cppljevans · March 6, 2020 12:19 · Mar 6, 2020
diff --git a/sehe-coliru-stackoverflow-answer.cpp b/sehe-coliru-stackoverflow-answer.cpp
@@ -0,0 +1,133 @@
+//OriginalCode:
+//  http://coliru.stacked-crooked.com/a/d149f05af81bf0cf
+//WhichWasAnswerTo:
+//  https://stackoverflow.com/questions/45899090/recursive-rule-in-spirit-x3/45900983
+//DownloadedOn:
+//  2020-03-06.0512CST
+//Modifications:
+//  WHAT:
+//    * added macro, USE_TOP_LEVEL_SKIPPER
+//    * depending on defined(USE_TOP_LEVEL_SKIPPER)
+//      used skipper top level or at lower level
+//  WHY:
+//    to see if https://github.com/boostorg/spirit/pull/237 solves problem
+//    mentioned in the stackoverflow question.
+//Conclusion:
+//  #if defined(USE_TOP_LEVEL_SKIPPER)
+//     runs OK
+//  #else
+//     fails at compile time with:
+/*
+/usr/include/boost/spirit/home/x3/support/traits/attribute_of.hpp:35:51: fatal error: recursive template instantiation exceeded
+      maximum depth of 200
+                typename Component::attribute_type>::type>
+                ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^
+ */
+//=============
+//#define USE_TOP_LEVEL_SKIPPER
+//#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(':') >>
+    #ifndef USE_TOP_LEVEL_SKIPPER
+      x3::skip(x3::space) [
+    #endif
+      ("[" >> value_ % "," >> "]")
+    #ifndef USE_TOP_LEVEL_SKIPPER
+      ]
+    #endif
+      ;
+
+    const auto value__def
+        = "float" >> (':' >> float_)
+        | "int" >> (':' >> x3::int_)
+        | o_tuple_
+        ;
+
+    BOOST_SPIRIT_DEFINE(value_, o_tuple_)
+
+    const auto entry_point = 
+    #ifdef USE_TOP_LEVEL_SKIPPER
+       x3::skip(x3::space) [ value_ ]
+    #else
+       value_
+    #endif
+       ;
+}
+
+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";
+    }
+}