From e6f439cb43ee51b13ed4d29ec84ac5450556b163 Mon Sep 17 00:00:00 2001
From: Johannes Stoelp <johannes.stoelp@gmail.com>
Date: Wed, 24 Aug 2022 18:09:52 +0200
Subject: c++: add any_of meta fn example

---
 src/development/c++.md       | 14 +++++++++++++-
 src/development/c++/meta.cc  |  4 ++++
 src/development/c++/meta3.cc | 22 ++++++++++++++++++++++
 3 files changed, 39 insertions(+), 1 deletion(-)
 create mode 100644 src/development/c++/meta3.cc

diff --git a/src/development/c++.md b/src/development/c++.md
index 4ce1538..aff9ccd 100644
--- a/src/development/c++.md
+++ b/src/development/c++.md
@@ -16,7 +16,19 @@ typename decltype(foo)::_;
 {{#include c++/meta.cc:3:}}
 ```
 
-## [SFINAE][sfinae] example ([enable_if][enable-if])
+## Example: `is_any_of` template meta function
+
+```cpp
+{{#include c++/meta3.cc:3:}}
+```
+
+## Example: [SFINAE][sfinae] ([enable_if][enable-if])
+
+Provide a single entry point `Invoke` to call some `Operations`.
+Use `enable_if` to enable/disable the template functions depending on the two
+available traits an operation can have:
+- Operation returns a result
+- Operation requires a context
 
 ```cpp
 {{#include c++/meta2.cc:3:}}
diff --git a/src/development/c++/meta.cc b/src/development/c++/meta.cc
index e674cfd..efb3b22 100644
--- a/src/development/c++/meta.cc
+++ b/src/development/c++/meta.cc
@@ -2,6 +2,8 @@
 
 #include <iostream>
 
+// -- Example 1 - print template value arguments.
+
 // Base case with one parameter.
 template<int P>
 void show_int() {
@@ -15,6 +17,8 @@ void show_int() {
     show_int<P1, Params...>();
 }
 
+// -- Example 2 - print values of different types.
+
 // Base case with one parameter.
 template<typename T>
 void show(const T& t) {
diff --git a/src/development/c++/meta3.cc b/src/development/c++/meta3.cc
new file mode 100644
index 0000000..5dce54a
--- /dev/null
+++ b/src/development/c++/meta3.cc
@@ -0,0 +1,22 @@
+// Copyright (C) 2022 johannst
+
+#include <type_traits>
+
+template<typename T, typename... U>
+struct any_of : std::false_type {};
+
+// Found our type T in the list of types U.
+template<typename T, typename... U>
+struct any_of<T, T, U...> : std::true_type {};
+
+// Pop off the first element in the list of types U,
+// since it didn't match our type T.
+template<typename T, typename U0, typename... U>
+struct any_of<T, U0, U...> : any_of<T, U...> {};
+
+// Convenience template variable to invoke meta function.
+template<typename T, typename... U>
+constexpr bool any_of_v = any_of<T, U...>::value;
+
+static_assert(any_of_v<int, char, bool, int>, "");
+static_assert(!any_of_v<int, char, bool, float>, "");
-- 
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