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Namespaces introduce new declarative regions for declarations, reducing the likelihood of conflicting identifiers with other declarative regions. One feature of namespaces is that they can be further extended, even within separate translation units. For instance, the following declarations are well-formed:.

namespace MyNamespace {
int ilength;
}
 
namespace MyNamespace {
int iwidth;
}
 
void f() {
  MyNamespace::ilength = MyNamespace::iwidth = 12;
}

The standard library introduces the namespace std for standards-provided declarations such as std::string, std::vector, and std::for_each. However, it is undefined behavior to introduce new declarations in namespace std except under special circumstances. The C++ Standard, [namespace.std], paragraphs 1 and 2 [ISO/IEC 14882-2014], states the following:

¹ The behavior of a C++ program is undefined if it adds declarations or definitions to namespace std or to a namespace within namespace std unless otherwise specified. A program may add a template specialization for any standard library template to namespace std only if the declaration depends on a user-defined type and the specialization meets the standard library requirements for the original template and is not explicitly prohibited.

2 The ² The behavior of a C++ program is undefined if it declares

— an explicit specialization of any member function of a standard library class template, or
— an explicit specialization of any member function template of a standard library class or class template, or
— an explicit or partial specialization of any member class template of a standard library class or class template.

In addition to restricting extensions to the the namespace std, the C++ Standard, [namespace.posix], paragraph 1, further states the following:

The behavior of a C++ program is undefined if it adds declarations or definitions to namespace posix or to a namespace within namespace posix unless otherwise specified. The namespace posix is reserved for use by ISO/IEC 9945 and other POSIX standards.

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In this noncompliant code example, the declaration of x is added to the namespace std, resulting in undefined behavior:.

namespace std {
int x;
}

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In this compliant solution, a specialization of std::plus is added to the std namespace, but the specialization depends on a user-defined type and meets the Standard Template Library requirements for the original template, and so it complies with this rule. However, because MyString can be constructed from std::string, this compliant solution involves invoking a converting constructor whereas the previous compliant solution does not.

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Altering the standard namespace can cause undefined behavior in the C++ standard library.

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

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Bibliography

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