# Extra Clang Tools 18.1.8 documentation

## clang-tidy - cert-dcl58-cpp

# cert-dcl58-cpp

Modification of the `std` or `posix` namespace can result in undefined behavior. This check warns for such modifications. The `std` (or `posix`) namespace is allowed to be extended with (class or function) template specializations that depend on an user-defined type (a type that is not defined in the standard system headers).

The check detects the following (user provided) declarations in namespace `std` or `posix`:

- Anything that is not a template specialization.

- Explicit specializations of any standard library function template or class template, if it does not have any user-defined type as template argument.

- Explicit specializations of any member function of a standard library class template.

- Explicit specializations of any member function template of a standard library class or class template.

- Explicit or partial specialization of any member class template of a standard library class or class template.

Examples:

    namespace std {
      int x; // warning: modification of 'std' namespace can result in undefined behavior [cert-dcl58-cpp]
    }

    namespace posix::a { // warning: modification of 'posix' namespace can result in undefined behavior
    }

    template <>
    struct ::std::hash<long> { // warning: modification of 'std' namespace can result in undefined behavior
      unsigned long operator()(const long &K) const {
        return K;
      }
    };

    struct MyData { long data; };

    template <>
    struct ::std::hash<MyData> { // no warning: specialization with user-defined type
      unsigned long operator()(const MyData &K) const {
        return K.data;
      }
    };

    namespace std {
      template <>
      void swap<bool>(bool &a, bool &b); // warning: modification of 'std' namespace can result in undefined behavior

      template <>
      bool less<void>::operator()<MyData &&, MyData &&>(MyData &&, MyData &&) const { // warning: modification of 'std' namespace can result in undefined behavior
        return true;
      }
    }

This check corresponds to the CERT C++ Coding Standard rule DCL58-CPP. Do not modify the standard namespaces.