Extract code that makes class shared_ptr construct-able only

Question

I've tried using CRTP, but the forces the class to befriend CRTP Base.

template <typename T>
class SharedConstructable
    : public std::enable_shared_from_this<T>
{
    typedef std::shared_ptr<T> ptr_type;

public:

    struct Ptr
        : ptr_type
    {
        template<typename... Args>
        Ptr(Args&&... args)
            : ptr_type(new T(args...))
        {
        }
    };
};

class SharedOnly
    : SharedConstructable<SharedOnly>
{
private:
    friend class SharedConstructable<SharedOnly>;
    SharedOnly(int) {}
}

Usage:

SharedOnly x(5); // Error Constructor is private
SharedOnly::Ptr(5); // Correct usage;

Is there a cleaner/better way of doing this (without the friendship)?

Answer 1

If I follow you correctly, you're looking for a clean way to have a private constructor that can only be invoked by std::make_shared or std::allocate_shared. Unfortunately this does not appear to be portable or easy, and I don't see a way to avoid your request to avoid friendship. However you can make the friend something that other people cannot easily co-opt. I'd call this mildly better, but it definitely has some downsides.

As this is not portable, the scenario I'm showing here is specific to the compiler I had handy: Visual Studio 2012. First I created a class like this:

class SomeClass
{
public:
    int GetValue() const { return m_val; }
    // optionally provide Ptr like yours?
    // static std::shared_ptr<SomeClass> Ptr(int val) { return std::make_shared<SomeClass>(val); }

private:
    SomeClass(int val) { m_val = val; }
    SomeClass(const SomeClass&); // = delete;
    SomeClass(SomeClass&&); // = delete;
    ~SomeClass() {}

    int m_val;
};

This is enough to prevent usage like SomeClass thing(5); Then I wanted to try to befriend std::make_shared. When I tried to compile code using this definition and std::make_shared<SomeClass>(5), I got an error pointing to what needed access:

error C2248 [...] cannot access private member [SomeClass::~SomeClass ...] while compiling class template member function 'void std::_Ref_count_obj<_Ty>::_Destroy(void)'

After befriending class std::_Ref_count_obj<SomeClass> (I first tried befriending the specific method, but that created worse problems), I also decided to fix the warning by befriending std::_Get_align<SomeClass>:

warning C4624: 'std::_Get_align<_Ty [= SomeClass]>' : destructor could not be generated because a base class destructor is inaccessible

This left me with the following additions to this class that allows std::make_shared<SomeClass>(5) but not SomeClass obj(5).

class SomeClass
{
    : : :    
    // Allow use in VS2012 implementation of std::make_shared
    friend class std::_Ref_count_obj<SomeClass>;
    friend struct std::_Get_align<SomeClass>;
    : : :
};

I'll leave other compilers up to you as you need them. It may be worth collecting a series of macro alternatives that you can put in any class that needs this capability.