Owning instance of object vs owning pointer

Question

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.

Answer 1

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.