Proper initialisation of smart pointers array

Question

For such case:

    class A
    {
        //implementation
    };

    class B
    {
     public:
         B();
        ~B();
     private:
         std::vector<std::shared_ptr<A>> _innerArray;
    };

what should I do in the B() to create an object with valid state? Do I need to manually call default constructor for every A object in array? And do I need to do something special in ~B()? If B class is example of bad design, feel free to say how to make it better. Thanks.

Edit So, here is a scheme of what I really need here.

So real values stored only in array of A and all other objects are for storing connections. The easiest example - A = dot, B = Line (or curve) going via selected dots and C = a plane described by lines. Hope it makes question more exact.

Answer 1

To create a B object in a valid state you do not have to do anything more. You even do not have to declare and implement constructor and destructor for B. std::vector<std::shared_ptr<A>> that is a member of B will be default initialized in B's constructor which means it will not have any elements in a container yet. It will also be properly deleted in ~B thanks to std::vector and std::shared_ptr destructors.

On the other hand if you for example want to initialize it somehow (i.e. 3 values) you can use std::vector's std::initializer_list constructor in a B's constructor initialization list. For example:

class B
{
 public:
     B(): _innerArray{ std::make_shared<A>(),
                       std::make_shared<A>(),
                       std::make_shared<A>() } {}
    ~B() {}
 private:
     std::vector<std::shared_ptr<A>> _innerArray;
};

Remember that std::make_shared uses perfect forwarding so you pass A's constructor arguments as the function arguments and not the class object itself.

Answering your concerns about the design I would like to encourage you to first think about the exclusive ownership of members in a vector before you decide to share them.

class B
{
 public:
     B();
    ~B();
 private:
     std::vector<std::unique_ptr<A>> _innerArray;
};

Above implementation is more effective on many grounds. First of all it makes your design more clear on who is responsible for the lifetime of As. Next std::unique_ptr is faster because it does not demand thread safe reference counting. And last but not least it does not cost any additional memory (compared to regular C pointer) while std::shared_ptr may take tens of bytes (24-48) to store shared state data which is highly ineffective when you operate on small classes. That is why I always use std::unique_ptr as my first resort smart pointer and I only fallback to std::shared_ptr when it is really needed.

EDIT:

Answering your edit I would create 3 containers of classes A, B, C. Depending of the fact if you need them to be polymorphic or not I would store either values like that (non-polymorphic types):

std::deque<A> as;
std::deque<B> bs;
std::deque<C> cs;

or (polymorphic types):

std::vector<std::unique_ptr<A>> as;
std::vector<std::unique_ptr<B>> bs;
std::vector<std::unique_ptr<C>> cs;

in that order (as must live longer than bs and bs must live longer than cs). Then I would just have std::vector<A*> inside B class and std::vector<B*> inside C class without any smart pointers usage.

I hope that helps.

EDIT:

Changed std::vector to std::deque in the first case which allows references/pointers to container elements survive containers extensions with push_back(). However they will not survive erasing elements, sorting or other stuff.