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 (local) scope can only be accessed from the point of declaration until the end of the block in which they are declared (including nested blocks). This includes:
- Local variables
- Function parameters
- 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 until the end of the file. This includes:
- Global variables
- Functions
- Program-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 a declaration of that same identifier in a different scope refers to the same entity (object, function, reference, etc…) or not.
Local variables have no linkage. Each declaration of an identifier with no linkage refers to a unique object or function.
- An identifier with no linkage means another declaration of the same identifier refers to a unique entity. Entities whose identifiers have no linkage include:
- Local variables
- Program-defined type identifiers (such as enums and classes) declared inside a block
- 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 (initialized or uninitialized)
- Static functions
- Const global variables
- Unnamed namespaces and anything defined within them
- An identifier with external linkage means a declaration of the same identifier within the entire program refers to the same object or function. Entities whose identifiers have external linkage include:
- Non-static functions
- Non-const global variables (initialized or uninitialized)
- Extern const global variables
- Inline const global variables
- Namespaces
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 local variable | int* x { new int{} }; | Block | Dynamic | None | |
| Function parameter | void foo(int x) | Block | Automatic | None | |
| Internal non-const global variable | static int g_x; | Global | Static | Internal | Initialized or uninitialized |
| External non-const global variable | int g_x; | Global | Static | External | Initialized or uninitialized |
| Inline non-const global variable (C++17) | inline int g_x; | Global | Static | External | Initialized or uninitialized |
| Internal constant global variable | constexpr int g_x { 1 }; | Global | Static | Internal | Must be initialized |
| External constant global variable | extern const int g_x { 1 }; | Global | Static | External | Must be initialized |
| Inline constant global variable (C++17) | inline constexpr int g_x { 1 }; | Global | Static | External | Must be initialized |
Forward declaration summary
You can use a forward declaration to access a function or variable in another file. The scope of the declared variable is as per usual (global scope for globals, block scope for locals).
| Type | Example | Notes |
|---|---|---|
| Function forward declaration | void foo(int x); | Prototype only, no function body |
| Non-constant variable forward declaration | extern int g_x; | Must be uninitialized |
| Const variable forward declaration | extern const int g_x; | Must be uninitialized |
| Constexpr variable forward declaration | extern constexpr int g_x; | Not allowed, constexpr cannot be forward declared |
A constexpr variable (which is implicitly const) can be forward declared using a const variable forward declaration. When accessed through the forward declaration, the variable will be considered const (not constexpr).
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 | |
| 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.