see #12: translate ch10

book/en-us/10-cpp20.md

@@ -6,10 +6,97 @@ order: 10
 
 # Chapter 10 Outlook: Introduction of C++20
 
+[TOC]
 
+C++20 seems to be an exciting update.
+For example, as early as C++11, the `Concept`, 
+which was eager to call for high-altitude but ultimately lost, is now on the line.
+The C++ Organizing Committee decided to vote to finalize C++20 with many proposals, 
+such as **Concepts**/**Module**/**Coroutine**/**Ranges**/ and so on.
+In this chapter we'll take a look at some of the important features that 
+C++20 will introduce.
+
+## Concept
+
+Concept is a further enhancement to C++ template programming. 
+In simple terms, the concept is a compile-time feature.
+It allows the compiler to evaluate template parameters at compile time, 
+greatly enhancing our experience with template programming in C++.
+When programming with templates, we often encounter a variety of heinous errors.
+This is because we have so far been unable to check and limit template parameters.
+For example, the following two lines of code can cause a lot of 
+almost unreadable compilation errors:
+
+```cpp
+#include <list>
+#include <algorithm>
+int main() {
+    std::list<int> l = {1, 2, 3};
+    std::sort(l.begin(), l.end());
+    return 0;
+}
+```
+
+The root cause of this code error is that `std::sort` must provide 
+a random iterator for the sorting container, otherwise it will not be used, 
+and we know that `std::list` does not support random access.
+In the conceptual language, the iterator in `std::list` does not satisfy 
+the constraint of the concept of random iterators in `std::sort`.
+After introducing the concept, we can constrain the template parameters 
+like this:
+
+```cpp
+template <typename T>
+requires Sortable<T> // Sortable is a concept
+void sort(T& c);
+```
+
+abbreviate as:
+
+```cpp
+template<Sortable T> // T is a Sortable typename
+void sort(T& c)
+```
+
+Even use it directly as a type:
+
+```cpp
+void sort(Sortable& c); // c is a Sortable type object
+```
+
+Let's look at a practical example.
+
+TODO
+
+## Module
+
+TODO
+
+## Contract
+
+TODO
+
+## Range
+
+TODO
+
+## Coroutine
+
+TODO
+
+## Conclusion
+
+In general, I finally saw the exciting features of Concepts/Ranges/Modules in C++20.
+This is still full of charm for a programming language that is already in its thirties.
 
 [Table of Content](./toc.md) | [Previous Chapter](./09-others.md) | [Next Chapter](./appendix1.md)
 
+## Further Readings
+
+- [Why Concepts didn't make C++17？](http://honermann.net/blog/2016/03/06/why-concepts-didnt-make-cxx17/)
+- [C++11/14/17/20 Compiler Support](http://en.cppreference.com/w/cpp/compiler_support)
+- [C++ History](https://en.cppreference.com/w/cpp/language/history)
+
 ## Licenses
 
 <a rel="license" href="http://creativecommons.org/licenses/by-nc-nd/4.0/"><img alt="Creative Commons License" style="border-width:0" src="https://i.creativecommons.org/l/by-nc-nd/4.0/88x31.png" /></a><br />This work was written by [Ou Changkun](https://changkun.de) and licensed under a <a rel="license" href="http://creativecommons.org/licenses/by-nc-nd/4.0/">Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License</a>. The code of this repository is open sourced under the [MIT license](../../LICENSE).