Martin Sebor wrote:
>
>Travis Vitek wrote:
>>
>>
>> Not always. If the template is instantiated the error would
>> not be seen.
>
>The reason I ask is because I couldn't get even a simple SFINAE
>test program to compile. I think the program is well-formed even
>though gcc 4.3.0 chokes on too (albeit for a different reason --
>see http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36797)
>
Hah, I was just filing a nearly identical bug with gcc when I got this
message.
I happened to be playing around with this a little bit earlier today.
You might find this interesting. If I use the pseudo-function
__is_empty() directly, the error message says there is no matching
function, but if I wrap it in a template, I get multiple matches (after
the incomplete class type message I complained about earlier).
This all seems to work when using the trait with a non-template type. If
you want to use a template, it has to be instantiated first. I think I'm
going to put the Edison compiler port on hold for now.
$ cat t.cpp
template <bool B, class T = void>
struct enable_if {
typedef T type;
};
template <class T>
struct enable_if<false, T> {
};
#ifdef _TEMPLATE
template <class T>
struct is_empty {
enum { val = __is_empty(T) };
};
template <class T>
int enabled_if_empty (typename enable_if< is_empty<T>::val>::type* = 0)
{
return 1;
}
template <class T>
int enabled_if_empty (typename enable_if<!is_empty<T>::val>::type* = 0)
{
return 0;
}
#else // !_TEMPLATE
template <class T>
int enabled_if_empty (typename enable_if< __is_empty(T)>::type* = 0) {
return 1;
}
template <class T>
int enabled_if_empty (typename enable_if<!__is_empty(T)>::type* = 0) {
return 0;
}
#endif // !_TEMPLATE
template <int> struct S { };
#include <assert.h>
//template struct S<1>;
int main () {
assert (0 == enabled_if_empty< S<1> >());
assert (1 == enabled_if_empty< long >());
return 0;
}
$ eccp -A t.cpp
"t.cpp", line 50: error: no instance of overloaded function
"enabled_if_empty"
matches the argument list
assert (0 == enabled_if_empty< S<1> >());
^
1 error detected in the compilation of "t.cpp".
$ eccp -A -D_TEMPLATE t.cpp
"t.cpp", line 15: error: an incomplete class type is not allowed
enum { val = __is_empty(T) };
^
detected during instantiation of class "is_empty<T> [with
T=S<1>]"
at line 50
"t.cpp", line 50: error: more than one instance of overloaded function
"enabled_if_empty" matches the argument list:
function template "int
enabled_if_empty<T>(enable_if<is_empty<T>::val,
void>::type *)"
function template "int
enabled_if_empty<T>(enable_if<<expression>,
void>::type *)"
assert (0 == enabled_if_empty< S<1> >());
^
2 errors detected in the compilation of "t.cpp".
>
