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Unnamed namespaces are used to define a namespace that is unique to the translation unit, where the names contained within have internal linkage by default. The C++ Standard, [namespace.unnamed], paragraph 1 [ISO/IEC 14882-2014], states the following:

An unnamed-namespace-definition behaves as if it were replaced by:

  inline namespace unique { /* empty body */ }
  using namespace unique ;
  namespace unique { namespace-body }

where inline appears if and only if it appears in the unnamed-namespace-definition, all occurrences of unique in a translation unit are replaced by the same identifier, and this identifier differs from all other identifiers in the entire program.

...

When executed, this program prints the following:.

f(): 0
g(): 0
f(): 42
g(): 100

...

In this compliant solution, v is defined in only one translation unit but is externally visible to all translation units, resulting in the expected behavior:.

// a.h
#ifndef A_HEADER_FILE
#define A_HEADER_FILE
 
extern int v;
 
#endif // A_HEADER_FILE
 
// a.cpp
#include "a.h"
#include <iostream>

int v; // Definition of global variable v
 
void f() {
  std::cout << "f(): " << v << std::endl;
  v = 42;
  // ...
}
 
// b.cpp
#include "a.h"
#include <iostream>
 
void g() {
  std::cout << "g(): " << v << std::endl;
  v = 100;
}
 
int main() {
  extern void f();
  f(); // Prints v, sets it to 42
  g(); // Prints v, sets it to 100
  f(); // Prints v, sets it back to 42
  g(); // Prints v, sets it back to 100
}

When executed, this program prints the following:.

f(): 0
g(): 42
f(): 100
g(): 42

...

In this compliant solution, v is defined in only one translation unit but is externally visible to all translation units and can be accessed from the inline get_v() function:.

// a.h
#ifndef A_HEADER_FILE
#define A_HEADER_FILE
 
extern int v;

inline int get_v() {
  return v;
}

#endif // A_HEADER_FILE
 
// a.cpp
#include "a.h"
 
// Externally used by get_v();
int v;
 
void f() {
  int i = get_v();
  // ...
}
 
// b.cpp
#include "a.h"
 
void g() {
  int i = get_v();
  // ...
}

...

In this compliant solution, f() is not defined with an unnamed namespace and is instead defined as an inline function. Inline functions are required to be defined identically in all the translation units in which they are used, which allows an implementation to generate only a single instance of the function at runtime in the event the body of the function does not get generated for each call site.

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