# Extra Clang Tools 21.1.8 documentation

## clang-tidy - bugprone-reserved-identifier

# bugprone-reserved-identifier

cert-dcl37-c and cert-dcl51-cpp redirect here as an alias for this check.

Checks for usages of identifiers reserved for use by the implementation.

The C and C++ standards both reserve the following names for such use:

- identifiers that begin with an underscore followed by an uppercase letter;

- identifiers in the global namespace that begin with an underscore.

The C standard additionally reserves names beginning with a double underscore, while the C++ standard strengthens this to reserve names with a double underscore occurring anywhere.

Violating the naming rules above results in undefined behavior.

    namespace NS {
      void __f(); // name is not allowed in user code
      using _Int = int; // same with this
      #define cool__macro // also this
    }
    int _g(); // disallowed in global namespace only

The check can also be inverted, i.e. it can be configured to flag any identifier that is not a reserved identifier. This mode is for use by e.g. standard library implementors, to ensure they don’t infringe on the user namespace.

This check does not (yet) check for other reserved names, e.g. macro names identical to language keywords, and names specifically reserved by language standards, e.g. C++ ‘zombie names’ and C future library directions.

This check corresponds to CERT C Coding Standard rule DCL37-C. Do not declare or define a reserved identifier as well as its C++ counterpart, DCL51-CPP. Do not declare or define a reserved identifier.

## Options

Invert

  If true, inverts the check, i.e. flags names that are not reserved. Default is false.

AllowedIdentifiers

  Semicolon-separated list of regular expressions that the check ignores. Default is an empty list.