http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58713
--- Comment #6 from Daniel Krügler <daniel.kruegler at googlemail dot com> ---
(In reply to Daniel Krügler from comment #5)
> Thanks for your test, Marc. I will reflect upon the problem in a bit more
> detail
My current guess is that my suggested approach should work, assuming a proper
helper trait template will be used. Here is the model code that I used for
testing:
//---------------------------
#include <type_traits>
#include <utility>
namespace xstd {
template<class CharT = char>
struct OStream
{
OStream& operator<<(int);
OStream& operator<<(short);
OStream& operator<<(double);
OStream& operator<<(const void*);
};
struct is_ostreamable_impl
{
template<class CharT, class T, class =
decltype(std::declval<OStream<CharT>&>() << std::declval<T>())>
static std::true_type test(int);
template<class, class>
static std::false_type test(...);
};
template<class CharT, class T>
struct is_ostreamable
: decltype(is_ostreamable_impl::test<CharT, T>(0))::type
{
};
template<class CharT, class T,
typename std::enable_if<
is_ostreamable<CharT, const T&>::value,
bool
>::type = false>
OStream<CharT>& operator<<(OStream<CharT>&& os, const T& t)
{
return os << t;
}
OStream<> out;
}
struct A {};
int main() {
A a;
xstd::out << 1;
xstd::out << a;
}
//---------------------------
The diagnostics of my gcc current 4.9.0 is
<quote>
main.cpp||In function 'int main()':|
main.cpp|50|error: no match for 'operator<<' (operand types are
'xstd::OStream<>' and 'A')|
main.cpp|50|note: candidates are:|
main.cpp|9|note: xstd::OStream<CharT>& xstd::OStream<CharT>::operator<<(int)
[with CharT = char]|
main.cpp|9|note: no known conversion for argument 1 from 'A' to 'int'|
main.cpp|10|note: xstd::OStream<CharT>& xstd::OStream<CharT>::operator<<(short
int) [with CharT = char]|
main.cpp|10|note: no known conversion for argument 1 from 'A' to 'short int'|
main.cpp|11|note: xstd::OStream<CharT>&
xstd::OStream<CharT>::operator<<(double) [with CharT = char]|
main.cpp|11|note: no known conversion for argument 1 from 'A' to 'double'|
main.cpp|12|note: xstd::OStream<CharT>& xstd::OStream<CharT>::operator<<(const
void*) [with CharT = char]|
main.cpp|12|note: no known conversion for argument 1 from 'A' to 'const
void*'|
main.cpp|36|note: template<class CharT, class T, typename
std::enable_if<xstd::is_ostreamable<CharT, const T&>::value, bool>::type
<anonymous> > xstd::OStream<CharT>& xstd::operator<<(xstd::OStream<CharT>&&,
const T&)|
main.cpp|36|note: template argument deduction/substitution failed:|
main.cpp|35|error: no type named 'type' in 'struct std::enable_if<false,
bool>'|
main.cpp|35|note: invalid template non-type parameter|
</quote>
