using boost visitor to convert between types

Question

Let's say that I have a

boost::variant<std::string, int> myVariant;

In this object I keep data from a database, which is usually integer or text, but sometimes is a time stored in the database as text. So I wonder if I can create a visitor, that when visiting the variant object with a string into it, returns a struct of type 'tm'. Something like that:

class timeVisitor : public boost::static_visitor<boost::shared_ptr<tm> >
{
public:
    boost::shared_ptr<tm> operator()(string &str) const
    {
        boost::shared_ptr<tm> dst(new tm());
        strptime(str.c_str(), "%Y-%m-%d", dst.get());
        return dst;
    }
};

Then in order to use it:

boost::shared_ptr<tm> result = boost::apply_visitor( timeVisitor(), myVariant );

The thing is, that I don't want to create the tm struct into the visitor and mess around with some shared pointers and stuff. I prefer to give an already created one to the visitor and inside just to be initialized. Something like (in the sense of usage):

tm result;
int returnCode = boost::apply_visitor( timeVisitor(result), myVariant );

The visitor will just initialize with strptime my result tm struct and will even return if there was a problem with the conversion into returnCode. Does anyone know how this can be achieved? Can I define somehow visitor that take two parameters ... or maybe something else?

Answer 1

Your straightforward example call should work. Add a constructor to the visitor that takes a reference and stores it, like:

 tm* target;
 timeVisitor( tm& a ) : target(&a) {}
 int operator()(string &str) const {
      strptime(str.c_str(), "%Y-%m-%d", target);
 }

Answer 2

Indeed, it's perfectly allowed to give the visitor an argument at creation. The code you wrote at the end of your question is the good way to do it :

tm result;
int returnCode = boost::apply_visitor( timeVisitor(result), myVariant );

Here is how the visitor should looks like : (not tested on my side, slight syntax error possible)

class timeVisitor : public boost::static_visitor<bool>
{
public:
    timeVisitor(tm& s):m_tm(s) {}

    bool operator()(string &str) const
    {
        return strptime(str.c_str(), "%Y-%m-%d", m_tm.get());
        // in case of error, NULL pointer is converted to false automatically
    }
protected:
    tm& m_tm;
};