Background: I'm trying to create perfect-forwarding factory methods for creating shared pointers of classes, where it's very clear when someone is calling one that might have a side-effect by taking in a non-const lvalue as a constructor parameter. See: SFINAE enable_if for variadic perfect forwarding template on reference/pointer const-ness So far so good.
However, now my problem is that when using make_shared to create classes that have private or protected constructors, even if they friend my 'builder' class template instance for themselves, the inner make_shared doesn't have access to the constructor.
I had hoped I could SFINAE on that too, by making a wrapper class which itself tries to use make_shared to create a member variable in the same fashion as my builder plans to, and then use that in an enable_if<is_constructible<...>> but that returns true (in gcc 4.8.2) even if trying to actually instantiate such a class doesn't compile:
template<typename R>
struct builder {
class MakeSharedConstructTest
{
public:
template<typename... Args>
MakeSharedConstructTest(Args&&... args)
: m_R(std::make_shared<R>(std::forward<Args>(args)...))
{}
private:
std::shared_ptr<R> m_R;
};
};
struct NonPublic
{
friend class builder<NonPublic>;
private:
NonPublic() {};
};
. . .
// prints 1, ie yes, constructible
std::cout << std::is_constructible<builder<NonPublic>::MakeSharedConstConstructTest>::value << "\n";
// test.cpp:134:3: error: ‘NonPublic::NonPublic()’ is private
builder<NonPublic>::MakeSharedConstConstructTest q;
What am I missing here?
