Lecture 5 - Memory Ownership

Question 1

Why do we use the C programming language?

Answer 1

It is powerful, fast and very versatile (used in games , kernels, compilers, networking etc.)
It is also quite lightweight and can be used in constrained systems.

Question 2

What does C build in a person?

Answer 2

Understanding of resource management.

Question 3

What is a massive drawback of C?

Answer 3

The lack of garbage collection, we have to sanitize the program ourselves via malloc and free.

Question 4

How many times can we call free on a certain memory location?

Answer 4

Once , otherwise we will get an error

Question 5

Why are multiple pointers to the same memory location problematic?

Answer 5

As we only need to call free once, so it is very easy to call it a second time when there is multiple pointers.

Question 6

What is a good practice when freeing nodes?

Answer 6

To set the pointer variable to NULL.

Question 7

What is a double-free error?

Answer 7

Calling free twice on one memory location (has been freed the first time and not malloced).

Question 8

Is calling free with NULL ok?

Answer 8

Yes

Question 9

What are dangling pointers?

Answer 9

Memory locations not freed, but also have no pointers pointing to them.

Question 10

What is a memory leak?

Answer 10

When free is never called on a malloced piece of memory.

Question 11

What is the most common cause of memory leaks?

Answer 11

Reassigning a pointer , thus creating a dangling pointer.

Question 12

What is ownership?

Answer 12

A concept related to memory managment. It states that we identify a single entity that is responsible for managing a location in memory.

Question 13

Tree example of ownership.

Answer 13

we might say that the parent in a tree owns its children
i.e. it is responsible for allocating and freeing the memory used by its children

Question 14

Equivalent of printf in C++?

Answer 14

std::cout &laquo_space;””

Question 15

How do you create an auto variable in C++?

Answer 15

auto v = ;

Question 16

Does C enforce an ownership model?

Answer 16

No, that’s why we are learning C++.

Question 17

What is C++?

Answer 17

C++ is a superset of C (provides more features and keywords).

Question 18

What is the difference between C and C++?

Answer 18

C++ provides OOP, focusing on objects rather than functions. C++ also provides ownership!

Question 19

Do you still use clang to compile C++?

Answer 19

We use clang++.

Question 20

What does the -std=x flag specify?

Answer 20

The version of the language standard, used with clang and clang++.

Question 21

What is ownership called in C++?

Answer 21

RAll = Resource Acquisition is initialization

Question 22

What is std :: cin&raquo_space; in C++?

Answer 22

Equivalent to scanf in C.

Question 23

What is sleep() in C++?

Answer 23

Pauses execution for some period specified as an argument.

Question 24

What happens in RAll?

Answer 24

• We tie the management of a resource to the lifetime of a variable on the stack
• The allocation of the resource (e.g. the malloc call) is done when we create the
  variable
• The deallocation of the resource (e.g. the free call) is done when the variable is
  destroyed

Question 25

What is RAll represented by (in C++.) HINT : OBJECTS

Answer 25

struct datatypes and their constructor+destructor

Question 26

What is the constructor?

Answer 26

A function called when creating variables

Question 27

What is a destructor?

Answer 27

A function called when variable is deleted.

Question 28

CLICK FOR EXAMPLE.

Answer 28

struct ints_on_the_heap {
int * ptr;

ints_on_the_heap(int size) {
ptr = (int*)malloc(sizeof(int) * size);
}

~ints_on_the_heap() { free(ptr); }
};

typedef struct ints_on_the_heap ints_on_the_heap;

Question 29

Does the destructor run automatically?

Answer 29

yes, this happens at end of the block

Question 30

What is std :: vector in C++?

Answer 30

dynamic array which will store values of the same typ

Question 31

Is this concept of ownership only applicable to memory managment?

Answer 31

No, but resource managment in general.

e.g. closing and opening a file

Question 32

Does C++ provide RAll data types?

Answer 32

YEs, these will not leak memory and are easy to use.

Question 33

How do we store a single value on the heap?

Answer 33

using one of the smart pointers:

• std::unique_ptr for unique ownership
  the default case, when a single variable owns the value on the heap
  (if parent variable goes then so do the children)
• std::share_ptr for shared ownership
  when it is not possible to identify a single owner (e.g. multi-threaded progs)

We alternatively use plain pointers when we have a non-owning relationship.

Question 34

How do you make a unique pointer in C++?

Answer 34

std :: make_unique<struct>(value_ptr)</struct>