What is external linkage and internal linkage?
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 object file from your implementation file plus all the headers you #include
d 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's 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 isn't specified then the default linkage is extern
for non- const
symbols and static
(internal) for const
symbols.
// in namespace 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 foo(); // extern by default
static int bar(); // explicitly static
Note that instead of using static
for internal linkage it is better to use anonymous namespaces into which you can also put class
es. The linkage for anonymous namespaces has changed between C++98 and C++11 but the main thing is that they are unreachable from other translation units.
namespace {
int i; // external linkage but unreachable from other translation units.
class invisible_to_others { };
}
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);
//...
}
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;
}
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.
Hope that helps!
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