Smart pointers in C++

Smart pointer is a class object that acts like a pointer but it has some additional features.

For ex: in below function:

Normal pointers

Each time this function is called it allocates memory from heap but never frees memory. We can sort this out by putting one additional statement before return statement :

delete ps;

We have to remember all the time about memory allocation.

Suppose before deleting ps if some exception is thrown then we would not be able to delete that memory.

Normal pointers with delete statement.

When function func will terminate then all the local variables will be removed from stack. Memory occupied by pointer ps will be freed but it would be nice if memory pointed by ps will also be freed. If ps had a destructor then we can free the memory pointed by ps so the main problem is ps is only a pointer not a class object having a destructor.

This is the idea behind the smart pointers😃.

Smart pointer

So smart pointers are like pointer objects in which destructor will call delete when pointer will go out of scope.

Why multiple types of smart pointers??

Example using auto_ptr

If p1 and p2were normal pointers then two pointers will be pointing to same string object and program will try to delete same object twice ,once when p1 will go out of scope and another when p2 will go out of scope.

Here p2takes the ownership of object and p1 losses ownership so it’s now the responsibility of p2 to delete this object.

But what will happen if program will try to use data pointed by p1??🤷‍♀️

Consider this ex:

Example using auto_ptr

Segmentation fault will occur in both examples..!!!

Here comes unique pointer in picture:

Unique_ptr

“If a program attempts to assign one unique_ptr to another, the compiler allows it if the source object is a temporary rvalue and disallows it if the source object has some duration.”

Unique pointer

If program needs more than one pointer for an object then shared pointer should be used.

shared_ptr uses reference counting. For every new pointer to the same object count will be increased by one and will be decreased by one if pointer will expire. When final pointer will expire then delete will be invoked.

Note:

1 . C++11 standard made auto_ptr deprecated.

2. std::move() function can allow assignment of one unique_ptr to other.

Way ahead: move semantics and rvalue reference.

Thanks for reading!!😃