# Extra Clang Tools 4 documentation

## clang-tidy - misc-definitions-in-headers

# misc-definitions-in-headers

Finds non-extern non-inline function and variable definitions in header files, which can lead to potential ODR violations in case these headers are included from multiple translation units.

    // Foo.h
    int a = 1; // Warning: variable definition.
    extern int d; // OK: extern variable.

    namespace N {
      int e = 2; // Warning: variable definition.
    }

    // Warning: variable definition.
    const char* str = "foo";

    // OK: internal linkage variable definitions are ignored for now.
    // Although these might also cause ODR violations, we can be less certain and
    // should try to keep the false-positive rate down.
    static int b = 1;
    const int c = 1;
    const char* const str2 = "foo";

    // Warning: function definition.
    int g() {
      return 1;
    }

    // OK: inline function definition is allowed to be defined multiple times.
    inline int e() {
      return 1;
    }

    class A {
    public:
      int f1() { return 1; } // OK: implicitly inline member function definition is allowed.
      int f2();

      static int d;
    };

    // Warning: not an inline member function definition.
    int A::f2() { return 1; }

    // OK: class static data member declaration is allowed.
    int A::d = 1;

    // OK: function template is allowed.
    template<typename T>
    T f3() {
      T a = 1;
      return a;
    }

    // Warning: full specialization of a function template is not allowed.
    template <>
    int f3() {
      int a = 1;
      return a;
    }

    template <typename T>
    struct B {
      void f1();
    };

    // OK: member function definition of a class template is allowed.
    template <typename T>
    void B<T>::f1() {}

## Options

`HeaderFileExtensions```

  A comma-separated list of filename extensions of header files (the filename extensions should not include “.” prefix). Default is “h,hh,hpp,hxx”. For header files without an extension, use an empty string (if there are no other desired extensions) or leave an empty element in the list. e.g., “h,hh,hpp,hxx,” (note the trailing comma).

`UseHeaderFileExtension```

  When non-zero, the check will use the file extension to distinguish header files. Default is 1.