What is external linkage and internal linkage?


Question

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.

1
312
5/7/2017 6:26:33 AM

Accepted Answer

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 #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).

249
3/9/2016 11:25:41 AM

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 classes. 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 { };
}

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