C++ std -- range

Range libcxx implementation.

Let’s walk through a simple example.

#include <iostream>
#include <ranges>
using namespace std;

int main() {
  vector<int> a = {1, 2, 3, 4};
  int n = 3;
  for (const auto& x : a | views::take(n)) { cout << x << ","; }
  return 0;
}

The first note is about the pipe operator. It acts between an vector and a range adaptor. The definition in libcxx is here. The left side of | is any type that satisfies range concept, namely, any type that defines the begin and end functions. Actually, my statement is not rigorous. The range library defines 4 valid cases. See code. So it works too if we change vector to int a[] = {1, 2, 3, 4}; The right side is a range adaptor, which is simply a functor. x | f means f(x) and the result is a new range.

Secondly, how does views::take(n) works? The definition is here. It uses a C++23 feature std::bind_back to store the parameter n in the functor and then call views::taken(a, n). The final result can be the original range type, or iota range, take_view, etc. For the above case, the result is a take_view. If we change a to be auto a = views::iota(1, 5), then the result is a iota_view.