String-bool comparsion - why?

Question

I was working with boost::variant<int,std::string,bool> and its visitors when I runned into an unexpected behavior: the string and bool values were comparable. I don't know, why does it work like this, but I found it interesting. My only idea is that the variant with the bool value was interpreted as a char? Someone could explain it to me? The comparsion visitor:

#include <iostream>
#include <algorithm>
#include <vector>
#include <boost/variant.hpp>
#include <boost/function.hpp>

struct my_less : boost::static_visitor<bool*>
{
   template<typename T>
   bool* operator()(T a, T b) const
   {
       return a<b ? new bool(true) : new bool(false);
   }

   template<typename T, typename U>
   bool* operator()(T a, U b) const
   {
       return NULL;
   }
};

int main()
{
typedef boost::variant<int,bool,std::string> datatype;
datatype *a = new datatype(false);
datatype *b = new datatype("abc");

my_less cmp;

bool* val = boost::apply_visitor(cmp,*a,*b);

if(val)
{
    std::cout << *val;
}
else
{
    std::cout << "NULL";
}

}

EDIT Here is an extended main function with some test cases:

void show_result(bool* val)
{
if(val)
{
    std::cout << *val << std::endl;
}
else
{
    std::cout << "NULL" << std::endl;
}
}

int main()
{
//std::string a = "bbb";
//bool b = true;
//std::cout << b<a;      //compilation error

typedef boost::variant<int,bool,std::string> datatype;
datatype int_value_1(4);
datatype int_value_2(3);
datatype string_value("abc");
datatype bool_value(true);
my_less cmp;

std::cout<<"First result, compare ints 4 and 3:"<<std::endl;
bool* val = boost::apply_visitor(cmp,int_value_1,int_value_2);
show_result(val);

std::cout<<"Second result, compare int to string 4 to abc " << std::endl;
val = boost::apply_visitor(cmp,int_value_1,string_value);
show_result(val);

std::cout <<"Third result, int 4 to bool true:" << std::endl;
val = boost::apply_visitor(cmp,int_value_1,bool_value);
show_result(val);

std::cout<<"Fourth result, string abc to bool true" << std::endl;
val = boost::apply_visitor(cmp,string_value,bool_value);
show_result(val);

}

The output:

First result, compare ints 4 and 3:
0
Second result, compare int to string 4 to abc
NULL
Third result, int 4 to bool true:
NULL
Fourth result, string abc to bool true
0

Answer 1

OK, now that you've completely changed your program, let me try again.

The problem is:

datatype *b = new datatype("abc");

"abc" is a const char*, not a std::string. If you want to create a std::string variant, you need to do so explicitly. Otherwise, you'll end up creating a bool variant because all pointers are convertible to bool, including const char* pointers.

Try this

datatype *b = new datatype(std::string("abc"));

---

This interaction between bool and std::string is apparently well-known and somewhat irritating. boost::variant provides a templated constructor, but the resolution rules prefer the built-in converstion to bool and there's no way in C++ to specify a template specialization on a constructor. It is possible to explicitly specialize assignment, so you can write:

datatype b;
b.operator=<std::string>("abc");

which might be marginally more efficient but much less readable than

datatype b;
b = std::string("abc");

If you don't include bool as a variant, then string literals do automatically convert to std::string. Maybe it's possible to use some sort of proxy pseudo-boolean class. I've never tried.