pass more than single param to functor

Question

I did overload the operator() for my functor that now it receives two params. I know it possible, but how I invoke it? as far as I see, functions as std::transform or std::for_each iterate over single param each time.

minimal example:

struct Functor
{
      Functor(double epsilon, double delta): ...
      float operator()(int a, int b) const;

      private:
      _epsilon, _delta;
}

Functor f(.1, .2);
std::vector v = {1, 2, 3, 4}; // a should be taken from here each time
int b = 10;
std::vector k; // save result into k
std::transform(v.begin(), v.end(), b, k, f);

thank you!

edit: I do it because I want a to be taken each time from v, whereas b will be constant

What would you expect `a` and `b` to be for each iteration of `transform`, assuming there's a way to do what you want. — Stephen Newell, May 24 '21 at 23:53
Does this answer your question? [Functors with two variables or more](https://stackoverflow.com/questions/40451263/functors-with-two-variables-or-more) — Rushikesh Talokar, May 24 '21 at 23:57
Your intent is to have four invocations of `f(x, b)`, where `b` is constant and `a` takes its values from the elements of `v`? For each `x` in `v`, call `f(x, b)`? — JaMiT, May 25 '21 at 00:02

eerorika · Accepted Answer · 2021-05-25T11:05:03.170

2

You don't need a standard algorithm to call a functor. You can use the function call operator and pass as many arguments as the functor expects:

Functor{.1, .2}(1, 2);

You've simply chosen two standard algorithms that use a unary functor as your examples. There are other standard algorithms that use functors of different arity. For example, std::find uses a binary functor such as the one in your example.

I want ... b ... be constant

This is what functors are for. They can be composed. Here is an example of creating a unary functor from your binary one. I'll use a lambda:

auto unary = [](int a) {
    return Functor{.1, .2}(a, 2);
};

edited May 25 '21 at 11:05

answered May 24 '21 at 23:50

eerorika

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ah ty i see now. do you know a suitable algorithm if I want the first param to be taken from a vector whereas the second to be constant? – Nissim Levy May 25 '21 at 00:01
1

@NissimLevy you don't need another algorithm. See edit. – eerorika May 25 '21 at 00:03
1

`std::transform(v.begin(), v.end(), std::back_inserter(k), [](int a){return f(a, b);})`? – Mooing Duck May 25 '21 at 00:36

pass more than single param to functor

1 Answers1