boost variant type collision

问题

Follow-Up Question

So, I've been playing with the

Boost Mini C Tutorial

What I have done is added a rule to parse string literals. The purpose is so that I can parse and compile programs like (functionality already built-in):

int ret(int x) {
    return x;
}

int main() {
   int x = 5;
   return ret(x)*2;
}

As well as (want to add this functionality),

string print(string s) {
   return s;
}

int main() {
   string foo = "bar";
   print(foo);
   return 0;
}

Whether or not the last two examples compile with say gcc, is inconsequential.

So, the gist of what I added is the following:

Within the file expression_def.hpp (production rule 'quoted_string' has been added):

quoted_string = '"' >> *('\\' >> char_ | ~char_('"')) >> '"'; // ADDED THIS

            primary_expr =
            uint_ 
            |   quoted_string // ADDED THIS
            |   function_call
            |   identifier
            |   bool_
            |   '(' > expr > ')'
            ;

within ast.hpp, the variant type 'std:string' has been added:

typedef boost::variant<
            nil
            , bool
            , unsigned int
            , std::string // ADDED THIS
            , identifier
            , boost::recursive_wrapper<unary>
            , boost::recursive_wrapper<function_call>
            , boost::recursive_wrapper<expression>
        >
        operand;

Here is the rule declaration for the addition, as well as the rule it's colliding with:

qi::rule<Iterator, std::string(), skipper<Iterator> > identifier;
qi::rule<Iterator, std::string()> quoted_string; // declaring this without the skipper 
                             // lets us avoid the lexeme[] incantation (thanks @sehe).

The problem now, is that the compiler confuses what should be an 'identifier' for a 'quoted_string' - or actually just a std::string.

My guess is, the fact that they both have a std::string signature return type is the cause of the problem, but I don't know a good workaround here. Additionally, the 'identifier' struct has a data member of type std::string that it is initialized with, so really the compiler cannot tell between the two and the variant std::string ends up being the better match.

Now, if I change std::string to char* like so:

typedef boost::variant<
                nil
                , bool
                , unsigned int
                , char* // CHANGED, YET AGAIN
                , identifier
                , boost::recursive_wrapper<unary>
                , boost::recursive_wrapper<function_call>
                , boost::recursive_wrapper<expression>
            >
            operand;

it will compile and work with integers, bet then I am unable to parse strings (in fact, VS will call abort()) It should be noted that because each variant needs an overload, I have something in my code along the lines of:

bool compiler::operator()(std::string const& x)
    {
        BOOST_ASSERT(current != 0);
        current->op(op_string, x);
        return true;
    }

and

 void function::op(int a, std::string const& b)
        {
            code.push_back(a);
            code.push_back(b.size());
            for (uintptr_t ch : b)
            {
                code.push_back(ch);
            }
            size_ += 2 + b.size();
        }

These both work swimmingly when I need to parse strings (of course sacrificing the ability to handle integers).

Their integer equivalents are (and found in compiler.cpp)

bool compiler::operator()(unsigned int x)
        {
            BOOST_ASSERT(current != 0);
            current->op(op_int, x);
            return true;
        }

and of course:

  void function::op(int a, int b)
        {
            code.push_back(a);
            code.push_back(b);
            size_ += 2;
        }

If I have to change the variant type from std::string to char*, then I have to update the overloads, and because of C legacies, it gets to look a bit ugly.

I understand this might be a bit daunting and not really appealing to comb through the source, but I assure you it really isn't. This compiler tutorial simply pushes bytecode into a vector, which by design only handles integers. I am trying to modify it to handle strings, as well, hence the additions and overloads, as well as the need for unintptr_t. Anyone familiar with the material and/or Boost will likely know exactly what they are looking at (ehem, @sehe, ehem!).

来源：https://stackoverflow.com/questions/34167067/boost-variant-type-collision

标签

c++

boost

compiler-construction

boost-variant