boost::signals2; no matching function for adding signal slot

Question

I have a project where I have a problem with boost::signals2. To give a short example I've shortened the code to following three classes (All classes are simplified):

class SignalArgs
{
    int a_number_;
};

class Plugin
{

    protected:
        typedef boost::signals2::signal<void (SignalArgs& args)> Event;
    public:
        typedef Event::slot_type EventHandler;



    public:
        void addEventHandler(EventHandler& handler)
        {
            onEvent.connect(handler);
        }

    private:
        Event onEvent;
};

class PluginManager_
{
public:
    PluginManager_()
    {
        p = new Plugin();
        // the error occurs in the following line
        p->addEventHandler(boost::bind(&PluginManager_::plugin_event_handler, this, _1)); 
    }
private:
    Plugin* p;

    void plugin_event_handler(SignalArgs& args);
};

While compiling, I always get the error error: no matching function for call to 'Plugin::addEventHandler(boost::_bi::bind_t, boost::_bi::list2, boost::arg<1> > >)'

Okay, the error is simple: The compiler didn't find a function with the same type and number of params. I tried to bypass that with defining an object with the exact type of Plugin::EventHandler (Which itself is a typedef to boost::signals2::slot1<void, SignalArgs, boost::function<void(SignalArgs)> >& ), but without success.

Did I have overseen something?

Answer 1

boost::bind returns a temporary (a bind object). In addition, the conversion to slot_type likely incurs another implicit conversion - resulting in yet another temporary.

Temporaries can only (portably) bind to const-references, which means that you should change the argument to either

    void addEventHandler(EventHandler handler);

or

    void addEventHandler(EventHandler const& handler);

In C++03 code the latter might eliminate a copy, whereas under C++11 the first one will invoke move-construction since the parameter is an rvalue.

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