Re: [boost] Suggestions for TMP/function introspection problem?

Subject: Re: [boost] Suggestions for TMP/function introspection problem?
From: Hans Dembinski (hans.dembinski_at_[hidden])
Date: 2018-11-08 17:27:15

Hi Steven,

> On 7. Nov 2018, at 21:38, Steven Watanabe via Boost <boost_at_[hidden]> wrote:
> 
> On 11/07/2018 01:18 PM, Hans Dembinski via Boost wrote:
>> 
>> I am trying to figure out how to solve an introspection problem since a few
>> days, and I am not making progress. I could use some help, it is for
>> boost.histogram. Maybe what I want is not possible...
>> 
> 
> I don't know what you need this for, but
> I would recommend reconsidering why you
> need this in the first place.

It is for generic support of arbitrary accumulators per bin. Normally, a histogram gets some values equal to the number of axes, finds a corresponding bin and increments a counter for that bin. More generally, one can replace the counter by an accumulator that can accept an arbitrary number of values (the standard histogram is then the special case where the accumulator accepts zero arguments). Extra arguments passed to the histogram are forwarded to the accumulator. For example, an accumulator could compute a mean for values which end up in the same bin. This is what people in high energy physics call a "profile".

// bin in 'x', compute a mean of 'y' for each bin in 'x'
auto h = make_histogram_with(std::vector<mean_accumulator>(), axis::regular<>(10, 0, 1));

// first argument is used to find the bin, second argument is passed to the mean_accumulator instance of that bin
h(1, 2);

The operator() of the accumulator could accept more than one value. Maybe you want to bin in 'x' and compute means for 'y' and 'z' at the same time. This should work and then you would pass three values to the histogram in total. I want to use the introspection for the accumulator to detect the number of arguments. I don't always know at compile-time how many arguments should be passed to the histogram axes (because one may use a dynamic histogram where the number of axis is only known at runtime), so I cannot just subtract the arguments that go to the axes and push the rest to the accumulator. But I could do it the other way round, subtract the arguments that need to go to the accumulator and pass the rest to the histogram axes.

>> Here is my problem. I have some classes A, B, ... which have some
>> overloaded methods, let's call them `foo`.
>> 
>> struct A {
>> void foo(int, char);
>> void foo(bool, int, char);
>> };
>> 
>> struct B {
>> void foo(double);
>> void foo(bool, double);
>> };
>> 
>> I need a way to detect how many arguments the shorter methods has. If the
>> shorter method is foo(Ts...), the longer method is always (bool, Ts...).
>> This is guaranteed. I do not know what Ts... is, it can be anything.
>> 
> 
> What if Ts... has a bool as the first item?

This case can never happen. bool was a placeholder for the special weight_type. It never makes sense to pass two weight_type's to an accumulator.

>> Can't I match the member function with a template of the form
>> something(bool, Ts...) somehow?
> 
> Like this?
> 
> template<class R, class C, class... T>
> constexpr int arity_impl(R (C::*)(bool, T...))
> { return sizeof...(T); }
> 
> template<int N>
> struct int_ {};
> 
> struct Tester {
>    void fun(double);
>    void fun(bool, double);
> };
> 
> int main() {
>    int_<arity_impl(&Tester::fun)> x;
> }

Yes, great, thanks.

Hans