Hey,
As to your suggestion: I don't think you can overload connect()
specifically for a lambda with a specific signature ?
As to how you can make it work: you can use std::bind() to bind away the
argument. See for example the second example at :
http://www.webtoolkit.eu/widgets/forms/push-button
Regards,
koen
2013/3/27 Nathan Ridge <[email protected]>
>
> Hello,
>
> Suppose I am writing an event handler for a push button. The expected
> event handler type is a function with one argument of type WMouseEvent.
> Often I don't care about the WMouseEvent and would like to pass in
> a zero-argument function instead.
>
> If I use bind() to create my handler, such as in the following code,
> then I can simply pass a zero-argument function:
>
>
> #include <functional>
> #include <Wt/WApplication>
> #include <Wt/WPushButton>
> using namespace Wt;
>
> struct Application : public WApplication
> {
> Application(const WEnvironment& env) : WApplication(env)
> {
> WPushButton* button = new WPushButton(root());
> button->clicked().connect(std::bind(&Application::onClicked,
> this));
> }
>
> void onClicked()
> {
> }
> };
>
>
> (In this example I used the C++11 std::bind(), but it's the same for
> boost::bind() and boost::phoenix::bind()).
>
> This works because the result of a bind() call is a function object
> that accepts any number of arguments, and ignores the ones it doesn't
> use. So, even though a zero-argument function was bound and Wt passes
> in the WMouseEvent argument, it compiles.
>
> However, if I try to do the same thing with a C++11 lambda, it doesn't
> compile:
>
>
> #include <functional>
> #include <Wt/WApplication>
> #include <Wt/WPushButton>
> using namespace Wt;
>
> struct Application : public WApplication
> {
> Application(const WEnvironment& env) : WApplication(env)
> {
> WPushButton* button = new WPushButton(root());
> button->clicked().connect([this]{ this->onClicked(); });
> }
>
> void onClicked()
> {
> }
> };
>
>
> The compiler error is:
>
> In file included from test.cpp:2:
> In file included from wt/include/Wt/WApplication:26:
> In file included from wt/include/Wt/WCssStyleSheet:14:
> In file included from wt/include/Wt/WBreak:10:
> In file included from wt/include/Wt/WWebWidget:14:
> In file included from wt/include/Wt/WWidget:14:
> In file included from wt/include/Wt/WSignal:17:
> In file included from boost/signal.hpp:20:
> In file included from boost/signals/signal1.hpp:24:
> In file included from boost/signals/signal_template.hpp:37:
> In file included from boost/function/function1.hpp:11:
> In file included from boost/function/detail/maybe_include.hpp:18:
> boost/function/function_template.hpp:153:33: error: no matching function
> for call to object of type '<lambda at test.cpp:11:35>'
> BOOST_FUNCTION_RETURN((*f)(BOOST_FUNCTION_ARGS));
> ^~~~
> boost/function/function_template.hpp:75:36: note: expanded from macro
> 'BOOST_FUNCTION_RETURN'
> # define BOOST_FUNCTION_RETURN(X) X
> ^
> boost/function/function_template.hpp:934:53: note: in instantiation of
> member function
> 'boost::detail::function::void_function_obj_invoker1<<lambda at
> test.cpp:11:35>, void, Wt::WMouseEvent>::invoke' requested here
> { { &manager_type::manage }, &invoker_type::invoke };
> ^
> boost/function/function_template.hpp:722:13: note: in instantiation of
> function template specialization 'boost::function1<void,
> Wt::WMouseEvent>::assign_to<<lambda at test.cpp:11:35> >' requested here
> this->assign_to(f);
> ^
> boost/signals/slot.hpp:111:24: note: in instantiation of function template
> specialization 'boost::function1<void, Wt::WMouseEvent>::function1<<lambda
> at test.cpp:11:35> >' requested here
> slot(const F& f) :
> slot_function(BOOST_SIGNALS_NAMESPACE::get_invocable_slot(f,
> BOOST_SIGNALS_NAMESPACE::tag_type(f)))
> ^
> wt/include/Wt/WSignal:1021:27: note: in instantiation of function template
> specialization 'boost::slot<boost::function1<void, Wt::WMouseEvent>
> >::slot<<lambda at test.cpp:11:35> >' requested here
> return dynamic_.connect(function, boost::signals::at_front);
> ^
> test.cpp:11:27: note: in instantiation of function template specialization
> 'Wt::EventSignal<Wt::WMouseEvent>::connect<<lambda at test.cpp:11:35> >'
> requested here
> button->clicked().connect([this]{ this->onClicked(); });
> ^
> test.cpp:11:35: note: candidate function not viable: requires 0 arguments,
> but 1 was provided
> button->clicked().connect([this]{ this->onClicked(); });
> ^
>
>
> The reason this doesn't work is that, unlike the result of bind(),
> a lambda is not callable with extra arguments that it ignores.
>
> I think it would be nice if Wt supported passing in a zero-argument
> lambda in such a case, just like it supports passing in a zero-
> argument bound function.
>
> What do you think? Should I file a ticket for this?
>
> Thanks,
> Nate
>
>
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