Question

There is a signal and several objects with slots. I want to implement the behavior when one object calls signal and blocks its own connection. I guess a small snippet will be more informative:

typedef boost::signal<void()> TSignal;

template<class TSignal>
class SlotObject
{
public:

    void Connect(boost::shared_ptr<TSignal> pSignal, boost::function slot)
    {
        m_connection = pSignal->connect(slot);
        m_pSignal = pSignal;
    }

    // How to define TSignal signature here?
    VOID Call()
    {
        m_connection.block();
        (*m_pSignal)();
        m_connection.unblock();
    }

    boost::shared_ptr<TSignal> m_pSignal;
    boost::signals::connection m_connection;
};

The questions:

1. Is there a standard approach with some boost stuff? Do I reinventing the wheel?
2. How to define Call method with TSignal signature?

Answer 1

For your first question: I’m not aware of a “standard boost way” to achieve what you want. You may post your question to the boost users mailing list.

For your second question: Without varidic templates and rvalue references, forwarding is always cumbersome.

A few suggestions, in no particular order:

1) You may look at the boost/signal.hpp and the files in boost/signals/ to get an idea of how this kind of stuff can be done with the preprocessor, but here’s a partial implementation to show the idea(warning: untested):

template<size_t Arity, class SignalT>
struct SlotBase;

template<class SignalT>
struct SlotBase<0, SignalT>
{
    typedef SignalT::slot_function_type SlotType;

    SlotBase(boost::shared_ptr<SignalT> S, SlotType F)
        : m_Signal(S), m_Connection(S->connect(F))){};

    void operator()()const
    {
        m_Connection.block();
        m_Signal();
        m_Connection.unblock()
    };

private:
    boost::shared_ptr<SignalT> > m_Signal;
    boost::signals::connection m_Connection;
};

template<class SignalT>
struct SlotBase<1, SignalT>
{
    // as above, except for operator()
    // ...

    void operator()(typename SignalT::arg1_type arg1)
    {
        m_Connection.block();
        m_Signal(arg1);
        m_Connection.unblock();
    };
};

template<class SignalT>
struct SlotBase<2, SignalT>
{
    // as above, except for operator()
    // ...

    void operator()(typename SignalT::arg1_type arg1, typename SignalT::arg2_type arg2)
    {
        m_Connection.block();
        m_Signal(arg1, arg2);
        m_Connection.unblock()
    };
};

// repeat for other arities
// ...

template<class SignalT>
class SlotObject : public SlotBase<SignalT::arity, SignalT>
{
    typedef SlotBase<SignalT::arity, SignalT> BaseType;

public:
    Slot(boost::shared_ptr<SignalT>S, 
         typename SignalT::slot_function_type F
    ) : BaseType(S, F)
    {}
};

2) If you are willing to give up a bit of syntax nicety for the users of SlotObject, other things are possible. One is to wrap the call to the signal using the technique shown in boost::shared_ptr documentation (http://www.boost.org/doc/libs/1_40_0/libs/smart_ptr/sp_techniques.html#wrapper), ie, your Call() method would block the m_connection, and return a shared_ptr to m_signal having a custom deleter that unblocks m_connection.

Sadly, this does not give a nice syntax to the caller. It would look like:

SlotObject<signal<void(int, float)> > s = ...;
s.Call()->operator()(1, 1.234);

3) Another alternative is to ask the user to package the arguments in a tuple (I’m using a boost::fusion::vector below) at the call site, and use boost::fusion:::fused to unpack them and call the signal.

#include <boost/function_types/parameter_types.hpp>
#include <boost/fusion/include/vector.hpp>
#include <boost/fusion/include/mpl.hpp>
#include <boost/fusion/include/fused.hpp>
#include <boost/signal.hpp>
#include <boost/shared_ptr.hpp>

// Metafunction to extract the Signature template parameter
// from a boost::signal instantiation
// For example, SignatureOf<signal<void(int, float)>::type 
// is "void(int, float)"
template<class SignalT>
struct SignatureOf;

template<
    typename Signature, typename Combiner, typename Group,
    typename GroupCompare, typename SlotFunction
>
struct SignatureOf<
    boost::signal<Signature, Combiner, Group, GroupCompare, SlotFunction>
>
{
    typedef Signature type;
};

// The SlotObject    
template<class SignalT>
class SlotObject
{
public:
    typedef typename SignatureOf<SignalT>::type SignatureType;

    // Defines the "packed" parameters type corresponding
    // to the slot's signature
    // For example, for a SignalT of boost::signal<void(int, float)>
    // ArgsType is "boost::fusion::vector<int, float>"
    typedef typename boost::fusion::result_of::as_vector<
        typename boost::function_types::parameter_types<SignatureType>::type
    >::type ArgsType;

    void Call(ArgsType P)
    {
        m_Connection.block();
        boost::fusion::fused<SignalT&> f(*m_Signal);
        f(P);
        m_Connection.unblock();
    }

    //...
};

This would be used as:

typedef SlotObject<boost::signal<void(int, float)> > SlotType;
SlotType s = ...;
s.Call(SlotType::ArgsType(1, "foo"));