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## Class template slot

boost::signals2::slot — Pass slots as function arguments, and associate tracked objects with a slot.

## Synopsis

    // In header: <boost/signals2/slot.hpp>

    template<typename Signature,   // Function type R (T1, T2, ..., TN)
             typename SlotFunction = boost::function<R (T1, T2, ..., TN)> >
    class slot : public boost::signals2::slot_base {
    public:
      // types
      typedef R            result_type;
      typedef T1           argument_type;         // Exists iff arity == 1
      typedef T1           first_argument_type;   // Exists iff arity == 2
      typedef T2           second_argument_type;  // Exists iff arity == 2
      typedef Signature    signature_type;
      typedef SlotFunction slot_function_type;

      // static constants
      static const int arity = N;  // The number of arguments taken by the slot.

      // member classes/structs/unions
      template<unsigned n>
      class arg {
      public:
        // types
        typedef Tn type;  // The type of the slot's (n+1)th argument
      };

      // construct/copy/destruct
      template<typename Slot> slot(const Slot &);
      template<typename OtherSignature, typename OtherSlotFunction>
        slot(const slot<OtherSignature, OtherSlotFunction> &);
      template<typename Func, typename Arg1, typename Arg2, ..., typename ArgN>
        slot(const Func &, const Arg1 &, const Arg2 &, ..., const ArgN &);

      // invocation
      result_type operator()(arg<0>::type, arg<1>::_type, ..., arg<N-1>::type);
      result_type operator()(arg<0>::type, arg<1>::_type, ..., arg<N-1>::type) const;

      // tracking
      slot & track(const weak_ptr<void> &);
      slot & track(const signals2::signal_base &);
      slot & track(const signals2::slot_base &);

      // slot function access
      slot_function_type & slot_function();
      const slot_function_type & slot_function() const;
    };

## Description

A slot consists of a polymorphic function wrapper (boost::function by default) plus a container of `weak_ptr`s which identify the slot's "tracked objects". If any of the tracked objects expire, the slot will automatically disable itself. That is, the slot's function call operator will throw an exception instead of forwarding the function call to the slot's polymorphic function wrapper. Additionally, a slot will automatically lock all the tracked objects as `shared_ptr` during invocation, to prevent any of them from expiring while the polymorphic function wrapper is being run.

The slot constructor will search for signals2::signal and signals2::trackable inside incoming function objects and automatically track them. It does so by applying a visitor to the incoming functors with boost::visit_each.

### `slot` public construct/copy/destruct

1.     template<typename Slot> slot(const Slot & target);

   Effects: | Initializes the `SlotFunction` object in `this` with `target`, which may be any function object with which a `SlotFunction` can be constructed. In this special case where the template type parameter `Slot` is a compatible signals2::signal type, the signal will automatically be added to the slot's tracked object list. Otherwise, the slot's tracked object list is initially empty.

2.     template<typename OtherSignature, typename OtherSlotFunction>
         slot(const slot<OtherSignature, OtherSlotFunction> & other_slot);

   Effects: | Initializes `this` with a copy of `other_slot`'s `OtherSlotFunction` object and tracked object list.

3.     template<typename Func, typename Arg1, typename Arg2, ..., typename ArgN>
         slot(const Func & f, const Arg1 & a1, const Arg2 & a2, ..., const ArgN & aN);

   Effects: | Syntactic sugar for `bind()` when the constructor is passed more than one argument. As if: `slot(boost::bind(f, a1, a2, ..., aN))`

### `slot` invocation

1.     result_type operator()(arg<0>::type a1, arg<1>::_type a2, ...,
                              arg<N-1>::type aN);
       result_type operator()(arg<0>::type a1, arg<1>::_type a2, ...,
                              arg<N-1>::type aN) const;

   Effects: | Calls the slot's `SlotFunction` object.
   Returns: | The result returned by the slot's `SlotFunction` object.
   Throws: | Any exceptions thrown by the slot's `SlotFunction` object. boost::signals2::expired_slot if any object in the tracked object list has expired.
   Notes: | If you have already used lock to insure the tracked objects are valid, it is slightly more efficient to use the slot_function() method and call the slot's `SlotFunction` directly.

### `slot` tracking

1.     slot & track(const weak_ptr<void> & tracked_object);
       slot & track(const signals2::signal_base & tracked_signal);
       slot & track(const signals2::slot_base & tracked_slot);

   Effects: | Adds object(s) to the slot's tracked object list. Should any of the tracked objects expire, then subsequent attempts to call the slot's `operator()` or `lock()` methods will throw an signals2::expired_slot exception. When tracking a signal, a shared_ptr internal to the signal class is used for tracking. The signal does not need to be owned by an external `shared_ptr`. In the case of passing another slot as the argument to `track()`, only the objects currently in the other slot's tracked object list are added to the tracked object list of `this`. The other slot object itself is not tracked.
   Returns: | `*this`

### `slot` slot function access

1.     slot_function_type & slot_function();
       const slot_function_type & slot_function() const;

   Returns: | A reference to the slot's underlying SlotFunction object.