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.

Production-quality C++ code frequently uses header files as a means to share code between translation units. A header file is any file that is inserted into a translation unit through an #include directive. Do not define an unnamed namespace in a header file. When an unnamed namespace is defined in a header file, it can lead to surprising results. Due to default internal linkage, each translation unit will define its own unique instance of members of the unnamed namespace that are ODR-used within that translation unit. This can cause unexpected results, bloat the resulting executable, or inadvertently trigger undefined behavior due to one-definition rule (ODR) violations.

Noncompliant Code Example

In this noncompliant code example, the variable v is defined in an unnamed namespace within a header file and is accessed from two separate translation units. Each translation unit prints the current value of v and then assigns a new value into it. However, because v is defined within an unnamed namespace, each translation unit operates on its own instance of v, resulting in unexpected output.

// a.h
#ifndef A_HEADER_FILE
#define A_HEADER_FILE
 
namespace {
int v;
}
 
#endif // A_HEADER_FILE
 
// a.cpp
#include "a.h"
#include <iostream>
 
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();
  g();
}

When executed, this program prints the following.

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

Compliant Solution

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

Noncompliant Code Example

In this noncompliant code example, the variable v is defined in an unnamed namespace within a header file, and an inline function, get_v(), is defined, which accesses that variable. ODR-using the inline function from multiple translation units (as shown in the implementation of f() and g()) violates the one-definition rule because the definition of get_v() is not identical in all translation units due to referencing a unique v in each translation unit.

// a.h
#ifndef A_HEADER_FILE
#define A_HEADER_FILE
 
namespace {
int v;
}
 
inline int get_v() { return v; }
 
#endif // A_HEADER_FILE
 
// a.cpp
#include "a.h"
 
void f() {
  int i = get_v();
  // ...
}
 
// b.cpp
#include "a.h"
 
void g() {
  int i = get_v();
  // ...
}

See DCL60-CPP. Obey the one-definition rule for more information on violations of the one-definition rule.

Compliant Solution

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();
  // ...
}

Noncompliant Code Example

In this noncompliant code example, the function f() is defined within a header file. However, including the header file in multiple translation units causes a violation of the one-definition rule that usually results in an error diagnostic generated at link time due to multiple definitions of a function with the same name.

// a.h
#ifndef A_HEADER_FILE
#define A_HEADER_FILE
 
void f() { /* ... */ }
 
#endif // A_HEADER_FILE
 
// a.cpp
#include "a.h"
// ...
 
// b.cpp
#include "a.h"
// ...

Noncompliant Code Example

This noncompliant code example attempts to resolve the link-time errors by defining f() within an unnamed namespace. However, it produces multiple, unique definitions of f() in the resulting executable. If a.h is included from many translation units, it can lead to increased link times, a larger executable file, and reduced performance.

// a.h
#ifndef A_HEADER_FILE
#define A_HEADER_FILE
 
namespace { 
void f() { /* ... */ }
}
 
#endif // A_HEADER_FILE
 
// a.cpp
#include "a.h"
// ...
 
// b.cpp
#include "a.h"
// ...

Compliant Solution

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.

// a.h
#ifndef A_HEADER_FILE
#define A_HEADER_FILE
 
inline void f() { /* ... */ }
 
#endif // A_HEADER_FILE
 
// a.cpp
#include "a.h"
// ...
 
// b.cpp
#include "a.h"
// ...

Risk Assessment

Defining an unnamed namespace within a header file can cause data integrity violations and performance problems but is unlikely to go unnoticed with sufficient testing. One-definition rule violations result in undefined behavior.

Automated Detection 

Tool	Version	Checker	Description
Astrée		unnamed-namespace-header	Fully checked
Axivion Bauhaus Suite		CertC++-DCL59
Clang		cert-dcl59-cpp	Checked by clang-tidy
CodeSonar		LANG.STRUCT.DECL.ANH	Anonymous Namespace in Header File
Helix QAC		C++2518
Klocwork		MISRA.NAMESPACE.UNMD
LDRA tool suite		286 S, 512 S	Fully implemented
Parasoft C/C++test		CERT_CPP-DCL59-a	There shall be no unnamed namespaces in header files
Polyspace Bug Finder		CERT C++: DCL59-CPP	Checks for unnamed namespaces in header files (rule fully covered)
RuleChecker		unnamed-namespace-header	Fully checked
SonarQube C/C++ Plugin		UnnamedNamespaceInHeader
PVS-Studio		V1068

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Related Guidelines

Bibliography