Introduction
C++20 was a landmark release, enriching the language with modern abstractions that streamline generic programming, improve performance, and simplify async programming. Three of its most impactful additions are Concepts, Ranges, and Coroutines. This article dissects each feature, shows practical usage, and explains how they integrate with existing C++ tooling.
1. Concepts – Compile-Time Interface Contracts
1.1 What Are Concepts?
A concept is a compile-time predicate that validates a type’s suitability for a template. It serves as a contract, replacing fragile enable_if checks and providing expressive diagnostics.
template <typename T>
concept Incrementable = requires(T x) {
++x;
x++;
};1.2 Benefits
- Readable Constraints: The template signature becomes self-documenting.
- Better Error Messages: Failed concepts produce specific diagnostic messages.
- SFINAE-Free: Eliminates boilerplate
enable_if.
1.3 Example: Generic sort with Constraints
#include <algorithm>
#include <concepts>
#include <vector>
template <std::ranges::input_range R>
requires std::sortable<std::ranges::iterator_t<R>>
void quick_sort(R& r) {
std::sort(std::ranges::begin(r), std::ranges::end(r));
}1.4 Integrating with Existing Code
Concepts are additive; you can keep using enable_if or SFINAE for backward compatibility. Libraries like Ranges-v3 already expose concepts and can be gradually upgraded.
2. Ranges – The New STL Paradigm
2.1 From Iterators to Views
Before C++20, you manipulated iterators manually. Ranges introduce views—lazy, composable transformations that operate on ranges.
#include <vector>
#include <ranges>
#include <iostream>
std::vector <int> data{1, 2, 3, 4, 5};
auto evens = data | std::views::filter([](int n){ return n % 2 == 0; });
for (int n : evens) {
std::cout << n << ' ';
}2.2 Built-in Views
| View | Description |
|---|---|
std::views::filter |
Filters elements |
std::views::transform |
Applies a unary operation |
std::views::reverse |
Reverse traversal |
std::views::take / drop |
Slicing |
std::views::join |
Flatten nested ranges |
2.3 Combining Views
Views are composable; the pipeline above can be extended:
auto processed = data
| std::views::transform([](int n){ return n * 2; })
| std::views::filter([](int n){ return n % 3 == 0; });
for (int x : processed) std::cout << x << ' ';2.4 Performance Considerations
- Lazy Evaluation: No intermediate containers.
- Short-Circuiting:
std::ranges::any_ofstops at the first match. - Custom Views: Implement
begin()andend()to create specialized views without runtime overhead.
3. Coroutines – Simplified Asynchronous Programming
3.1 The Coroutine Skeleton
Coroutines are functions that can suspend and resume. The language introduces the co_await, co_yield, and co_return keywords.
#include <coroutine>
#include <iostream>
struct Task {
struct promise_type {
Task get_return_object() { return {}; }
std::suspend_never initial_suspend() { return {}; }
std::suspend_never final_suspend() noexcept { return {}; }
void return_void() {}
void unhandled_exception() {}
};
};
Task async_print() {
std::cout << "Before suspension\n";
co_await std::suspend_always{};
std::cout << "After suspension\n";
}3.2 Common Use Cases
- Async I/O:
asio::awaitableor custom awaitables. - Generators:
co_yieldto produce sequences. - State Machines: Encapsulate complex state transitions.
3.3 Example: Asynchronous File Reader
#include <asio.hpp>
#include <iostream>
asio::awaitable<std::string> async_read_file(const std::string& path) {
asio::ip::tcp::socket sock{co_await asio::this_coro::executor};
// Setup I/O, then read...
co_return "file content";
}
int main() {
asio::io_context io{1};
asio::co_spawn(io, async_read_file("data.txt"), asio::detached);
io.run();
}3.4 Integrating Coroutines with Ranges
Combine lazy evaluation with async streams:
auto async_numbers = async_generator([]{ co_yield 1; co_yield 2; });
for (auto n : async_numbers | std::views::filter([](int x){ return x % 2 == 0; })) {
std::cout << n << '\n';
}4. Practical Migration Path
| Feature | Steps | Notes |
|---|---|---|
| Concepts | Define concepts for existing templates; replace enable_if. |
Requires C++20 compiler support. |
| Ranges | Refactor loops using std::ranges::for_each; replace manual iterators. |
std::ranges works with existing containers. |
| Coroutines | Start with simple co_yield generators; then use libraries (e.g., Boost.Coroutine2). |
Must handle thread safety and synchronization. |
5. Summary
C++20’s Concepts, Ranges, and Coroutines elevate the language’s expressiveness:
- Concepts make generic programming safer and more readable.
- Ranges provide a lazy, composable way to work with containers.
- Coroutines simplify asynchronous workflows without sacrificing performance.
Adopting these features can dramatically improve code clarity, maintainability, and runtime efficiency. Whether you’re writing high-performance systems or concise utilities, C++20 offers the tools to do it elegantly.