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20.2. CPP/C 20.md

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+## Modules
+
+Modules provide a new way to organize and manage large codebases, helping to improve build times, reduce code duplication, and increase modularity.
+```cpp
+// Example: math.cppm  
+export module math;  
+  
+export int square(int x) {  
+  return x * x;  
+}  
+  
+export int cube(int x) {  
+  return x * x * x;  
+}  
+  
+// Example: main.cpp  
+import math;  
+  
+int main() {  
+  int result = math::square(5);  
+  // result = 25  
+  int result2 = math::cube(5);  
+  // result2 = 125  
+}
+```
+
+## Concepts
+
+Concepts provide a way to specify **_constraints_** on templates, making it easier to write **_generic_** code that works with a wide range of types.
+```cpp
+template <typename T>  
+concept Addable = requires(T a, T b) {  
+  { a + b } -> T;  
+};  
+  
+template <Addable T>  
+T add(T a, T b) {  
+  return a + b;  
+}  
+  
+int main() {  
+  int x = 1, y = 2;  
+  auto result = add(x, y);  
+  // result = 3  
+}
+```
+
+## Ranges
+
+Ranges provide a new way to manipulate **_sequences_** of data in C++, making it easier to write clean, readable, and efficient code.
+```cpp
+#include <ranges>  
+#include <iostream>  
+  
+int main() {  
+  std::vector<int> v{1, 2, 3, 4, 5};  
+  auto even = v | std::ranges::views::filter([](int x) { return x % 2 == 0; });  
+  for (int x : even) {  
+    std::cout << x << '\n';  
+  }  
+}
+```
+
+## Contract Programming
+
+Contract programming allows developers to specify **_preconditions_**, **_postconditions_**, and **_assertions_** in their code, making it easier to **catch** bugs early and reduce the number of runtime errors.
+```cpp
+#include <iostream>  
+  
+[[nodiscard]] int foo() {  
+  return 42;  
+}  
+  
+int main() {  
+  int x = foo();  
+  std::cout << x << '\n';  
+}
+```
+
+## Improved Template Metaprogramming
+
+C++20 includes several imrovements to the way templates can be used in C++, making it easier to write **generic** code that can be used with a wide range of types.
+
+```cpp
+#include <type_traits>  
+  
+template <typename T>  
+struct is_integral : std::false_type {};  
+  
+template <>  
+struct is_integral<int> : std::true_type {};  
+  
+int main() {  
+    static_assert(is_integral<int>::value, "");  
+    static_assert(!is_integral<float>::value, "");  
+}
+```