Viewing file:  sfinae64.C (2.22 KB) -rw-r--r--
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// PR c++/87748
// { dg-do compile { target c++11 } }
template <class T> T&& declval();
template <bool B, class T> struct enable_if;
template <class T> struct enable_if <true, T> { typedef T type; };
struct true_type { static const bool value = true; };
struct false_type { static const bool value = false; };
struct string
{
string (const char *p);
};
/// Template to map all type arguments to void, useful for SFINAE, see also WG21 N3911.
template<class...> using void_t = void;
/// REQUIRES<value> - Simplified version of enable_if<> to use SFINAE in function templates.
template<bool value> using REQUIRES = typename ::enable_if<value, bool>::type;
/// DerivesString<T> - Check if @a T is of type 'string'.
template<class T> struct DerivesString
{
static const int value = __is_same_as (T, string);
};
/// Has__aida_visit__<T,Visitor> - Check if @a T provides a member template __aida_visit__<>(Visitor).
template<class, class, class = void> struct Has__aida_visit__ : false_type {};
template<class T, class V>
struct Has__aida_visit__<T, V, void_t< decltype (declval<T>().template __aida_visit__<V> (*(V*) 0)) >> : true_type {};
struct Foo {
template<class V> void __accept__ (V*);
};
/// Base template for Visitor classes, dispatches operator() to visit_<type>() methods.
template<class DerivedVisitor>
class VisitorDispatcher {
DerivedVisitor* derived () { return static_cast<DerivedVisitor*> (this); }
protected:
typedef const char* Name; ///< Member name argument type for visit() methods.
public:
// dispatch for calls like: visitor (field, "field");
template<class A,
REQUIRES< (!Has__aida_visit__<A, DerivedVisitor>::value) > = true>
void operator() (A &a, Name n)
{ return derived()->visit_string (a, n); }
};
class PspecVisitor : public VisitorDispatcher<PspecVisitor> {
public:
void visit_string (::string &a, Name name);
};
int
main (int argc, char *argv[])
{
#ifdef WITHASSERT // define this to fix g++-8.1.1
static_assert (Has__aida_visit__<string, PspecVisitor>::value == false, "");
#endif
PspecVisitor v;
string str ("A string");
v (str, "some_string");
static_assert (Has__aida_visit__<string, PspecVisitor>::value == false, "");
return 0;
}
// g++ -std=gnu++14 -Wall -O2 aidavisit.cc && ./a.out
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