How to make apply-visitor not discard const qualifier?

Question

Is it possible to write const function with apply_visitor inside? For example, this code compiles without errors:

#include <stdio.h>
#include <stdlib.h>
#include <string>
#include <boost/variant.hpp>
using namespace std;

typedef boost::variant<int,string> vTypeVariants;

struct vType_toN : boost::static_visitor<int>
{
    int operator()(int& i) const {
        return i;
    }
    int operator()(const string& str) const{
        return str.length();
    }
};

class vType{
public:
    vType(const int& src) : data(src){}
    vType(const std::string& src) : data(src){}

    int getLength(){
        return boost::apply_visitor(vType_toN(),data);
    }
private:
    vTypeVariants data;
};

int main(int argc, char ** argv)
{
  vType x = string("2");
  printf("L=%d",x.getLength());
  return(0);
}

Unless you will add const to getLength():

int getLength() const{
        return boost::apply_visitor(vType_toN(),data);
}

In such case an error with vast description (2 pages) appears complaining about problem with initializing first argument.

So, the question is: How to use apply_visitor inside const function?

Answer 1

Found out myself. Forgot const before int in static_visitor class operator definition. Maybe someone will find this useful as it was not easy to find this out (my original class is much bigger).