Is there a method to declare a namespace with the extern modifier so that the namespace's entire content is externally linked?
You seem to be asking two questions. One regarding external linkage and one regarding the extern specifier.
Regarding the linkage, there is not really a need for such a syntax.
External linkage is already the default at namespace scope, whether in the global namespace or another namespace, except for const non-template non-inline variables and members of anonymous unions. (https://en.cppreference.com/w/cpp/language/storage_duration#internal_linkage)
So the other way around, syntax to make all names in a namespace have internal linkage, is more likely to be required and this is possible with unnamed namespaces
namespace {
//...
}
in which all declared names have internal linkage.
Regarding the extern specifier used to turn a variable definition into just a declaration at namespace scope or to give it external linkage explicitly, I am not aware of any syntax to apply it to a whole namespace. You will have to specify it explicitly on each variable.
So I read that:
An identifier’s linkage determines whether other declarations of that name refer to the same object or not.
source
and that variables especially local ones:
have no linkage
int main()
{
int x;
{
int x;
}
}
Concerning the fact that local variables have no linkage is it because when we declare a variable it also defines it, and so both x declarations will never refer to the same object(or else naming collision will occur)?
Can a local variable have linkage if it is specified as extern?
Thanks in advance.
Based on the comments I received, I formulated the following answer:
Local variables have a scope which means that they don't exist out of this scope, thus are not exposed to linkage.
Local variables cannot be exposed to translation units, contradicting the definition of linkage.
Local variables are not fixed because of their limited lifetime. Thus are unstable and not suited for linkage.
I have gone through following two questions:
static and extern global variables in C and C++
global variable in C are static or not?
Both questions says the two things in different way.
Question 1's Answer:
Global variables are not extern nor static by default on C and C++.
Question 2's Answer:
If you do not specify a storage class (that is, the extern or static keywords), then by default global variables have external linkage
I need to know the following:
Are global variables extern by default in linkage (or) is it equivalent to declaring variables by specifying extern storage class?
Are global variables static by default in scope (or) is it equivalent to declaring variables by specifying static storage class?
Is there any difference with C or C++? Can anyone please clarify?
is global variables are extern by default in linkage (or) it is equivalent to declaring variable by specifying extern storage class?
The default storage duration, scope and linkage of variables declared outside any block, at the outer most level, have static storage duration, file scope and external linkage. C11 standard says that:
6.2.1 Scopes of identifiers (p4):
[...] If the declarator or type specifier that declares the identifier appears outside of any block or list of parameters, the identifier has file scope, which terminates at the end of the translation unit. [...]
6.2.2 Linkages of identifiers (p5):
[...] If the declaration of an identifier for an object has file scope and no storage-class specifier, its linkage is external.
6.2.4 Storage durations of objects (p3):
An object whose identifier is declared without the storage-class specifier
_Thread_local, and either with external or internal linkage or with the storage-class specifier static, has static storage duration.
So, if x is global
int x;
then its storage duration, scope and linkage is equivalent to x in
extern int x;
is global variables are static by default in scope (or) it is equivalent to declaring variable by specifying static storage class?
No. As I stated above that its duration is static and it has file scope.
If there is any c or c++ difference please clarify?
No difference. Rule is same in both languages.
is global variables are extern by default in linkage (or) it is equivalent to declaring variable by specifying extern storage class?
Unless otherwise specified, they have external linkage (except in C++, where they have internal linkage if they're constant).
The first answer you link to is saying that it's not equivalent to declaring it extern (which makes it a pure declaration, not a definition); not that it doesn't have external linkage.
is global variables are static by default in scope (or) it is equivalent to declaring variable by specifying static storage class?
In C++, they have internal linkage (as if declared static) if they are constant, external linkage otherwise. In C, they always have external linkage.
If there is any c or c++ difference please clarify?
As mentioned above, the default is always external linkage in C, while in C++ it's internal for constant variables.
Verified by using gcc version9.3.0:
Global variables are extern by default.
What is the difference between static global and non-static global identifier in C++?
Static limits the scope of the variable to the same translation unit.
A static global variable has internal linkage.
A non-static global variable has external linkage.
Good Read:
What is external linkage and internal linkage?
A global static variable is only available in the translation unit (i.e. source file) the variable is in. A non-static global variable can be referenced from other source files.
Global Non static variables are accessable from other files whereas static global variables are not
If you don't know what the difference is, correct answer will probably be even more confusing to you. In short, statics of a class aren't realted to statics at file scope. Statics of a class are esentially identical to regular variables, but they will have to be referenced by prefixing them with class name. Statics at file scope are regular variables that are local to the file only. To understand what that means, try to add two variables with the same name into a single project. You will get linker errors because there are multiple identical symbols. By making symbols static you will avoid that problems and variable's name won't be accessible from outside the file.
I want to understand the external linkage and internal linkage and their difference.
I also want to know the meaning of
const variables internally link by default unless otherwise declared as extern.
When you write an implementation file (.cpp, .cxx, etc) your compiler generates a translation unit. This is the source file from your implementation plus all the headers you #included in it.
Internal linkage refers to everything only in scope of a translation unit.
External linkage refers to things that exist beyond a particular translation unit. In other words, accessible through the whole program, which is the combination of all translation units (or object files).
As dudewat said external linkage means the symbol (function or global variable) is accessible throughout your program and internal linkage means that it is only accessible in one translation unit.
You can explicitly control the linkage of a symbol by using the extern and static keywords. If the linkage is not specified then the default linkage is extern (external linkage) for non-const symbols and static (internal linkage) for const symbols.
// In namespace scope or global scope.
int i; // extern by default
const int ci; // static by default
extern const int eci; // explicitly extern
static int si; // explicitly static
// The same goes for functions (but there are no const functions).
int f(); // extern by default
static int sf(); // explicitly static
Note that instead of using static (internal linkage), it is better to use anonymous namespaces into which you can also put classes. Though they allow extern linkage, anonymous namespaces are unreachable from other translation units, making linkage effectively static.
namespace {
int i; // extern by default but unreachable from other translation units
class C; // extern by default but unreachable from other translation units
}
A global variable has external linkage by default. Its scope can be extended to files other than containing it by giving a matching extern declaration in the other file.
The scope of a global variable can be restricted to the file containing its declaration by prefixing the declaration with the keyword static. Such variables are said to have internal linkage.
Consider following example:
1.cpp
void f(int i);
extern const int max = 10;
int n = 0;
int main()
{
int a;
//...
f(a);
//...
f(a);
//...
}
The signature of function f declares f as a function with external linkage (default). Its definition must be provided later in this file or in other translation unit (given below).
max is defined as an integer constant. The default linkage for constants is internal. Its linkage is changed to external with the keyword extern. So now max can be accessed in other files.
n is defined as an integer variable. The default linkage for variables defined outside function bodies is external.
2.cpp
#include <iostream>
using namespace std;
extern const int max;
extern int n;
static float z = 0.0;
void f(int i)
{
static int nCall = 0;
int a;
//...
nCall++;
n++;
//...
a = max * z;
//...
cout << "f() called " << nCall << " times." << endl;
}
max is declared to have external linkage. A matching definition for max (with external linkage) must appear in some file. (As in 1.cpp)
n is declared to have external linkage.
z is defined as a global variable with internal linkage.
The definition of nCall specifies nCall to be a variable that retains its value across calls to function f(). Unlike local variables with the default auto storage class, nCall will be initialized only once at the first invocation of f(). The storage class specifier static affects the lifetime of the local variable and not its scope.
NB: The keyword static plays a double role. When used in the definitions of global variables, it specifies internal linkage. When used in the definitions of the local variables, it specifies that the lifetime of the variable is going to be the duration of the program instead of being the duration of the function.
In terms of 'C' (Because static keyword has different meaning between 'C' & 'C++')
Lets talk about different scope in 'C'
SCOPE: It is basically how long can I see something and how far.
Local variable : Scope is only inside a function. It resides in the STACK area of RAM.
Which means that every time a function gets called all the variables
that are the part of that function, including function arguments are
freshly created and are destroyed once the control goes out of the
function. (Because the stack is flushed every time function returns)
Static variable: Scope of this is for a file. It is accessible every where in the file
in which it is declared. It resides in the DATA segment of RAM. Since
this can only be accessed inside a file and hence INTERNAL linkage. Any
other files cannot see this variable. In fact STATIC keyword is the
only way in which we can introduce some level of data or function
hiding in 'C'
Global variable: Scope of this is for an entire application. It is accessible form every
where of the application. Global variables also resides in DATA segment
Since it can be accessed every where in the application and hence
EXTERNAL Linkage
By default all functions are global. In case, if you need to
hide some functions in a file from outside, you can prefix the static
keyword to the function. :-)
Before talking about the question, it is better to know the term translation unit, program and some basic concepts of C++ (actually linkage is one of them in general) precisely. You will also have to know what is a scope.
I will emphasize some key points, esp. those missing in previous answers.
Linkage is a property of a name, which is introduced by a declaration. Different names can denote same entity (typically, an object or a function). So talking about linkage of an entity is usually nonsense, unless you are sure that the entity will only be referred by the unique name from some specific declarations (usually one declaration, though).
Note an object is an entity, but a variable is not. While talking about the linkage of a variable, actually the name of the denoted entity (which is introduced by a specific declaration) is concerned. The linkage of the name is in one of the three: no linkage, internal linkage or external linkage.
Different translation units can share the same declaration by header/source file (yes, it is the standard's wording) inclusion. So you may refer the same name in different translation units. If the name declared has external linkage, the identity of the entity referred by the name is also shared. If the name declared has internal linkage, the same name in different translation units denotes different entities, but you can refer the entity in different scopes of the same translation unit. If the name has no linkage, you simply cannot refer the entity from other scopes.
(Oops... I found what I have typed was somewhat just repeating the standard wording ...)
There are also some other confusing points which are not covered by the language specification.
Visibility (of a name). It is also a property of declared name, but with a meaning different to linkage.
Visibility (of a side effect). This is not related to this topic.
Visibility (of a symbol). This notion can be used by actual implementations. In such implementations, a symbol with specific visibility in object (binary) code is usually the target mapped from the entity definition whose names having the same specific linkage in the source (C++) code. However, it is usually not guaranteed one-to-one. For example, a symbol in a dynamic library image can be specified only shared in that image internally from source code (involved with some extensions, typically, __attribute__ or __declspec) or compiler options, and the image is not the whole program or the object file translated from a translation unit, thus no standard concept can describe it accurately. Since symbol is not a normative term in C++, it is only an implementation detail, even though the related extensions of dialects may have been widely adopted.
Accessibility. In C++, this is usually about property of class members or base classes, which is again a different concept unrelated to the topic.
Global. In C++, "global" refers something of global namespace or global namespace scope. The latter is roughly equivalent to file scope in the C language. Both in C and C++, the linkage has nothing to do with scope, although scope (like linkage) is also tightly concerned with an identifier (in C) or a name (in C++) introduced by some declaration.
The linkage rule of namespace scope const variable is something special (and particularly different to the const object declared in file scope in C language which also has the concept of linkage of identifiers). Since ODR is enforced by C++, it is important to keep no more than one definition of the same variable or function occurred in the whole program except for inline functions. If there is no such special rule of const, a simplest declaration of const variable with initializers (e.g. = xxx) in a header or a source file (often a "header file") included by multiple translation units (or included by one translation unit more than once, though rarely) in a program will violate ODR, which makes to use const variable as replacement of some object-like macros impossible.
I think Internal and External Linkage in C++ gives a clear and concise explanation:
A translation unit refers to an implementation (.c/.cpp) file and all
header (.h/.hpp) files it includes. If an object or function inside
such a translation unit has internal linkage, then that specific
symbol is only visible to the linker within that translation unit. If
an object or function has external linkage, the linker can also see it
when processing other translation units. The static keyword, when used
in the global namespace, forces a symbol to have internal linkage. The
extern keyword results in a symbol having external linkage.
The compiler defaults the linkage of symbols such that:
Non-const global variables have external linkage by default
Const global variables have internal linkage by default
Functions have external linkage by default
Basically
extern linkage variable is visible in all files
internal linkage variable is visible in single file.
Explain: const variables internally link by default unless otherwise declared as extern
by default, global variable is external linkage
but, const global variable is internal linkage
extra, extern const global variable is external linkage
A pretty good material about linkage in C++
http://www.goldsborough.me/c/c++/linker/2016/03/30/19-34-25-internal_and_external_linkage_in_c++/
Linkage determines whether identifiers that have identical names refer to the same object, function, or other entity, even if those identifiers appear in different translation units. The linkage of an identifier depends on how it was declared.
There are three types of linkages:
Internal linkage : identifiers can only be seen within a translation unit.
External linkage : identifiers can be seen (and referred to) in other translation units.
No linkage : identifiers can only be seen in the scope in which they are defined.
Linkage does not affect scoping
C++ only : You can also have linkage between C++ and non-C++ code fragments, which is called language linkage.
Source :IBM Program Linkage
In C++
Any variable at file scope and that is not nested inside a class or function, is visible throughout all translation units in a program. This is called external linkage because at link time the name is visible to the linker everywhere, external to that translation unit.
Global variables and ordinary functions have external linkage.
Static object or function name at file scope is local to translation unit. That is
called as Internal Linkage
Linkage refers only to elements that have addresses at link/load time; thus, class declarations and local variables have no linkage.