[Development] [Interest] Qt5: Problem connecting QTimer::timeout() to lambda

[Thiago Macieira]

On sexta-feira, 2 de novembro de 2012 09.41.06, Olivier Goffart wrote:
> > Why doesn't the code use the number of arguments from the receiver instead
> > of the sender? We did that for regular functions and member functions.
> 
> Because for regular functions slot, I can knw that.    The arguments are in
> the template argument.
> 
> When I do  QtPrivate::FunctionPointer<&MyObject::mySlot>::AgumentCount,  the
> partial template specialisation  is chosen  (imagine mySlot has two
> arguments)
> 
> template<class Obj, typename Ret, typename Arg1, typename Arg2>
> struct FunctionPointer<Ret (Obj::*) (Arg1, Arg2)> {
>   enum {ArgumentCount = 2};   ... }
> 
> Hence, I know I can call the function with just the two first argument of
> the signal.

I don't care about C++98 for this functionality. It's provided as a "best 
effort" basis, with known limitations. Nominally, the new syntax of the 
connection mechanism is a feature that requires C++11.

So please tell me how the above works with variadic templates.

> However, in the case of a functor, I have not found a way.
> struct MyFunctor{ void operator()(); };

There is a function, it's just MyFunctor::operator(). Why can't we do the 
same?

> QtPrivate::FunctionPointer<&MyFunctor::operator()> will not work in case
> there are several operator().

That is a good point, but an acceptable compromise.

> With C++11, it would be possible to do something like
> 
> template<typename Functor, typename ... SignalArgs>
> test(Functor&f, SignalArgs... &args) ->
> enable_if<(sizeof(f(args...))>0),yes>;
> 
> And if it does not exist, try removing the last argument from signal args
> and see if that exist.
> 
> Something like that could be done for Qt 5.1  (or even maybe Qt 5.0)  But I
> don't know how to do it without C++11.

Then let's do it, since we don't need C++98 support for this.

> And connecting to functor works without C++11  (now, it is probably ok to
> say that connecting to functor with less arguments than the signal is only
> working with C++11)
-- 
Thiago Macieira - thiago.macieira (AT) intel.com
  Software Architect - Intel Open Source Technology Center
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