| Home | Libraries | People | FAQ | More

## Class template recursive_wrapper

boost::recursive_wrapper — Solves circular dependencies, enabling recursive types.

## Synopsis

    // In header: <boost/variant/recursive_wrapper.hpp>

    template<typename T>
    class recursive_wrapper {
    public:
      // types
      typedef T type;

      // construct/copy/destruct
      recursive_wrapper();
      recursive_wrapper(const recursive_wrapper &);
      recursive_wrapper(const T &);
      ~recursive_wrapper();

      // modifiers
      void swap(recursive_wrapper &);
      recursive_wrapper & operator=(const recursive_wrapper &);
      recursive_wrapper & operator=(const T &);

      // queries
      T & get();
      const T & get() const;
      T * get_pointer();
      const T * get_pointer() const;
    };

## Description

The `recursive_wrapper` class template has an interface similar to a simple value container, but its content is allocated dynamically. This allows `recursive_wrapper` to hold types `T` whose member data leads to a circular dependency (e.g., a data member of `T` has a data member of type `T`).

The application of `recursive_wrapper` is easiest understood in context. See the section called “Recursive types with `recursive_wrapper`” for a demonstration of a common use of the class template.

Notes:

- Any type specified as the template argument to `recursive_wrapper` must be capable of construction via `operator new`. Thus, for instance, references are not supported.

### `recursive_wrapper` public construct/copy/destruct

1.     recursive_wrapper();

   Initializes `*this` by default construction of `T`.

   Requires: | `T` must fulfill the requirements of the DefaultConstructible [20.1.4] concept.
   Throws: | May fail with any exceptions arising from the default constructor of `T` or, in the event of insufficient memory, with `std::bad_alloc`.

2.     recursive_wrapper(const recursive_wrapper & other);

   Copies the content of `other` into `*this`.

   Throws: | May fail with any exceptions arising from the copy constructor of `T` or, in the event of insufficient memory, with `std::bad_alloc`.

3.     recursive_wrapper(const T & operand);

   Copies `operand` into `*this`.

   Throws: | May fail with any exceptions arising from the copy constructor of `T` or, in the event of insufficient memory, with `std::bad_alloc`.

4.     ~recursive_wrapper();

   Deletes the content of `*this`.

   Throws: | Will not throw.

### `recursive_wrapper` modifiers

1.     void swap(recursive_wrapper & other);

   Exchanges contents of `*this` and `other`.

   Throws: | Will not throw.

2.     recursive_wrapper & operator=(const recursive_wrapper & rhs);

   Assigns the content of `rhs` to the content of `*this`.

   Requires: | `T` must fulfill the requirements of the Assignable concept.
   Throws: | May fail with any exceptions arising from the assignment operator of `T`.

3.     recursive_wrapper & operator=(const T & rhs);

   Assigns `rhs` into the content of `*this`.

   Requires: | `T` must fulfill the requirements of the Assignable concept.
   Throws: | May fail with any exceptions arising from the assignment operator of `T`.

### `recursive_wrapper` queries

1.     T & get();
       const T & get() const;

   Returns a reference to the content of `*this`.

   Throws: | Will not throw.

2.     T * get_pointer();
       const T * get_pointer() const;

   Returns a pointer to the content of `*this`.

   Throws: | Will not throw.