Scope, Duration and Linkage Flashcards
What is meant by a variable’s scope?
Scope determines where a variable is accessible.
Explain duration of a variable
Duration determines when a variable is created and destroyed
What is the linkage of a variable?
Linkage determines whether the variable can be exported to another file or not
What is the scope of global variables?
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
What is the duration of a global variable?
They have static duration, which means they are created when the program is started, and destroyed when it ends
What is the linkage of a global variable?
They can have either internal or external linkage, via the static and external keywords respectively
How should you define a global constant in C++17
pragma once
Define them as inline constexpr global variables in a header file
Constants.h
constexpr int MAX_VALUE = 100;
What is static duration?
When a variable is created when the program starts and destroyed when it ends
What is internal linkage?
When an identifier can only be used in the file in which it is defined
What is automatic duration?
When a local variable is created at point of definition and destroyed when the block is exited
What is automatic duration?
When a local variable is created at point of definition and destroyed when the block is exited
What effect does using keyword static have on a global variable?
It gives it internal linkage; meaning the variable cannot be exported to other files
What effect does using keyword static have on a local variable?
It gives it static duration, meaning the variable will only be created once, and will not be destroyed until the end of the program
Variable with block/local scope can only be accessed from point of declaration until the end of the block. Three examples are:
Local variables, function parameters and program defined type definitions (such as enums and classes) declared inside a block
Variables and functions with global scope can be accessed from the point of declaration to the end of the file. This includes (3)
Global variables
functions
program defined type definitions (such as enums or classes) declared inside a namespace or in the global scope
Define automatic duration and give two examples
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
Define static duration and give two examples of variables with static duration
Variables which are created when the program begins and destroyed when it ends.
E.g.
global variables
static local variables
Define static duration and give two examples
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
Define dynamic duration and give an example
Variables are created and destroyed by programmer request. E.g. dynamically allocated variables
Define no linkage and give 2 examples
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
Define internal linkage and give 5 examples
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
Define external linkage and give 4 examples
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
What is extern keyword?
This means static storage duration and external linkage
static keyword
Static storage duration and internal linkage
thread_local keyword
Thread storage duration - which is when the object is alive as long as the thread still exists
mutable keyword
Object allowed to be modified even if containing class is const
auto keyword
Automatic storage duration (deprecated in C++11)
register keyword
Automatic storage duration and hint to the compiler to place in a register (deprecated in C++17)
What is a compound statement/block?
A group of statements treated by the compiler as a single statement. Begin with { and end with }
What is a user defined namespace?
Namespaces defined by you for your own declarations. Doesn’t include ones provided by c++ or third party libraries
What is the scope resolution operator?
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
What is shadowing / name hiding?
When a variable in a nested block can obscure an identically named variable in an outer block. This should be avoided
Define file scope
When a variable is visible from point of declaration until end of the file
Why should you initialise static variables on the line they are defined?
If you initialise them dynamically other parts of your code might access them before they are defined which could have unexpected results
Why should non const global variables be avoided?
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
What is a qualified name?
A name that includes an associate scope e.g. std::string
What is an unqualified name
A name that does not include a scoping qualifier e.g. string vs std::string
Why should using statements be avoided?
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
What is an in-line function
It exempts a function from the one definition rule, allowing its definition to be imported into multiple code files. Typically defined in header files
What is a constexpr function?
A function whose return value may be computed at compile time
constexpr int square(int x)
{
return x * x;
}
Unnamed namespace
Implicitly treats all content of namespace as if it had internal linkage
consteval keyword
Indicates a function must evaluate at compile time. Otherwise a compile error. Called immediate functions
consteval int square(int x)
{
return x * x;
}
What is the cost of creating a variable e.g.
int i {}
There is no cost, it is the initialisation that has a cost
