这是 X3 中递归的常见问题。还没有解决。
我想我理解这个问题是因为x3::skip 改变了上下文对象¹。确实，删除它会使东西编译，并成功解析一些琐碎的测试用例：
"float: 3.14",
"int: 3.14",
"tuple: [float: 3.14,int: 3]",
但是，如果没有船长，显然以下内容无法解析：
// the following _should_ have compiled with the original skip() configuration:
"tuple: [ float: 3.14,\tint: 3 ]",
现在，我冒昧地说，您可以通过在顶层应用船长来解决这个问题（这意味着实例化“循环”中涉及的所有规则的上下文都是相同的）。如果这样做，您将立即开始在输入中接受更灵活的空格：
// the following would not have parsed with the original skip() configuration:
"float:3.14",
"int:3.14",
"tuple:[float: 3.14,int: 3]",
"tuple:[float:3.14,int:3]",
"tuple: [ float:3.14,\tint:3 ]",
即使编译成功，这些都不会用原始方法解析。
需要什么
这是我对代码所做的一些调整。
删除了无用的赋值运算符value::operator=（我不知道你为什么有它们）
-
添加代码以打印任何value 的调试转储：
friend std::ostream& operator<<(std::ostream& os, base_type const& v) {
struct {
std::ostream& operator()(float const& f) const { return _os << "float:" << f; }
std::ostream& operator()(int const& i) const { return _os << "int:" << i; }
std::ostream& operator()(std::vector<value> const& v) const {
_os << "tuple: [";
for (auto& el : v) _os << el << ",";
return _os << ']';
}
std::ostream& _os;
} vis { os };
return std::visit(vis, v);
}
-
去掉skipper，从: interpunction 中拆分出关键字：
namespace x3 = boost::spirit::x3;
x3::rule<struct value_class, value> const value_ = "value";
x3::rule<struct o_tuple_class, std::vector<value> > o_tuple_ = "tuple";
x3::real_parser<float, x3::strict_real_policies<float> > float_;
const auto o_tuple__def = "tuple" >> x3::lit(':') >> ("[" >> value_ % "," >> "]");
const auto value__def
= "float" >> (':' >> float_)
| "int" >> (':' >> x3::int_)
| o_tuple_
;
BOOST_SPIRIT_DEFINE(value_, o_tuple_)
-
现在，关键步骤：在顶层添加船长：
const auto entry_point = x3::skip(x3::space) [ value_ ];
-
创建好的测试驱动main():
int main()
{
for (std::string const str : {
"",
"float: 3.14",
"int: 3.14",
"tuple: [float: 3.14,int: 3]",
// the following _should_ have compiled with the original skip() configuration:
"tuple: [ float: 3.14,\tint: 3 ]",
// the following would not have parsed with the original skip() configuration:
"float:3.14",
"int:3.14",
"tuple:[float: 3.14,int: 3]",
"tuple:[float:3.14,int:3]",
"tuple: [ float:3.14,\tint:3 ]",
// one final show case for good measure
R"(
tuple: [
int : 4,
float: 7e9,
tuple: [float: -inf],
int: 42
])"
}) {
std::cout << "============ '" << str << "'\n";
//using boost::spirit::x3::parse;
auto first = str.begin(), last = str.end();
value val;
if (parse(first, last, parser::entry_point, val))
std::cout << "Parsed '" << val << "'\n";
else
std::cout << "Parse failed\n";
if (first != last)
std::cout << "Remaining input: '" << std::string(first, last) << "'\n";
}
}
现场演示
看Live On Coliru
//#define BOOST_SPIRIT_X3_DEBUG
#include <iostream>
#include <boost/fusion/adapted.hpp>
#include <boost/spirit/home/x3.hpp>
#include <string>
#include <vector>
#include <variant>
struct value: std::variant<int,float,std::vector<value>>
{
using base_type = std::variant<int,float,std::vector<value>>;
using base_type::variant;
friend std::ostream& operator<<(std::ostream& os, base_type const& v) {
struct {
std::ostream& operator()(float const& f) const { return _os << "float:" << f; }
std::ostream& operator()(int const& i) const { return _os << "int:" << i; }
std::ostream& operator()(std::vector<value> const& v) const {
_os << "tuple: [";
for (auto& el : v) _os << el << ",";
return _os << ']';
}
std::ostream& _os;
} vis { os };
return std::visit(vis, v);
}
};
namespace parser {
namespace x3 = boost::spirit::x3;
x3::rule<struct value_class, value> const value_ = "value";
x3::rule<struct o_tuple_class, std::vector<value> > o_tuple_ = "tuple";
x3::real_parser<float, x3::strict_real_policies<float> > float_;
const auto o_tuple__def = "tuple" >> x3::lit(':') >> ("[" >> value_ % "," >> "]");
const auto value__def
= "float" >> (':' >> float_)
| "int" >> (':' >> x3::int_)
| o_tuple_
;
BOOST_SPIRIT_DEFINE(value_, o_tuple_)
const auto entry_point = x3::skip(x3::space) [ value_ ];
}
int main()
{
for (std::string const str : {
"",
"float: 3.14",
"int: 3.14",
"tuple: [float: 3.14,int: 3]",
// the following _should_ have compiled with the original skip() configuration:
"tuple: [ float: 3.14,\tint: 3 ]",
// the following would not have parsed with the original skip() configuration:
"float:3.14",
"int:3.14",
"tuple:[float: 3.14,int: 3]",
"tuple:[float:3.14,int:3]",
"tuple: [ float:3.14,\tint:3 ]",
// one final show case for good measure
R"(
tuple: [
int : 4,
float: 7e9,
tuple: [float: -inf],
int: 42
])"
}) {
std::cout << "============ '" << str << "'\n";
//using boost::spirit::x3::parse;
auto first = str.begin(), last = str.end();
value val;
if (parse(first, last, parser::entry_point, val))
std::cout << "Parsed '" << val << "'\n";
else
std::cout << "Parse failed\n";
if (first != last)
std::cout << "Remaining input: '" << std::string(first, last) << "'\n";
}
}
打印
============ ''
Parse failed
============ 'float: 3.14'
Parsed 'float:3.14'
============ 'int: 3.14'
Parsed 'int:3'
Remaining input: '.14'
============ 'tuple: [float: 3.14,int: 3]'
Parsed 'tuple: [float:3.14,int:3,]'
============ 'tuple: [ float: 3.14, int: 3 ]'
Parsed 'tuple: [float:3.14,int:3,]'
============ 'float:3.14'
Parsed 'float:3.14'
============ 'int:3.14'
Parsed 'int:3'
Remaining input: '.14'
============ 'tuple:[float: 3.14,int: 3]'
Parsed 'tuple: [float:3.14,int:3,]'
============ 'tuple:[float:3.14,int:3]'
Parsed 'tuple: [float:3.14,int:3,]'
============ 'tuple: [ float:3.14, int:3 ]'
Parsed 'tuple: [float:3.14,int:3,]'
============ '
tuple: [
int : 4,
float: 7e9,
tuple: [float: -inf],
int: 42
]'
Parsed 'tuple: [int:4,float:7e+09,tuple: [float:-inf,],int:42,]'
¹ 其他指令也是如此，例如 x3::with<>。问题是上下文在每个实例化级别上得到扩展，而不是“修改”以恢复原始上下文类型，并结束实例化周期。