String-bool comparsion - why?

Question

I was working with boost::variant and its visitors when I runned into an unexpected behavior: the string and bool values were comparable

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.