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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], statesstate 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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