Searching/Iterating boost::spirit::qi::symbols

Question

If I have a symbol table:

struct MySymbols : symbols<char, MyEnum::Fruits>
{
    MySymbols ()
        : symbols<char, MyEnum::Fruits>(std::string("MySymbols"))
    {
        add("apple", MyEnum::Apple)
           ("orange", MyEnum::Orange);
    }
};

I want to iterate over the table in order to search for a symbol by data value. I cannot use lambda expressions so I implemented a simple class:

template<typename T>
struct SymbolSearcher
{
    SymbolSearcher::SymbolSearcher(T searchFor)
        : _sought(searchFor)
    {
        // do nothing
    }

    void operator() (std::basic_string<char> s, T ct)
    {
        if (_sought == ct)
        {
            _found = s;
        }
    }

    std::string found() const { return _found; }

private:
    T _sought;
    std::string _found;
};

And I'm using it as follows:

SymbolSearcher<MyEnum::Fruits> search(ct);
MySymbols symbols;

symbols.for_each(search);
std::string symbolStr = search.found();

If I set a breakpoint on _found = s I can confirm that _found is getting set, however search.found() always returns an empty string. I'm guessing it has something to do with the way the functor is being called inside for_each but I don't know.

What am I doing wrong?

Answer 1

It could be that

• the actual value of the string is the empty string (unlikely)

• the functor is being passed by value, making the stateful functor useless (as the original state will not actually be passed).

You could make the _found field a reference (requiring you to make sure to respect the Rule-Of-Three to make it work).

Here's a demonstration that shows the principle by asserting the result of a roundtrip via SymbolSearcher: http://liveworkspace.org/code/4qupWC$1

#include <boost/spirit/include/qi.hpp>

namespace qi     = boost::spirit::qi;

template<typename T>
struct SymbolSearcher
{
    SymbolSearcher(T searchFor, std::string& result) : _sought(searchFor), _found(result)
    {
    }

    void operator() (std::basic_string<char> s, T ct)
    {
        if (_sought == ct)
        {
            _found = s;
        }
    }

    std::string found() const { return _found; }

private:
    T _sought;
    std::string& _found;
};

int main()
{
    const std::string str("mies");

    typedef std::string::const_iterator It;
    It begin = str.cbegin();
    It end   = str.cend();

    qi::symbols<char, int> symbols;
    symbols.add("aap", 1)("noot", 2)("mies", 3);

    int out;
    bool ok = qi::parse(begin, end, symbols, out);
    assert(ok);

    std::string found;
    SymbolSearcher<int> sf(out, found);
    symbols.for_each(sf);

    assert(str == sf.found());
}