[boost] Implicit cast rules

Subject: [boost] Implicit cast rules
From: Gregory Crosswhite (gcross_at_[hidden])
Date: 2011-05-24 02:06:34

Hey everyone,

Would someone please explain to me the implicit cast rule that prevents 
the following from compiling, and/or if there is a way around it?

#include <boost/optional.hpp>
#include <iostream>

using namespace boost;
using namespace std;

struct C {
   operator optional<int>() { return none; }
};

int main() {
   C c;
   cout << static_cast<optional<int> >(c) << endl;
   return 0;
}

Error message:

In file included from /usr/include/boost/optional.hpp:15,
                  from cast-to-optional.cpp:1:
/usr/include/boost/optional/optional.hpp: In member function ‘void 
boost::optional_detail::optional_base<T>::construct(const Expr&, const 
void*) [with Expr = C, T = int]’:
/usr/include/boost/optional/optional.hpp:262:   instantiated from 
‘boost::optional_detail::optional_base<T>::optional_base(const Expr&, 
const Expr*) [with Expr = C, T = int]’
/usr/include/boost/optional/optional.hpp:559:   instantiated from 
‘boost::optional<T>::optional(const Expr&) [with Expr = C, T = int]’
cast-to-optional.cpp:13:   instantiated from here
/usr/include/boost/optional/optional.hpp:392: error: cannot convert 
‘const C’ to ‘int’ in initialization

I can tell what is happening; it would seem that the implicit cast rule 
selects the optional() constructor before it even looks at my more 
specific cast operator.  I'm just confused about why this should be, 
since I see no reason why constructor casts should be prioritized over 
operator casts.  Is this just the way it is, a GCC quirk, something I'm 
doing wrong, etc?

Thanks!
Greg