Scope, Duration and Linkage

Question 1

What is meant by a variable’s scope?

Answer 1

Scope determines where a variable is accessible.

Question 2

Explain duration of a variable

Answer 2

Duration determines when a variable is created and destroyed

Question 3

What is the linkage of a variable?

Answer 3

Linkage determines whether the variable can be exported to another file or not

Question 4

What is the scope of global variables?

Answer 4

Global scope (a.k.a. file scope), which means they can be accessed from the point of declaration to the end of the file which they are declared

Question 5

What is the duration of a global variable?

Answer 5

They have static duration, which means they are created when the program is started, and destroyed when it ends

Question 6

What is the linkage of a global variable?

Answer 6

They can have either internal or external linkage, via the static and external keywords respectively

Question 7

How should you define a global constant in C++17

Answer 7

pragma once

Define them as inline constexpr global variables in a header file

Constants.h

constexpr int MAX_VALUE = 100;

Question 8

What is static duration?

Answer 8

When a variable is created when the program starts and destroyed when it ends

Question 9

What is internal linkage?

Answer 9

When an identifier can only be used in the file in which it is defined

Question 10

What is automatic duration?

Answer 10

When a local variable is created at point of definition and destroyed when the block is exited

Question 11

What is automatic duration?

Answer 11

When a local variable is created at point of definition and destroyed when the block is exited

Question 12

What effect does using keyword static have on a global variable?

Answer 12

It gives it internal linkage; meaning the variable cannot be exported to other files

Question 13

What effect does using keyword static have on a local variable?

Answer 13

It gives it static duration, meaning the variable will only be created once, and will not be destroyed until the end of the program

Question 14

Variable with block/local scope can only be accessed from point of declaration until the end of the block. Three examples are:

Answer 14

Local variables, function parameters and program defined type definitions (such as enums and classes) declared inside a block

Question 15

Variables and functions with global scope can be accessed from the point of declaration to the end of the file. This includes (3)

Answer 15

Global variables
functions
program defined type definitions (such as enums or classes) declared inside a namespace or in the global scope

Question 16

Define automatic duration and give two examples

Answer 16

Variables which are created at the point of definition, and destroyed when the block they are part of ends.
E.g. local variables and function parameters

Question 17

Define static duration and give two examples of variables with static duration

Answer 17

Variables which are created when the program begins and destroyed when it ends.
E.g.
global variables
static local variables

Question 18

Define static duration and give two examples

Answer 18

It is when a variable is created when the program begins and is destroyed when it ends. E.g. global variables and static local variables

Question 19

Define dynamic duration and give an example

Answer 19

Variables are created and destroyed by programmer request. E.g. dynamically allocated variables

Question 20

Define no linkage and give 2 examples

Answer 20

An identifier with no linkage means another declaration of the same identifier refers to a unique entity.

It can also be thought of as block scope

E.g.
local variables
Function parameters
program defined type identifiers (such as classes and enums) declared inside a block

Question 21

Define internal linkage and give 5 examples

Answer 21

An identifier with internal linkage means a declaration of the same identifier within the same translation unit refers to the same object or function. Entities whose identifiers have internal linkage include

Static global variables
Static local variables
Static functions
Const global variables
Functions declared in an unnamed namespace
Program defined type definitions (such as enums and classes) declared inside an unnamed namespace

Question 22

Define external linkage and give 4 examples

Answer 22

It means a declaration of the same identifier within the same program refers to the same object of function. E.g. functions, non const global variables, external const global variables, inline const global variables

Question 23

What is extern keyword?

Answer 23

This means static storage duration and external linkage

Question 24

static keyword

Answer 24

Static storage duration and internal linkage

Question 25

thread_local keyword

Answer 25

Thread storage duration - which is when the object is alive as long as the thread still exists

Question 26

mutable keyword

Answer 26

Object allowed to be modified even if containing class is const

Question 27

auto keyword

Answer 27

Automatic storage duration (deprecated in C++11)

Question 28

register keyword

Answer 28

Automatic storage duration and hint to the compiler to place in a register (deprecated in C++17)

Question 29

What is a compound statement/block?

Answer 29

A group of statements treated by the compiler as a single statement. Begin with { and end with }

Question 30

What is a user defined namespace?

Answer 30

Namespaces defined by you for your own declarations. Doesn’t include ones provided by c++ or third party libraries

Question 31

What is the scope resolution operator?

Answer 31

It is ::

It tells the compiler that the identifier specified by the right hand operand should be looked for in the scope of the left hand operand. If no left hand operand is provided, global namespace is assumed

Question 32

What is shadowing / name hiding?

Answer 32

When a variable in a nested block can obscure an identically named variable in an outer block. This should be avoided

Question 33

Define file scope

Answer 33

When a variable is visible from point of declaration until end of the file

Question 34

Why should you initialise static variables on the line they are defined?

Answer 34

If you initialise them dynamically other parts of your code might access them before they are defined which could have unexpected results

Question 35

Why should non const global variables be avoided?

Answer 35

Hard to know what other parts of program are changing variable, what value is expected, hard to debug when something goes wrong as have to check every place program access global var

Question 36

What is a qualified name?

Answer 36

A name that includes an associate scope e.g. std::string

Question 37

What is an unqualified name

Answer 37

A name that does not include a scoping qualifier e.g. string vs std::string

Question 38

Why should using statements be avoided?

Answer 38

They increase likelihood of naming collisions in your codebase and make it unclear if you are calling internal or external code. Also a using directive imports all identifiers into the scope which can be costly

Question 39

What is an in-line function

Answer 39

It exempts a function from the one definition rule, allowing its definition to be imported into multiple code files. Typically defined in header files

Question 40

What is a constexpr function?

Answer 40

A function whose return value may be computed at compile time

constexpr int square(int x)
{
return x * x;
}

Question 41

Unnamed namespace

Answer 41

Implicitly treats all content of namespace as if it had internal linkage

Question 42

consteval keyword

Answer 42

Indicates a function must evaluate at compile time. Otherwise a compile error. Called immediate functions

consteval int square(int x)
{
return x * x;
}

Question 43

What is the cost of creating a variable e.g.

int i {}

Answer 43

There is no cost, it is the initialisation that has a cost