Complex Use of Template in C++ - c++

Question

I come across a piece of code about template:
unroller.h
template <int It, int End> struct unroller {
template <typename Action> static void step(const Action &action) {
action(std::integral_constant<int, It>());
unroller<It + 1, End>::step(action);
}
};
template <int End> struct unroller<End, End> {
template <typename Action> static void step(const Action &) {}
};
snippet of the code that show how it is invoked:
constexpr int NUM_ROWS = 2;
constexpr int NUM_COLS = 2;
int input_idx = 0;
for (int pass = 0; pass < num_passes; pass++) {
unroller<0, NUM_ROWS>::step([&input_idx, input_accessor](auto i_idx) {
constexpr int i = i_idx.value;
unroller<0, NUM_COLS>::step([&input_idx, input_accessor](auto j_idx) {
constexpr int j = j_idx.value;
ProducerToConsumerPipeMatrix::pipe_at<i, j>::write(input_accessor[input_idx++]);
});
});
}
Question:
template <typename Action> static void step(const Action &action) is function template?
what is action(std::integral_constant<int,It>());? I thought action is pass by reference. However, i don't really understand it.
I don't understand the way it invoke unroller<0, NUM_ROWS>::step([&input_idx, input_accessor](autoi_idx). My understanding is in the angle bracket <> should be data types. In this case, it pass the value. Besides, what is ::step? Then, why it can pass 2 argument in the step(...)>
I am new to c++. Can someone explain in simple way?

Answer 1

I think you may need to start learning templates from the beginning.
Yes. It's a typical function template
No, it's passed by value, at least in your example is passed by value.
[&input_idx, input_accessor](auto j_idx) {
constexpr int j = j_idx.value;
ProducerToConsumerPipeMatrix::pipe_at<i, j>::write(input_accessor[input_idx++]);
});
this lambda is passed into higher-order function step as its parameter, it will later receieve a std::integral_constant<int, It> prvalue temporary as the parameter of the lambda.
std::integral_constant<int,It>() constructs a temporary object of std::integral_constant<int, It>, see this: https://en.cppreference.com/w/cpp/types/integral_constant
It's not two arguments, it's lambda capture, see this: https://en.cppreference.com/w/cpp/language/lambda
the parameters of a template don't need to be types, see this: https://en.cppreference.com/w/cpp/language/templates#Syntax

Answer 2

I am new to c++. Can someone explain in simple way?
I'm going to try, but this is template meta-programming that usually needs some kind of expertise mixed with functional style (which is another realm of expertise).
template static void step(const Action &action) is function template?
Yes, a function in which Action is a parameter.
what is action(std::integral_constant());? I thought action is
pass by reference. However, i don't really understand it.
The general idea of unroller is to unroll a loop. This to generate automatically a sequence of calls to something like:
action(0);
action(1);
action(2);
...
action(NUM_ROWS-1);
action is then a parameter to any function that is able to get an int value.
The function used here is:
[&input_idx, input_accessor](auto i_idx) {
constexpr int i = i_idx.value;
unroller<0, NUM_COLS>::step([&input_idx, input_accessor](auto j_idx) {
constexpr int j = j_idx.value;
ProducerToConsumerPipeMatrix::pipe_at<i,j>::write(input_accessor[input_idx++]);
}
}
which itself uses the unrolling! Looks like it unrolls a computation on 2D matrix elements. Theses functions are called lambdas, anonymous functions is you prefer. Then you are in the realm of functional programming.
I don't understand the way it invoke unroller<0, NUM_ROWS>::step([&input_idx, input_accessor](autoi_idx). My
understanding is in the angle bracket <> should be data types. In this
case, it pass the value. Besides, what is ::step? Then, why it can
pass 2 argument in the step(...)>
Template parameters can be types but also values of different kind...
unroller is a template type that takes two arguments: <int It, int End>.
It and End are typed as ints not class or typename (as it is with Action).