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Difference between revisions of "cpp/language/storage duration"

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< cpp‎ | language
(Removed the keyword revboxes again, no version marks needed.)
(Added CWG issue #2872 DR.)
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===Linkage===
 
===Linkage===
−
A name that denotes object, reference, function, type, template, namespace, or value, may have ''linkage''. If a name has linkage, it refers to the same entity as the same name introduced by a declaration in another scope. If a variable, function, or another entity with the same name is declared in several scopes, but does not have sufficient linkage, then several instances of the entity are generated.
+
A name can have ''external linkage''{{sep}}{{rev inl|since=c++20|, ''module linkage''}}, ''internal linkage'', or ''no linkage'':
 +
* An entity whose name has external linkage can be {{rlp|conflicting declarations#Multiple declarations of the same entity|redeclared}} in another {{rlp|translation phases|translation unit}}{{rev inl|since=c++20|, and the redeclaration can be {{rlp|modules#Module ownership|attached to a different module}}}}.
 +
{{rrev|since=c++20|
 +
* An entity whose name has module linkage can be redeclared in another translation unit, as long as the redeclaration is attached to the same module.
 +
}}
 +
* An entity whose name has internal linkage can be redeclared in another scope in the same translation unit.
 +
* An eneity whose name has no linkage can only be redeclared in the same scope.
  
 
The following linkages are recognized:
 
The following linkages are recognized:
  
 
====No linkage====
 
====No linkage====
−
The name can be referred to only from the scope it is in.
 
−
 
 
Any of the following names declared at block scope have no linkage:
 
Any of the following names declared at block scope have no linkage:
−
* variables that aren't explicitly declared {{c/core|extern}} (regardless of the {{c/core|static}} modifier);
+
* variables that are not explicitly declared {{c/core|extern}} (regardless of the {{c/core|static}} modifier);
 
* {{rlpsd|class#Local classes}} and their member functions;
 
* {{rlpsd|class#Local classes}} and their member functions;
 
* other names declared at block scope such as typedefs, enumerations, and enumerators.
 
* other names declared at block scope such as typedefs, enumerations, and enumerators.
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====Internal linkage====
 
====Internal linkage====
−
The name can be referred to from all scopes in the current translation unit.
 
−
 
 
Any of the following names declared at namespace scope have internal linkage:
 
Any of the following names declared at namespace scope have internal linkage:
 
* variables{{rev inl|since=c++14|, variable templates}}, functions, or function templates declared {{c/core|static}};
 
* variables{{rev inl|since=c++14|, variable templates}}, functions, or function templates declared {{c/core|static}};
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====External linkage====
 
====External linkage====
−
The name can be referred to from the scopes in the other translation units. Variables and functions with external linkage also have {{rlp|language linkage}}, which makes it possible to link translation units written in different programming languages.
+
Variables and functions with external linkage also have {{rlp|language linkage}}, which makes it possible to link translation units written in different programming languages.
  
 
Any of the following names declared at namespace scope have external linkage, unless they are declared in an unnamed namespace{{rev inl|since=c++20| or their declarations are attached to a named module and are not exported}}:
 
Any of the following names declared at namespace scope have external linkage, unless they are declared in an unnamed namespace{{rev inl|since=c++20| or their declarations are attached to a named module and are not exported}}:
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{{rrev|since=c++20|
 
{{rrev|since=c++20|
 
====Module linkage====
 
====Module linkage====
−
The name can be referred to only from the scopes in the same module unit or in the other translation units of the same named module.
+
Names declared at namespace scope have module linkage if their declarations are attached to a named module and are not exported, and do not have internal linkage.
−
 
+
−
Names declared at namespace scope have module linkage if their declarations are attached to a named module and are not exported, and don't have internal linkage.
+
 
}}
 
}}
  
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{{dr list item|wg=cwg|dr=2533|std=C++98|before=the storage duration of implicitly-<br>created objects were unclear|after=made clear}}
 
{{dr list item|wg=cwg|dr=2533|std=C++98|before=the storage duration of implicitly-<br>created objects were unclear|after=made clear}}
 
{{dr list item|wg=cwg|dr=2850|std=C++98|before=it was unclear when the storage for<br>function parameters are deallocated|after=made clear}}
 
{{dr list item|wg=cwg|dr=2850|std=C++98|before=it was unclear when the storage for<br>function parameters are deallocated|after=made clear}}
 +
{{dr list item|wg=cwg|dr=2872|std=C++98|before=the meaning of “can be referred to” was unclear|after=improved wording}}
 
{{dr list item|paper=P2788R0|std=C++20|before=declaring a const-qualified variable in a namespace<br>gave it internal linkage even in a module unit|after=internal linkage is not given}}
 
{{dr list item|paper=P2788R0|std=C++20|before=declaring a const-qualified variable in a namespace<br>gave it internal linkage even in a module unit|after=internal linkage is not given}}
 
{{dr list end}}
 
{{dr list end}}

Revision as of 22:32, 12 August 2024

 
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The storage class specifiers are a part of the decl-specifier-seq of a name's declaration syntax. Together with the scope of the name, they control two independent properties of the name: its storage duration and its linkage.

Contents

Storage duration

The storage duration is the property of an object that defines the minimum potential lifetime of the storage containing the object. The storage duration is determined by the construct used to create the object and is one of the following:

  • static storage duration
  • thread storage duration
(since C++11)
  • automatic storage duration
  • dynamic storage duration

Static, thread,(since C++11) and automatic storage durations are associated with objects introduced by declarations and with temporary objects. The dynamic storage duration is associated with objects created by a new expression or with implicitly created objects.

The storage duration categories apply to references as well.

The storage duration of subobjects and reference members is that of their complete object.

Specifiers

The following keywords are storage class specifiers :

  • auto
(until C++11)
  • register
(until C++17)
  • static
  • thread_local
(since C++11)
  • extern
  • mutable

In a decl-specifier-seq, there can be at most one storage class specifier, except that thread_local may appear with static or extern(since C++11).

mutable has no effect on storage duration. For its usage, see const/volatile.

Other storage class specifiers can appear in the decl-specifier-seq s of the following declarations:

Specifier Can appear in the decl-specifier-seq s of
Variable declarations Function declarations Structured binding declarations
(since C++17)
Non-member Member Non-member Member
Non-parameter Function parameter Non-static  Static  Non-static  Static 
auto Block scope only Yes No No No No No N/A
register Block scope only Yes No No No No No N/A
static Yes No Declares static Namespace scope only Declares static Yes
 thread_local  Yes No No Yes No No No Yes
extern Yes No No No Yes No No No

Anonymous unions can also be declared with static.

register is a hint that the variable so declared will be heavily used, so that its value can be stored in a CPU register. The hint can be ignored, and in most implementations it will be ignored if the address of the variable is taken. This use is deprecated.

(until C++17)

Static storage duration

A variable satisfying all following conditions have static storage duration :

  • It belongs to a namespace scope or are first declared with static or extern.
  • It does not have thread storage duration.
(since C++11)

The storage for these entities lasts for the duration of the program.

Thread storage duration

All variables declared with thread_local have thread storage duration.

The storage for these entities lasts for the duration of the thread in which they are created. There is a distinct object or reference per thread, and use of the declared name refers to the entity associated with the current thread.

(since C++11)

Automatic storage duration

The following variables have automatic storage duration :

  • Variables that belong to a block scope and are not explicitly declared static, thread_local,(since C++11) or extern. The storage for such variables lasts until the block in which they are created exits.
  • Variables that belong to a parameter scope (i.e. function parameters). The storage for a function parameter lasts until immediately after its destruction

Dynamic storage duration

Objects created by the following methods during program execution have dynamic storage duration :

Linkage

A name can have external linkage , module linkage(since C++20), internal linkage, or no linkage:

  • An entity whose name has module linkage can be redeclared in another translation unit, as long as the redeclaration is attached to the same module.
(since C++20)
  • An entity whose name has internal linkage can be redeclared in another scope in the same translation unit.
  • An eneity whose name has no linkage can only be redeclared in the same scope.

The following linkages are recognized:

No linkage

Any of the following names declared at block scope have no linkage:

  • variables that are not explicitly declared extern (regardless of the static modifier);
  • local classes and their member functions;
  • other names declared at block scope such as typedefs, enumerations, and enumerators.

Names not specified with external, module,(since C++20) or internal linkage also have no linkage, regardless of which scope they are declared in.

Internal linkage

Any of the following names declared at namespace scope have internal linkage:

  • variables, variable templates(since C++14), functions, or function templates declared static;
  • non-template (since C++14)variables of non-volatile const-qualified type, unless
  • they are inline,
(since C++17)
 (since C++20)
  • they are explicitly declared extern, or
  • they were previously declared and the prior declaration did not have internal linkage;

In addition, all names declared in unnamed namespace or a namespace within an unnamed namespace, even ones explicitly declared extern, have internal linkage.

(since C++11)

External linkage

Variables and functions with external linkage also have language linkage, which makes it possible to link translation units written in different programming languages.

Any of the following names declared at namespace scope have external linkage, unless they are declared in an unnamed namespace or their declarations are attached to a named module and are not exported(since C++20):

  • variables and functions not listed above (that is, functions not declared static, non-const variables not declared static, and any variables declared extern);
  • enumerations;
  • names of classes, their member functions, static data members (const or not), nested classes and enumerations, and functions first introduced with friend declarations inside class bodies;
  • names of all templates not listed above (that is, not function templates declared static).

Any of the following names first declared at block scope have external linkage:

  • names of variables declared extern;
  • names of functions.

Module linkage

Names declared at namespace scope have module linkage if their declarations are attached to a named module and are not exported, and do not have internal linkage.

(since C++20)
This section is incomplete
Reason: add the description of the behavior when an entity is declared with different linkages in the same translation unit (6.6 paragraph 6), note the difference between C++20 (ill-formed) and the current draft (well-formed)

Static block variables

Block variables with static or thread(since C++11) storage duration are initialized the first time control passes through their declaration (unless their initialization is zero- or constant-initialization, which can be performed before the block is first entered). On all further calls, the declaration is skipped.

  • If the initialization throws an exception, the variable is not considered to be initialized, and initialization will be attempted again the next time control passes through the declaration.
  • If the initialization recursively enters the block in which the variable is being initialized, the behavior is undefined.
  • If multiple threads attempt to initialize the same static local variable concurrently, the initialization occurs exactly once (similar behavior can be obtained for arbitrary functions with std::call_once).
  • Usual implementations of this feature use variants of the double-checked locking pattern, which reduces runtime overhead for already-initialized local statics to a single non-atomic boolean comparison.
(since C++11)

The destructor for a block variable with static storage duration is called at program exit, but only if the initialization took place successfully.

Variables with static storage duration in all definitions of the same inline function (which may be implicitly inline) all refer to the same object defined in one translation unit, as long as the function has external linkage.

Translation-unit-local entities

The concept of translation-unit-local entities is standardized in C++20, see this page for more details.

An entity is translation-unit-local (or TU-local for short) if

  • it has a name with internal linkage, or
  • it does not have a name with linkage and is introduced within the definition of a TU-local entity, or
  • it is a template or template specialization whose template argument or template declaration uses a TU-local entity.

Bad things (usually violation of ODR) can happen if the type of a non-TU-local entity depends on a TU-local entity, or if a declaration of, or a deduction guide for,(since C++17) a non-TU-local entity names a TU-local entity outside its

  • function-body for a non-inline function or function template
  • initializer for a variable or variable template
  • friend declarations in a class definition
  • use of value of a variable, if the variable is usable in constant expressions

Such uses are disallowed in a module interface unit (outside its private-module-fragment, if any) or a module partition, and are deprecated in any other context.

A declaration that appears in one translation unit cannot name a TU-local entity declared in another translation unit that is not a header unit. A declaration instantiated for a template appears at the point of instantiation of the specialization.

(since C++20)

Notes

Names at the top-level namespace scope (file scope in C) that are const and not extern have external linkage in C, but internal linkage in C++.

Since C++11, auto is no longer a storage class specifier; it is used to indicate type deduction.

In C, the address of a register variable cannot be taken, but in C++, a variable declared register is semantically indistinguishable from a variable declared without any storage class specifiers.

(until C++17)

In C++, unlike C, variables cannot be declared register.

(since C++17)

Names of thread_local variables with internal or external linkage referred from different scopes may refer to the same or to different instances depending on whether the code is executing in the same or in different threads.

The extern keyword can also be used to specify language linkage and explicit template instantiation declarations, but it's not a storage class specifier in those cases (except when a declaration is directly contained in a language linkage specification, in which case the declaration is treated as if it contains the extern specifier).

Storage class specifiers, except for thread_local, are not allowed on explicit specializations and explicit instantiations: 

template<class T>
struct S
{
    thread_local static int tlm;
};
 
template<>
thread_local int S<float>::tlm = 0; // "static" does not appear here

A const (may be implied by constexpr) variable template used to have internal linkage by default, which was inconsistent with other templated entities. Defect report CWG2387 corrected this.

(since C++14)
inline acts as a workaround for CWG2387 by giving external linkage by default. This is why the inline was added to many variable templates and then removed after having CWG2387 accepted. Standard library implementations also need to use inline as long as a supported compiler has not get CWG2387 implemented. See GCC Bugzilla #109126 and MSVC STL PR #4546. (since C++17)
Feature-test macro Value Std Feature
__cpp_threadsafe_static_init 200806L (C++11) Dynamic initialization and destruction with concurrency

Keywords

auto, register, static, extern, thread_local, mutable

Example

Run this code
#include <iostream>
#include <mutex>
#include <string>
#include <thread>
 
thread_local unsigned int rage = 1;
std::mutex cout_mutex;
 
void increase_rage(const std::string& thread_name)
{
    ++rage; // modifying outside a lock is okay; this is a thread-local variable
    std::lock_guard<std::mutex> lock(cout_mutex);
    std::cout << "Rage counter for " << thread_name << ": " << rage << '\n';
}
 
int main()
{
    std::thread a(increase_rage, "a"), b(increase_rage, "b");
 
    {
        std::lock_guard<std::mutex> lock(cout_mutex);
        std::cout << "Rage counter for main: " << rage << '\n';
    }
 
    a.join();
    b.join();
}

Possible output: 

Rage counter for a: 2
Rage counter for main: 1
Rage counter for b: 2

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
CWG 216 C++98 unnamed class and enumeration in class scope have
different linkage from those in namespace scope 		they all have external
linkage in these scopes
CWG 389 C++98 a name with no linkage should not be
used to declare an entity with linkage 		a type without linkage shall not be used
as the type of a variable or function
with linkage, unless the variable
or function has C language linkage
CWG 426 C++98 an entity could be declared with both internal
and external linkage in the same translation unit 		the program is ill-formed in this case
CWG 527 C++98 the type restriction introduced by the resolution of CWG
389 was also applied to variables and functions that
cannot be named outside their own translation units 		the restriction is lifted for these
variables and functions (i.e. with no
linkage or internal linkage, or declared
within unnamed namespaces)
CWG 809 C++98 register served very little function deprecated
CWG 1648 C++11 static was implied even if
thread_local is combined with extern 		implied only if no other storage
class specifier is present
CWG 1686 C++98
C++11 		the name of a non-static variable declared in namespace
scope had internal linkage only if it is explicitly
declared const (C++98) or constexpr (C++11) 		only required the type
to be const-qualified
CWG 2019 C++98 the storage duration of reference
members were unspecified 		same as their complete object
CWG 2387 C++14 unclear whether const-qualified variable
template have internal linkage by default 		const qualifier does not affect
the linkage of variable
templates or their instances
CWG 2533 C++98 the storage duration of implicitly-
created objects were unclear 		made clear
CWG 2850 C++98 it was unclear when the storage for
function parameters are deallocated 		made clear
CWG 2872 C++98 the meaning of “can be referred to” was unclear improved wording
P2788R0 C++20 declaring a const-qualified variable in a namespace
gave it internal linkage even in a module unit 		internal linkage is not given

See also

C documentation for storage duration
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