.. syntax:: union
  union type: type[|type...]
  array member type: <type[|type...]>
  :topics: types
  :highlighters: |
    <
    >

  Union types are a declaration at compile time that a variable, function parameter or function return value can be one of several types.

  .. todo:: if there were a pragma directive subclassing macro, we could link to the pragma def anytime one appears in text...

  Except for type checks using :pragma:`rtt_checks` they have no impact at runtime. There is no runtime union type, the concrete value type is one of the possibilities of the union type.

  Union types have no type names for themselves, they are declared anonymously with the declaration of the variable, function parameter or return type::

    int|string var;
    int|float fun(object|closure f);

  When using union types as array member types they must be enclosed with ``< >``::

    <int|string>* arr;    /* An array of ints and strings. */
    int*|string*  arr;    /* Either an array of ints or an array of strings.*/

    /*
      There must be a whitespace between two consecutive <to be not confused with the << operator:
    */
    < <int|string>*|object >* arr;