How do I use concepts in C++ to check if a type is a range?

Question

Ryan McCombe · Accepted Answer

Concepts in C++20 allow us to define requirements that types must satisfy. To determine if an object is a valid range using C++20 concepts, there are two steps:

Include <ranges> Header: Standard library concepts for testing ranges are in the std::ranges namespace.
Check Range Type: Use concepts like std::ranges::forward_range to determine the category.

Here's an example:

1#include <vector>
2#include <list>
3#include <forward_list>
4#include <ranges>
5#include <iostream>
6
7template <typename T>
8void Log(T Range) {
9  if constexpr (std::ranges::forward_range<T>) {
10    std::cout << " - Forward Range\n";
11  }
12  if constexpr (std::ranges::bidirectional_range<T>) {
13    std::cout << " - Bidirectional Range\n";
14  }
15  if constexpr (std::ranges::random_access_range<T>) {
16    std::cout << " - Random Access Range\n";
17  }
18}
19
20int main() {
21  std::cout << "std::forward_list<int>:\n";
22  Log(std::forward_list<int>{1, 2, 3});
23
24  std::cout << "\nstd::list<int>:\n";
25  Log(std::list<int>{1, 2, 3});
26
27  std::cout << "\nstd::vector<int>:\n";
28  Log(std::vector<int>{1, 2, 3});
29}

1std::forward_list<int>:
2 - Forward Range
3
4std::list<int>:
5 - Forward Range
6 - Bidirectional Range
7
8std::vector<int>:
9 - Forward Range
10 - Bidirectional Range
11 - Random Access Range

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