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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?


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