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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