribute
: boost::spirit::traits::build_std_vector<
typename boost::spirit::traits::attribute_of<Subject, Context, Iterator>::type>
{};
align_list_to_current_position_generator(Subject const& s)
: subject(s)
{}
// This function is called during the actual output generation process.
// It dispatches to the embedded generator while supplying a new
// delimiter to use
template <typename OutputIterator, typename Context
, typename Delimiter, typename Attribute>
bool generate(OutputIterator& sink, Context& ctx
, Delimiter const& delimiter, Attribute const& attr) const
{
using boost::spirit::karma::repeat;
using boost::spirit::karma::char_;
using boost::spirit::karma::eol;
using boost::spirit::karma::domain;
std::size_t column = sink.get_out_count();
//This would only work if you comment "private:" in line 82 of boost/spirit/home/karma/detail/output_iterator.hpp
// std::size_t column = sink.track_position_data.get_column()-1;
return boost::spirit::compile<domain>(subject%(eol << repeat(column)[char_(" ")])).generate(sink, ctx, delimiter, attr);
}
// This function is called during error handling to create
// a human readable string for the error context.
template <typename Context>
boost::spirit::info what(Context& ctx) const
{
return boost::spirit::info("align_list_to_current_position", subject.what(ctx));
}
Subject subject;
};
}
///////////////////////////////////////////////////////////////////////////////
// instantiation of the generator
namespace boost { namespace spirit { namespace karma
{
// This is the factory function object invoked in order to create
// an instance of our align_list_to_current_position_generator.
template <typename Subject, typename Modifiers>
struct make_directive<custom_generator::tag::align_list_to_current_position, Subject, Modifiers>
{
typedef custom_generator::align_list_to_current_position_generator<Subject> result_type;
result_type operator()(unused_type, Subject const& s, unused_type) const
{
return result_type(s);
}
};
}}}
//END OF ALIGN_LIST_TO_CURRENT_POSITION.HPP
int main() {
std::vector<int> v { 0, 1, 2, 3 };
{
namespace karma = boost::spirit::karma;
using custom_generator::align_list_to_current_position;
karma::rule<std::ostream_iterator<char>, std::vector<int>() > myRule =
karma::lit("Some text: ") << align_list_to_current_position[karma::int_];
karma::generate(std::ostream_iterator<char>(std::cout), myRule, v);
std::cout << std::endl;
//This rule would work if you make the changes mentioned in align_list_to_current_position_generator::generate
karma::rule<std::ostream_iterator<char>, std::vector<int>() > myRuleThatFails =
karma::lit(":_(") << karma::eol << karma::lit("Some text: ") << align_list_to_current_position[karma::int_ << karma::int_];
karma::generate(std::ostream_iterator<char>(std::cout), myRuleThatFails, v);
}
return 0;
}