parsing a single value into an ast node with a container - c++

Question

My problem is the following. I have an ast node which is defined as like the following:
struct foo_node{
std::vector<std::string> value;
}
and I have a parser like this for parsing into the struct, which works fine:
typedef x3::rule<struct foo_node_class, foo_node> foo_node_type;
const foo_node_type foo_node = "foo_node";
auto const foo_node_def = "(" >> +x3::string("bar") >> ")";
Now I want to achieve that the parser also parses "bar", without brackets, but only if its a single bar. I tried to do it like this:
auto const foo_node_def = x3::string("bar")
| "(" > +x3::string("bar") > ")";
but this gives me a compile time error, since x3::string("bar") returns a string and not a std::vector<std::string>.
My question is, how can I achieve, that the x3::string("bar") parser (and every other parser which returns a string) parses into a vector?

Answer 1

The way to parse a single element and expose it as a single-element container attribute is x3::repeat(1) [ p ]:
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#include <boost/fusion/adapted/struct.hpp>
#include <boost/spirit/home/x3.hpp>
#include <iostream>
namespace x3 = boost::spirit::x3;
struct foo_node {
std::vector<std::string> value;
};
BOOST_FUSION_ADAPT_STRUCT(foo_node, value)
namespace rules {
auto const bar
= x3::string("bar");
auto const foo_node
= '(' >> +bar >> ')'
| x3::repeat(1) [ +bar ]
;
}
int main() {
for (std::string const input : {
"bar",
"(bar)",
"(barbar)",
})
{
auto f = input.begin(), l = input.end();
foo_node data;
bool ok = x3::parse(f, l, rules::foo_node, data);
if (ok) {
std::cout << "Parse success: " << data.value.size() << " elements\n";
} else {
std::cout << "Parse failed\n";
}
if (f != l)
std::cout << "Remaining unparsed: '" << std::string(f,l) << "'\n";
}
}
Prints
Parse success: 1 elements
Parse success: 1 elements
Parse success: 2 elements