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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.

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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:

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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.

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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 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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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 

cert-dcl59-cpp

Related Vulnerabilities

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

Related Guidelines

Bibliography

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