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Motti Lanzkron · Guest

Hi, I've been trying to understand the rules of SFINAE (Substitution
Failure Is Not An Error).

What's bothering me is that I can't figure out what constitutes an
"acceptable" failure and what doesn't.

Here's my first attempt at is_pod:

template <class T>
class is_pod {
// If T is a POD we can build a pod.
union pod {
T t;
char c[2]; // sizeof(pod) > 1
};

static T make();

static pod check(T);
static char check(...);
public:
enum { value = sizeof(check(make())) != 1 };
};

#include <string>
int main()
{
const bool True = is_pod<int>::value; // OK
const bool False = is_pod<std::string>::value; // Ooops
}

I think this failed because pod was instantiated in any case, which
led to my second try.

template <class T>
class is_pod {
template <class U>
union pod {
U u;
char c[2];
};
static T make();

template <class U>
static pod<U> check(U);
static char check(...);
public:
enum { value = sizeof(check(make())) != 1 };
};

I thought that if the template check method isn't used then no-one
will instantiate pod<std::string>, needless to say I was wrong.

I would appreciate an explanation of the scope of SFINAE or a pointer
to such an explanation (google, for once, wasn't very helpful).

Thanks, Motti.

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Alf P. Steinbach · Guest

* Motti Lanzkron:

Motti Lanzkron · Guest

Alf P. Steinbach wrote:

Alf P. Steinbach · Guest

* Motti Lanzkron:

Allan W · Guest

[email]alfps (AT) start (DOT) no[/email] (Alf P. Steinbach) wrote