C++20: The Big Four & More
Discover C++20's transformative features: concepts, ranges, the spaceship operator, std::format, designated initializers, std::span, and more.
C++20: A Generational Leap
C++20 is the most significant update to C++ since C++11. It introduces four major features — Concepts, Ranges, Coroutines, and Modules — each of which fundamentally changes how C++ code is written. Beyond the "Big Four," C++20 also brings the spaceship operator, std::format, std::span, designated initializers, and many constexpr enhancements. This lesson covers everything except Coroutines and Modules, which have dedicated lessons.
Concepts: Constraining Templates
Concepts replace SFINAE with readable, composable constraints on template parameters. They produce clear error messages when constraints are not satisfied:
#include <iostream>
#include <concepts>
#include <string>
#include <vector>
#include <numeric>
// Define a custom concept
template<typename T>
concept Numeric = std::integral<T> || std::floating_point<T>;
// Use concept in a requires clause
template<typename T>
requires Numeric<T>
T add(T a, T b) {
return a + b;
}
// Shorthand syntax — concept as type constraint
auto multiply(Numeric auto a, Numeric auto b) {
return a * b;
}
// Concept with multiple requirements
template<typename Container>
concept Summable = requires(Container c) {
{ c.begin() } -> std::input_or_output_iterator;
{ c.end() } -> std::input_or_output_iterator;
{ c.size() } -> std::convertible_to<std::size_t>;
};
template<Summable Container>
auto sum_container(const Container& c) {
using T = typename Container::value_type;
return std::accumulate(c.begin(), c.end(), T{});
}
int main() {
std::cout << add(3, 4) << "\n"; // 7
std::cout << multiply(2.5, 4.0) << "\n"; // 10.0
std::vector<int> v{1, 2, 3, 4, 5};
std::cout << sum_container(v) << "\n"; // 15
// add(std::string("a"), std::string("b")); // Compile error!
// Error message: constraints not satisfied — Numeric<std::string> is false
return 0;
}Ranges: Composable Algorithms
Ranges bring a functional, pipeline-style approach to the STL. Views are lazy — they don't allocate or copy data, composing transformations that execute only when iterated:
#include <iostream>
#include <ranges>
#include <vector>
#include <string>
#include <algorithm>
int main() {
std::vector<int> numbers{1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
// Pipeline: filter even, square them, take first 3
auto result = numbers
| std::views::filter([](int n) { return n % 2 == 0; })
| std::views::transform([](int n) { return n * n; })
| std::views::take(3);
for (int n : result) {
std::cout << n << " "; // 4 16 36
}
std::cout << "\n";
// Range-based algorithms with projections
struct Employee {
std::string name;
int salary;
};
std::vector<Employee> team{
{"Alice", 90000}, {"Bob", 75000}, {"Carol", 120000}
};
// Sort by salary using a projection — no custom comparator needed
std::ranges::sort(team, {}, &Employee::salary);
for (const auto& e : team) {
std::cout << e.name << ": " << e.salary << "\n";
}
// iota generates an infinite sequence
for (int i : std::views::iota(1) | std::views::take(5)) {
std::cout << i << " "; // 1 2 3 4 5
}
std::cout << "\n";
return 0;
}Three-Way Comparison & Designated Initializers
The spaceship operator <=> generates all six comparison operators from a single declaration. Designated initializers bring named field initialization from C to C++ with added type safety:
#include <iostream>
#include <compare>
#include <string>
struct Version {
int major;
int minor;
int patch;
// One operator generates ==, !=, <, >, <=, >=
auto operator<=>(const Version&) const = default;
};
struct Config {
std::string host = "localhost";
int port = 8080;
bool ssl = false;
int max_connections = 100;
};
int main() {
Version v1{2, 0, 0};
Version v2{1, 9, 5};
if (v1 > v2) std::cout << "v1 is newer\n"; // true
if (v1 != v2) std::cout << "versions differ\n";
// Strong ordering gives you all 6 operators for free
auto cmp = v1 <=> v2;
if (cmp > 0) std::cout << "v1 > v2\n";
// Designated initializers — name the fields you're setting
Config cfg{
.host = "example.com",
.port = 443,
.ssl = true
// .max_connections keeps its default value (100)
};
std::cout << cfg.host << ":" << cfg.port
<< " SSL=" << std::boolalpha << cfg.ssl
<< " max=" << cfg.max_connections << "\n";
return 0;
}std::format & std::span
std::format brings Python-like string formatting to C++. std::span is a non-owning view over a contiguous sequence of elements, replacing raw pointer+size pairs:
#include <iostream>
#include <format>
#include <span>
#include <vector>
#include <array>
// std::span — works with any contiguous container
void print_values(std::span<const int> data) {
for (int v : data) {
std::cout << std::format("{:>5}", v);
}
std::cout << "\n";
}
int main() {
// std::format — type-safe, positional formatting
std::string msg = std::format("Hello, {}! You are {} years old.", "Alice", 30);
std::cout << msg << "\n";
// Format specifiers
std::cout << std::format("Hex: {:#x}, Oct: {:#o}, Bin: {:#b}\n", 255, 255, 255);
std::cout << std::format("Pi: {:.4f}\n", 3.14159265);
std::cout << std::format("{:<15} {:>10}\n", "Name", "Score");
std::cout << std::format("{:<15} {:>10}\n", "Alice", 95);
// std::span — unifies arrays, vectors, and raw buffers
int c_array[] = {1, 2, 3, 4, 5};
std::vector<int> vec{6, 7, 8, 9, 10};
std::array<int, 3> arr{11, 12, 13};
print_values(c_array); // works with C array
print_values(vec); // works with vector
print_values(arr); // works with std::array
// Subspan — slice without copying
std::span<int> full(vec);
auto first_three = full.subspan(0, 3);
print_values(first_three); // 6 7 8
return 0;
}constexpr Enhancements & Chrono
C++20 massively expands what can be constexpr: virtual functions, dynamic_cast, try-catch, and even std::vector and std::string can now be used in constexpr contexts. This means more computation moves to compile time, resulting in faster runtime code.
The 2024y/January/15, do calendar arithmetic, and convert between time zones — all type-safe and without external libraries.
Attributes: [[likely]] and [[unlikely]] hint to the compiler which branches are hot paths, enabling better code generation in performance-critical sections.
You don't have to use all of C++20 at once. Start with the features that give the biggest productivity boost with the least disruption:
1. Spaceship operator — add auto operator<=>(const T&) const = default; to your types immediately
2. Concepts — use standard concepts (std::integral, std::ranges::range) before writing custom ones
3. std::format — replace printf and stringstream formatting
4. Ranges — start with std::views::filter and std::views::transform in new code
5. Designated initializers — use for configuration structs and option types
Save Coroutines and Modules for when your team and toolchain are ready.
- Concepts replace SFINAE with readable, composable template constraints and clear error messages
- Ranges provide lazy, composable pipelines using the
|operator — no temporary containers needed - The spaceship operator
<=>generates all six comparison operators from one defaulted declaration std::formatbrings type-safe, Python-style string formatting to C++std::spanis a non-owning view over contiguous memory, replacing pointer+size pairs- Designated initializers (
{.field = value}) make struct construction self-documenting
Quiz — Test Your Knowledge
(20 XP)1. What does `auto operator<=>(const MyType&) const = default;` do?
2. What is the key benefit of Ranges views (like `std::views::filter`)?
3. What problem do C++20 Concepts solve?