Question

TOE

PART A - 50 Points A.1 10 pts Please explain in detail Scott Meyers's point: Use weak_ptr for shared_ptr like pointers that can dangle. Hint: a correct explanation should also discuss dangling raw pointer.

Answer 1

In context of C,pointer is a variable which holds the address of another variable.
In context of C++,A pointer which directly addresses memory is referred to as raw pointer.
A raw pointer can hold the address of an object.
Dangling pointer points to the memory location of the deallocated memory.
Suppose a pointer is pointing to a memory location where an object is stored,
Now object has been deleted, but still pointer is pointing to the memory location of the deallocated memory.
such pointers are known as dangling raw pointers.

In programming, memory consumed by the object should be deallocated only if,there is no logical entity using that object.when reference count becomes zero,then object should be deleted.

The shared_ptr is a smart pointer available in C++ library that is designed to deal with the situation where more than one owner have to manage the lifetime of the object in memory.

As shown in the image part -2,there are two pointers ptr1 and ptr2 pointing to a same object of Class named "SomeClass".Here the Reference count is 2, because there are two pointers ptr1 and ptr2.
ptr1 and ptr2 have share access to one "control block".this control block increments/decrements the reference count whenever a new shared_ptr is added/ goes out of scope/ or is reset.

It is the case of shared_ptr.

weak_ptr

What is weak reference?
Garbage collector deallocates the memory of object which is no longer(expired object)
If an object has weak reference associated with other object(s),garbage collector de allocates its memory.

As discussed earlier, if there is a increase in the number of pointers(except weak_ptr),reference count will be increased.
but
There may be a situation when an object should store a way to access the object of a shared_ptr without causing the reference count to be incremented(such situation arises when there is cyclic references between shared_ptr instances.).

weak_ptr is used to break reference cycles posed by objects managed by shared_ptr. If such cycle is not handled , the shared_ptr reference counts cannot reach zero and an undesirable situation arises. To prevent this, one of the pointers in the cycle can be made weak.
weak_ptr will be deleted by garbage collector.
In this way , system will get rid of reference cycle.
finally reference cycle is broken now.

this is all about weak_ptr.

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