2011/5/24 Gregory Crosswhite <gcross@phys.washington.edu>
Hey everyone,
Hi Greg
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?
The compiler sais "cannot convert 'const C' to 'int'. I think it means static_cast<> takes c as a const, and your conversion operator is not a const member, so the compiler cannot use it. Solution (not tested): operator optional<int>() const { return none; } Regards Kris