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6.11 — Scope, duration, and linkage summary

The concepts of scope, duration, and linkage cause a lot of confusion, so we’re going to take an extra lesson to summarize everything. Some of these things we haven’t covered yet, and they’re here just for completeness / reference later.

Scope summary

An identifier’s scope determines where the identifier can be accessed within the source code.

  • Variables with block scope / local scope can only be accessed within the block in which they are declared (including nested blocks). This includes:
    • Local variables
    • Function parameters
    • User-defined type definitions (such as enums and classes) declared inside a block
  • Variables and functions with global scope / file scope can be accessed anywhere in the file. This includes:
    • Global variables
    • Functions
    • User-defined type definitions (such as enums and classes) declared inside a namespace or in the global scope

Duration summary

A variable’s duration determines when it is created and destroyed.

  • Variables with automatic duration are created at the point of definition, and destroyed when the block they are part of is exited. This includes:
    • Local variables
    • Function parameters
  • Variables with static duration are created when the program begins and destroyed when the program ends. This includes:
    • Global variables
    • Static local variables
  • Variables with dynamic duration are created and destroyed by programmer request. This includes:
    • Dynamically allocated variables

Linkage summary

An identifier’s linkage determines whether multiple declarations of an identifier refer to the same identifier or not.

  • An identifier with no linkage means the identifier only refers to itself. This includes:
    • Local variables
    • User-defined type definitions (such as enums and classes) declared inside a block
  • An identifier with internal linkage can be accessed anywhere within the file it is declared. This includes:
    • Static global variables (initialized or uninitialized)
    • Static functions
    • Const global variables
    • Functions declared inside an unnamed namespace
    • User-defined type definitions (such as enums and classes) declared inside an unnamed namespace
  • An identifier with external linkage can be accessed anywhere within the file it is declared, or other files (via a forward declaration). This includes:
    • Functions
    • Non-const global variables (initialized or uninitialized)
    • Extern const global variables
    • Inline const global variables
    • User-defined type definitions (such as enums and classes) declared inside a namespace or in the global scope

Identifiers with external linkage will generally cause a duplicate definition linker error if the definitions are compiled into more than one .cpp file (due to violating the one-definition rule). There are some exceptions to this rule (for types, templates, and inline functions and variables) -- we’ll cover these further in future lessons when we talk about those topics.

Also note that functions have external linkage by default. They can be made internal by using the static keyword.

Variable scope, duration, and linkage summary

Because variables have scope, duration, and linkage, let’s summarize in a chart:

Type Example Scope Duration Linkage Notes
Local variable int x; Block Automatic None
Static local variable static int s_x; Block Static None
Dynamic variable int *x { new int{} }; Block Dynamic None
Function parameter void foo(int x) Block Automatic None
External non-constant global variable int g_x; File Static External Initialized or uninitialized
Internal non-constant global variable static int g_x; File Static Internal Initialized or uninitialized
Internal constant global variable constexpr int g_x { 1 }; File Static Internal Must be initialized
External constant global variable extern constexpr int g_x { 1 }; File Static External Must be initialized
Inline constant global variable inline constexpr int g_x { 1 }; File Static External Must be initialized
Internal constant global variable const int g_x { 1 }; File Static Internal Must be initialized
External constant global variable extern const int g_x { 1 }; File Static External Must be initialized at definition
Inline constant global variable inline const int g_x { 1 }; File Static External Must be initialized

Forward declaration summary

You can use a forward declaration to access a function or variable in another file:

Type Example Notes
Function forward declaration void foo(int x); Prototype only, no function body
Non-constant global variable forward declaration extern int g_x; Must be uninitialized
Const global variable forward declaration extern const int g_x; Must be uninitialized
Constexpr global variable forward declaration extern constexpr int g_x; Not allowed, constexpr cannot be forward declared

What the heck is a storage class specifier?

When used as part of an identifier declaration, the static and extern keywords are called storage class specifiers. In this context, they set the storage duration and linkage of the identifier.

C++ supports 4 active storage class specifiers:

Specifier Meaning Note
extern static (or thread_local) storage duration and external linkage
static static (or thread_local) storage duration and internal linkage
thread_local thread storage duration Introduced in C++11
mutable object allowed to be modified even if containing class is const
auto automatic storage duration Deprecated in C++11
register automatic storage duration and hint to the compiler to place in a register Deprecated in C++17

The term storage class specifier is typically only used in formal documentation.


6.12 -- Using statements
Index
6.10 -- Static local variables

96 comments to 6.11 — Scope, duration, and linkage summary

  • winterson

    hi . whats your mean "static storage duration"?
    is your mean the static keyword?

    • nascardriver

      See the table in section "Variable scope, duration, and linkage summary". Everything where it says "Static" in the "Duration" column has static storage duration.

  • Brion

    A company header file was recently changed from defining a set of function prototypes to defining a set of function pointers. One source file sets the pointer and another file calls the function pointed to by it. All source files that include this header get the function pointer declarations without any "extern", so I figured at link time there should be a collision with several source files each claiming to define these function pointers; however, somehow it just seems to work (compiled and linked using g++ on Linux). I always assumed one file should define a global variable and others should specify extern, but this seems to be working - how? Does the linker resolve such conflicts somehow so that all references point to the same memory location? What if they had all specified a different initialization value?

  • Zuhail

    In this phrase of yours
    "When used as part of an identifier declaration, the static and extern keywords are called storage class specifiers."
    shouldn't you say identifier's definition because
    e.g.
    main.cpp

    [/code]

    static int var;  //definition(as well as declaration)
    extern const var1; //forward declaration
    .........

    [/code]

    here :-
    static keyword only sets the linkage and duration of the variable 'var' but extern here forward declaring the const var only tells the compiler about its linkage it actually doesnt set its linkage .

  • sami

    Why in the table "Variable scope, duration, and linkage summary", there not any global variable defined using "const" keyword?

  • sami

    Hi,
    Does it mean when a class or enum is defined inside a function?

    "User-defined type definitions (such as enums and classes) declared inside a block"

  • Abhishek

    Hello,
    Can you please explain why constexpr variables cannot be forward declared ?

    • Jokerusho

      if you have some questions, you can try to search in stackOverflow, the best plateform for me.you just need to to search your question to find an other questions like this to resolve your question. (sorry for my eng)

      I just tap and replace :
      Hello,
      Can you please explain why constexpr variables cannot be forward declared ?  to  "why constexpr variables cannot be forward declared stackOverflow" in your browser.

      https://stackoverflow.com/questions/33197817/forward-declare-a-constexpr-variable-template

      i know im the best :).

  • Wambui

    string literals also have static storage duration - https://en.cppreference.com/w/cpp/language/string_literal

  • Ambareesh

    From the linkage definition under summary:
    " An identifier’s linkage determines whether multiple instances of an identifier refer to the same identifier or not "

    What is meant by "instance" of an identifier? Is it the occurrences of the identifier ? Say for variable name "length", is every occurrence of the word "length" in a file an instance? Or do you mean only declarations of variable with name "length" ?

  • Ayrton Fithiadi Sedjati

    In the Linkage summary section under the third bullet point, you wrote "An identifier*s* with external linkage...". I suppose you meant "An identifier with external linkage...".

  • Jimmy Hunter

    Just a small typo you might want to fix for readability.
    In the Linkage summary section under the external linkage bullet,
    you have "(such as enums and and classes)"
    I think you meant "(such as enums and classes)"

  • Anastasia

    Hi,
    I may be dumb, but in the linkage summary it's said that static functions are covered in chapter 7, but I can't see where it is and don't remember learning about them (except for member static functions, which I suppose are a totally different thing and they are covered later). Can anyone point me in the right direction?

    • Alex

      Hmmm, I think I actually don't cover these explicitly. I've removed the reference to chapter 7.

      But just like static global variables, static functions are only accessible in the file in which they are declared.

      • Anastasia

        There's a brief explanation (with an example) of static functions in lesson S.4.2 — Global variables and linkage (paragraph 'Function linkage'), it could be used as a reference instead (as a little refresher).

        Thanks!

  • mmp52

    Hello!
    On the end of the course, you've said that when we make forward declaration of a constant global variable in order to use it in another file, it should not be initialized. Shouldn't we initialize all constant variables in the moment of definition? What should I use if I want a constant global variable with external linkage and I want it to be initialized during definition?

    Thank you!

    • Alex

      Forward declarations just tell the compiler that an object (or function) exists (and what type it is). They don't define actual objects, and so they can't have initializers.

      The actual defined object (that the forward declaration is referencing) should be initialized.

  • SM.Haider

    Wait, what's the scope of this article?

  • Gabe

    "Identifiers with external linkage will generally cause a duplicate definition linker error if the definitions are compiled into more than one .cpp file (due to violating the one-definition rule)."

    Would you mind explaining this sentence? What does "if the definitions are compiled into more than one .cpp file"?

    • gabe.hpp

      gabe.cpp

      main.cpp

      `gabe` is now defined in "gabe.cpp" and "main.cpp" (Because `#include` simply copies a file's content). Variables with external linkage can only be defined once, so you're getting and error.

  • Samira Ferdi

    Hi, Alex and Nascardriver!

    What is normal function? I confused the term 'normal'. So, are there functions those are not normal?

    • Alex

      Normal functions are the kind of functions we've presented so far. There are other kinds of functions (member functions, inline functions, etc...) that you'll learn about later.

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