[ITK-users] Smartpointer as a default parameter for a function

[Williams, Norman K]

You do not need to explicitly initialize <itktype>::Pointer variables.
They have a default constructor that initializes the raw pointer member
m_Pointer to zero.

I would tend to use references to SmartPointers as parameters, because if
you pass a Pointer value, it creates a temporary Pointer on the stack, and
then assigns it to the member variable.  This isn¹t a huge overhead but it
does involve semaphore lock/unlock pairs protecting reference count
increment/decrement.
When you pass a Pointer by reference, it ends up doing this.

1. Construct anonymous temp Pointer.
2. assign <temp>.m_Pointer with <parameter>.m_Pointer (I.e. the actual
object pointed to by the smart pointer)
3. lock the object pointed to by <temp>.m_Pointer.
4. Increment the pointed-to object¹s reference count.
5. unlock pointed-to object.
6. Inside the method, assign <temp> to local or member variable, repeat
steps 2 through 5.
7. When the function exits, call <temp> destructor
8. lock the pointed-to object
9. decrement reference count
10. unlock pointed-to object

If you pass the smart pointer by reference, you skip creating and
destroying the temp, and only assign to a smart pointer once.

On 7/22/14, 2:37 AM, "ahmed at mufradat.com" <ahmed at mufradat.com> wrote:

>Hello All,
>
>Suppose I have class like
>
>class A{
>     public:
>     A(int a, ImageType::Pointer ptr){
>         // whatever!
>     }
>};
>
>My question is, what's the default value for that ptr? I'd like to be
>able to create an instance of that class using
>
>A myA(5);
>
>so for the definition of the constructor of A:
>
>A(int a, ImageType::Pointer ptr = WHAT?)
>
>Best regards,
>Ahmed
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