CODEWITHSUNDEEP
c++pointers
bhavesh
bhavesh

Reputation: 1406

Owning instance of object vs owning pointer

Let's say I have two class A and B. There are 2 ways in which class B can use A.

First:

class B
{
  A *a;
}

Second:

class B
{
  A a;
}

Why most of the C++ libraries prefer to use First version as compared to Second. What could be the disadvantage of using Second approach. Does it relate to Stack vs Heap assignment? Please clarify.

Upvotes: 4

Views: 1933

Answers (1)

Thomas
Thomas

Reputation: 182073

Some advantages of owning an instance (class B { A a; };):

  • No need to worry about creation and destruction of a because it happens automatically.
  • No need to worry that a might be a dangling or null pointer.
  • Memory locality: a lives where instances of B live. If you have a large array of Bs and access each B's A in turn, this could make a significant speed difference.
  • Memory efficiency: no storage for a pointer is needed.

To make a huge sweeping generalization, one could say that this approach is faster and safer.

Some advantages of owning a pointer (class B { A *a; };):

  • Polymorphism: a can actually point to a subclass of A.
  • a can be reassigned without needing to copy an instance of A.
  • a can live independently of B or even be owned by another object entirely.
  • a can be null, freeing up memory if it's not always needed.

To make another huge sweeping generalization, one could say that this approach is more flexible.

Upvotes: 8

Related Questions