从类似构造函数的调用中返回模板化智能指针

Question

I want to write some easy-to-call object initializiation and I would like those objects to be shared_ptr of MyClass.

Just for visualisiation purposes, imagine this class:

class MyClass
{
public:
  MyClass(int number);
  MyClass(wxString path);
}

int main()
{
  //For now, objects are created in c style like this:
  MyClass obj1("C:\\test");
  MyClass * obj2 = new MyClass(5);
}

Now I would like to change this with easy to use smart pointer initialization. My goal is something looking like a constructor call, so that very little has to be changed to update the old call to the new one:

Smart_MyClass obj1("C:\\test");
Smart_MyClass obj2(5);
//where Smart_MyClass is std::shared_ptr<MyClass> 

I could solve this with a function:

template<typename T>
inline Smart_MyClass Smart_MyClass_Fct(T t)
{
    return std::make_shared<MyClass>(t);
};

Smart_MyClass obj = Smart_MyClass_Fct(7);

How can I translate that into the form I need? I could think of using the operator(), that would require a struct/class and maybe inheritence of my actual class? And maybe casting it back to parent class? Pretty sure there is something more elegant. ;-)

Answer 1

You could try something along these lines:

template<typename T>
class SharedWrapper
{
public:
    template<typename... Args>
    SharedWrapper(Args... args) : 
        m_p{std::make_shared<T>(std::forward<Args>(args)...)}
    { }

    T& operator*() const noexcept { return *m_p; }
    T* operator->() const noexcept { return m_p.operator->(); }
    explicit operator bool() const noexcept { return static_cast<bool>(m_p); }

private:
    std::shared_ptr<T> m_p;
};

This just forwards the arguments of the constructor to the make_shared functions and then mirrors the interface of the smart pointer. (Here you might have to add additional forwarding methods as needed.)

With:

using Smart_MyClass = SharedWrapper<MyClass>;

you should get the desired behavior and can also use this for different classes.

However I don't see any possibility to have Smart_MyClass actually be a std::shared_ptr<MyClass> . Inheriting from shared_ptr is discouraged since it does not have a virtual destructor. This might also limit the usability of this approach.

Is there any reason besides style that makes you want to use this notation?