Intrusive lists

Question

I\'ve not been able to find too much information about them online. What are they and when are they typically used?

Thanks.

Answer 1

Here is a brief description that is valid for lists as well:

I. Intrusive containers.

Object to be stored contains additional information to allow integration in container. Example:

struct Node
{
    Node* next;   // additional
    Node* prev;   // information 
    T data;
} 

1. Pros:

• stores the objects themselves.


• doesn't involve memory management.


• iteration is faster.
• better exception guarantees.
• predictability in insertion and deletion of objects. (no additional (non-predictable) memory management is required.)
• better memory locality.

2. Cons:

• contains additional data for container integration. (every store type must be adapted (modified) to the container requirements.)
• caution with possible side effects when changing the contents of the stored object.(especially for associative containers.)
• lifetime management of the inserted object, independently from the container.
• object can be possibly disposed before erased from the container leading to iterator invalidation.
• intrusive containers are NON-copyable and NON-assignable.

II. Non-instrusive containers (C++ standard containers)

Object doesn't "know" and contain details about the container in which is to be stored. Example:

struct Node
{
    T data;
}

1. Pros:

• does not containe additional information regarding the container integration.
• object's lifetime managed by the container. (less complex.)

2. Cons:

• store copies of values passed by the user. (inplace construction possible.)
• an object can belong only to one container. (or the contaier should store pointers to objects.)
• overhead on storing copies. (bookkeeping on each allocation.)
• non-copyable or non-movable objects CAN'T be stored in non-intrusive containers.
• can't store derived object and still maintain its original type. (slicing - looses polymorphism.)