From 71c24af74f8fa4ce2e0a160f6209ed3222e17e67 Mon Sep 17 00:00:00 2001
From: Anthony Calandra <anthony@anthony-calandra.com>
Date: Fri, 31 Jan 2020 23:16:05 -0500
Subject: [PATCH] C++20 features: new attributes, deprecate implicit capture of
 this, and classes in non-type template parameters.

---
 CPP17.md  |  4 ++--
 CPP20.md  | 50 ++++++++++++++++++++++++++++++++++++++++++++++++++
 README.md | 54 ++++++++++++++++++++++++++++++++++++++++++++++++++++--
 3 files changed, 104 insertions(+), 4 deletions(-)

diff --git a/CPP17.md b/CPP17.md
index dcb1275..8a483a6 100644
--- a/CPP17.md
+++ b/CPP17.md
@@ -17,7 +17,7 @@ C++17 includes the following new language features:
 - [constexpr if](#constexpr-if)
 - [utf-8 character literals](#utf-8-character-literals)
 - [direct-list-initialization of enums](#direct-list-initialization-of-enums)
-- [new standard attributes](#new-standard-attributes)
+- [fallthrough, nodiscard, maybe_unused attributes](#fallthrough-nodiscard-maybe_unused-attributes)
 
 C++17 includes the following new library features:
 - [std::variant](#stdvariant)
@@ -260,7 +260,7 @@ byte d = byte{1}; // OK
 byte e = byte{256}; // ERROR
 ```
 
-### New standard attributes
+### fallthrough, nodiscard, maybe_unused attributes
 C++17 introduces three new attributes: `[[fallthrough]]`, `[[nodiscard]]` and `[[maybe_unused]]`.
 * `[[fallthrough]]` indicates to the compiler that falling through in a switch statement is intended behavior.
 ```c++
diff --git a/CPP20.md b/CPP20.md
index a1dae93..629e1fa 100644
--- a/CPP20.md
+++ b/CPP20.md
@@ -8,6 +8,9 @@ C++20 includes the following new language features:
 - [designated initializers](#designated-initializers)
 - [template syntax for lambdas](#template-syntax-for-lambdas)
 - [range-based for loop with initializer](#range-based-for-loop-with-initializer)
+- [likely and unlikely attributes](#likely-and-unlikely-attributes)
+- [deprecate implicit capture of this](#deprecate-implicit-capture-of-this)
+- [class types in non-type template parameters](#class-types-in-non-type-template-parameters)
 
 C++20 includes the following new library features:
 - [concepts library](#concepts-library)
@@ -195,6 +198,53 @@ for (auto v = std::vector{1, 2, 3}; auto& e : v) {
 // prints "123"
 ```
 
+### likely and unlikely attributes
+Provides a hint to the optimizer that the labelled statement is likely/unlikely to have its body executed.
+```c++
+int random = get_random_number_between_x_and_y(0, 3);
+[[likely]] if (random > 0) {
+  // body of if statement
+  // ...
+}
+
+[[unlikely]] while (unlikely_truthy_condition) {
+  // body of while statement
+  // ...
+}
+```
+
+### Deprecate implicit capture of this
+Implicitly capturing `this` in a lamdba capture using `[=]` is now deprecated; prefer capturing explicitly using `[=, this]` or `[=, *this]`.
+```c++
+struct int_value {
+  int n = 0;
+  auto getter_fn() {
+    // BAD:
+    // return [=]() { return n; };
+
+    // GOOD:
+    return [=, *this]() { return n; };
+  }
+};
+```
+
+### Class types in non-type template parameters
+Classes can now be used in non-type template parameters. Objects passed in as template arguments have the type `const T`, where `T` is the type of the object, and has static storage duration.
+```c++
+struct foo {
+  foo() = default;
+  constexpr foo(int) {}
+};
+
+template <foo f>
+auto get_foo() {
+  return f;
+}
+
+get_foo(); // uses implicit constructor
+get_foo<foo{123}>();
+```
+
 ## C++20 Library Features
 
 ### Concepts library
diff --git a/README.md b/README.md
index 5ab27c0..44b4fe6 100644
--- a/README.md
+++ b/README.md
@@ -8,6 +8,9 @@ C++20 includes the following new language features:
 - [designated initializers](#designated-initializers)
 - [template syntax for lambdas](#template-syntax-for-lambdas)
 - [range-based for loop with initializer](#range-based-for-loop-with-initializer)
+- [likely and unlikely attributes](#likely-and-unlikely-attributes)
+- [deprecate implicit capture of this](#deprecate-implicit-capture-of-this)
+- [class types in non-type template parameters](#class-types-in-non-type-template-parameters)
 
 C++20 includes the following new library features:
 - [concepts library](#concepts-library)
@@ -26,7 +29,7 @@ C++17 includes the following new language features:
 - [constexpr if](#constexpr-if)
 - [utf-8 character literals](#utf-8-character-literals)
 - [direct-list-initialization of enums](#direct-list-initialization-of-enums)
-- [new standard attributes](#new-standard-attributes)
+- [fallthrough, nodiscard, maybe_unused attributes](#fallthrough-nodiscard-maybe_unused-attributes)
 
 C++17 includes the following new library features:
 - [std::variant](#stdvariant)
@@ -287,6 +290,53 @@ for (std::vector v{1, 2, 3}; auto& e : v) {
 // prints "123"
 ```
 
+### likely and unlikely attributes
+Provides a hint to the optimizer that the labelled statement is likely/unlikely to have its body executed.
+```c++
+int random = get_random_number_between_x_and_y(0, 3);
+[[likely]] if (random > 0) {
+  // body of if statement
+  // ...
+}
+
+[[unlikely]] while (unlikely_truthy_condition) {
+  // body of while statement
+  // ...
+}
+```
+
+### Deprecate implicit capture of this
+Implicitly capturing `this` in a lamdba capture using `[=]` is now deprecated; prefer capturing explicitly using `[=, this]` or `[=, *this]`.
+```c++
+struct int_value {
+  int n = 0;
+  auto getter_fn() {
+    // BAD:
+    // return [=]() { return n; };
+
+    // GOOD:
+    return [=, *this]() { return n; };
+  }
+};
+```
+
+### Class types in non-type template parameters
+Classes can now be used in non-type template parameters. Objects passed in as template arguments have the type `const T`, where `T` is the type of the object, and has static storage duration.
+```c++
+struct foo {
+  foo() = default;
+  constexpr foo(int) {}
+};
+
+template <foo f>
+auto get_foo() {
+  return f;
+}
+
+get_foo(); // uses implicit constructor
+get_foo<foo{123}>();
+```
+
 ## C++20 Library Features
 
 ### Concepts library
@@ -545,7 +595,7 @@ byte d = byte{1}; // OK
 byte e = byte{256}; // ERROR
 ```
 
-### New standard attributes
+### fallthrough, nodiscard, maybe_unused attributes
 C++17 introduces three new attributes: `[[fallthrough]]`, `[[nodiscard]]` and `[[maybe_unused]]`.
 * `[[fallthrough]]` indicates to the compiler that falling through in a switch statement is intended behavior.
 ```c++