How do you create a generic parser using qi? - c++

Question

I am attempting to create generic parser-elements using qi as I unfortunately (MSVC must be supported) can not use X3.
The idea is to have a templated struct:
template<class T> struct parse_type;
Which I could use like this:
template<class T> T from_string(std::string const& s)
{
T res;
parse_type<T> t;
...
if (phrase_parse(...,parse_type<T>(),...,t))
}
or specialise like this
template<class T,class Alloc>
struct parse_type<std::vector<T,Alloc>>
{
// Parse a vector using rule '[' >> parse_type<T> % ',' > ']';
}
The primary purpose is to allow for easy parsing of e.g. std::tuple, boost::optional and boost::variant (The last one can not be automatic due to the greedy nature of qi).
I would appreciate feedback as to how approach this. Currently I base my struct on qi::grammar, but grammar is not supported in X3 and I would like to use X3 when MSVC compiles this, and I am also a little bit uncomfortable with having to provide the skipper.
An alternative would be to have a static function in parse_type that returns the appropriate rule. I am considering if this is a cleaner approach?
Any feedback will be appreciated.
Update2: Replaced code-snippet with compilable example that fails at runtime. Here is the code:
#include <boost/spirit/include/qi.hpp>
#include <boost/spirit/include/phoenix.hpp>
#include <string>
#include <string>
#include <iostream>
#include <iostream>
// Support to simplify
using iter = std::string::const_iterator;
void print(std::vector<int> const& v)
{
std::cout << '[';
for (auto i: v) std::cout << i << ',';
std::cout << "]";
}
namespace qi = boost::spirit::qi;
// My rule factory - quite useless if you do not specialise
template<class T> struct ps_rule;
// An example of using the factory
template<class T>
T from_string(std::string const& s)
{
T result;
iter first { std::begin(s) };
auto rule = ps_rule<T>::get();
phrase_parse(first,std::end(s),rule,qi::space,result);
return result;
}
// Specialising rule for int
template<>
struct ps_rule<int>
{
static qi::rule<iter,int()> get() { return qi::int_; }
};
// ... and for std::vector (where the elements must have rules)
template<class T,class Alloc>
struct ps_rule<std::vector<T,Alloc>>
{
static qi::rule<iter,std::vector<T,Alloc>()> get()
{
qi::rule<iter,std::vector<T,Alloc>()> res;
res.name("Vector");
res =
qi::lit('{')
>> ps_rule<T>::get() % ','
>> '}';
return res;
}
};
int main()
{
// This one works like a charm.
std::cout << ((from_string<int>("100") == 100) ? "OK\n":"Failed\n");
std::vector<int> v {1,2,3,4,5,6};
// This one fails
std::cout << ((from_string<std::vector<int>>("{1,2,3,4,5,6}") == v) ? "OK\n":"Failed\n");
}
The code fails in boost/function_template.hpp line 766:
result_type operator()(BOOST_FUNCTION_PARMS) const
{
if (this->empty())
boost::throw_exception(bad_function_call());
return get_vtable()->invoker
(this->functor BOOST_FUNCTION_COMMA BOOST_FUNCTION_ARGS);
}
This code is a member function in boost::function4
,boost::fusion::vector0 > &
,boost::spirit::unused_type const&>
and the problem is that get_vtable returns an invalid pointer.

Answer 1

Your main problem is that the copy constructor for qi::rule takes a reference to the original rule, which in your case is a local variable. One way you can avoid this problem is by using qi::rule's copy member function but this requires changing slightly the return type of your specialization of ps_rule.
static typename boost::proto::terminal<qi::rule<iter,std::vector<T,Alloc>()>>::type get()
{
//[...] (same as before)
return res.copy();
}
Once you do that, the same problem arises with your ps_rule<int> even though it seemed to work in isolation. You could do something analogous but in this case the rule is not required, it would be better (even from a performance point of view) to just use something like:
static qi::int_type get() { return qi::int_; }
Full Sample (Running on WandBox)
#include <boost/spirit/include/qi.hpp>
#include <string>
#include <iostream>
// Support to simplify
using iter = std::string::const_iterator;
void print(std::vector<int> const& v)
{
std::cout << '[';
for (auto i: v) std::cout << i << ',';
std::cout << "]";
}
namespace qi = boost::spirit::qi;
// My rule factory - quite useless if you do not specialise
template<class T> struct ps_rule;
// An example of using the factory
template<class T>
T from_string(std::string const& s)
{
T result;
iter first { std::begin(s) };
auto rule = ps_rule<T>::get();
qi::phrase_parse(first,std::end(s),rule,qi::space,result);
return result;
}
// Specialising rule for int
template<>
struct ps_rule<int>
{
static qi::int_type get() { return qi::int_; }
};
// ... and for std::vector (where the elements must have rules)
template<class T,class Alloc>
struct ps_rule<std::vector<T,Alloc>>
{
static typename boost::proto::terminal<qi::rule<iter,std::vector<T,Alloc>()>>::type get()
{
qi::rule<iter,std::vector<T,Alloc>()> res;
res.name("Vector");
res =
qi::lit('{')
>> ps_rule<T>::get() % ','
>> '}';
return res.copy();
}
};
int main()
{
// This one works like a charm.
std::cout << ((from_string<int>("100") == 100) ? "OK\n":"Failed\n");
std::vector<int> v {1,2,3,4,5,6};
std::cout << ((from_string<std::vector<int>>("{1,2,3,4,5,6}") == v) ? "OK\n":"Failed\n");
std::vector<std::vector<int> > vv {{1,2,3},{4,5,6}};
std::cout << ((from_string<std::vector<std::vector<int>>>("{{1,2,3},{4,5,6}}") == vv) ? "OK\n":"Failed\n");
}
PS: You can save lots of specializations if you use Spirit's own machinery to create parsers automatically in your primary template. Here is an example.