Checking a member exists, possibly in a base class

Question

In https://stackoverflow.com/a/1967183/134841, a solution is provided for statically checking whether a member exists, possibly in a subclass of a type:

template <typename Type> 
class has_resize_method
{ 
   class yes { char m;}; 
   class no { yes m[2];}; 
   struct BaseMixin 
   { 
     void resize(int){} 
   }; 
   struct Base : public Type, public BaseMixin {}; 
   template <typename T, T t>  class Helper{}; 
   template <typename U> 
   static no deduce(U*, Helper<void (BaseMixin::*)(), &U::foo>* = 0); 
   static yes deduce(...); 
public: 
   static const bool result = sizeof(yes) == sizeof(deduce((Base*)(0))); 
};

However, it doesn't work on C++11 final classes, because it inherits from the class under test, which final prevents.

OTOH, this one:

template <typename C>
struct has_reserve_method {
private:
    struct No {};
    struct Yes { No no[2]; };
    template <typename T, typename I, void(T::*)(I) > struct sfinae {};
    template <typename T> static No  check( ... );
    template <typename T> static Yes check( sfinae<T,int,   &T::reserve> * );
    template <typename T> static Yes check( sfinae<T,size_t,&T::reserve> * );
public:
    static const bool value = sizeof( check<C>(0) ) == sizeof( Yes ) ;
};

fails to find the reserve(int/size_t) method in baseclasses.

Is there an implementation of this metafunction that both finds reserved() in baseclasses of T and still works if T is final?

Answer 1

A version that also relies on decltype but not on passing arbitrary types to (...) [ which is in fact a non-issue anyway, see Johannes' comment ]:

template<typename> struct Void { typedef void type; };

template<typename T, typename Sfinae = void>
struct has_reserve: std::false_type {};

template<typename T>
struct has_reserve<
    T
    , typename Void<
        decltype( std::declval<T&>().reserve(0) )
    >::type
>: std::true_type {};

I'd like to point out according to this trait a type such as std::vector<int>& does support reserve: here expressions are inspected, not types. The question that this trait answers is "Given an lvalue lval for such a type T, is the expressions lval.reserve(0); well formed". Not identical to the question "Does this type or any of its base types has a reserve member declared".

On the other hand, arguably that's a feature! Remember that the new C++11 trait are of the style is_default_constructible, not has_default_constructor. The distinction is subtle but has merits. (Finding a better fitting name in the style of is_*ible left as an exercise.)

In any case you can still use a trait such as std::is_class to possibly achieve what you want.

Answer 2

Actually, things got much easier in C++11 thanks to the decltype and late return bindings machinery.

Now, it's just simpler to use methods to test this:

// Culled by SFINAE if reserve does not exist or is not accessible
template <typename T>
constexpr auto has_reserve_method(T& t) -> decltype(t.reserve(0), bool()) {
  return true;
}

// Used as fallback when SFINAE culls the template method
constexpr bool has_reserve_method(...) { return false; }

You can then use this in a class for example:

template <typename T, bool b>
struct Reserver {
  static void apply(T& t, size_t n) { t.reserve(n); }
};

template <typename T>
struct Reserver <T, false> {
  static void apply(T& t, size_t n) {}
};

And you use it so:

template <typename T>
bool reserve(T& t, size_t n) {
  Reserver<T, has_reserve_method(t)>::apply(t, n);
  return has_reserve_method(t);
}

Or you can choose a enable_if method:

template <typename T>
auto reserve(T& t, size_t n) -> typename std::enable_if<has_reserve_method(t), bool>::type {
  t.reserve(n);
  return true;
}

template <typename T>
auto reserve(T& t, size_t n) -> typename std::enable_if<not has_reserve_method(t), bool>::type {
  return false;
}

Note that this switching things is actually not so easy. In general, it's much easier when just SFINAE exist -- and you just want to enable_if one method and not provide any fallback:

template <typename T>
auto reserve(T& t, size_t n) -> decltype(t.reserve(n), void()) {
  t.reserve(n);
}

If substitution fails, this method is removed from the list of possible overloads.

Note: thanks to the semantics of , (the comma operator) you can chain multiple expressions in decltype and only the last actually decides the type. Handy to check multiple operations.