From 6d17d10604b57bd6e45ce5f234eb5a2719a18a1e Mon Sep 17 00:00:00 2001
From: LE GARREC Vincent <github@le-garrec.fr>
Date: Wed, 29 Jul 2020 18:37:01 +0200
Subject: [PATCH] T.5: Add an example for Type erasure (#1625)

Based on :
https://www.modernescpp.com/index.php/c-core-guidelines-type-erasure-with-templates

Co-authored-by: Vincent Legarrec <vincent.legarrec@csdental.com>
---
 CppCoreGuidelines.md | 41 +++++++++++++++++++++++++++++++++++++++++
 1 file changed, 41 insertions(+)

diff --git a/CppCoreGuidelines.md b/CppCoreGuidelines.md
index b9d08fd..391ec4d 100644
--- a/CppCoreGuidelines.md
+++ b/CppCoreGuidelines.md
@@ -16829,6 +16829,47 @@ Static helps dynamic: Use static polymorphism to implement dynamically polymorph
 Dynamic helps static: Offer a generic, comfortable, statically bound interface, but internally dispatch dynamically, so you offer a uniform object layout.
 Examples include type erasure as with `std::shared_ptr`'s deleter (but [don't overuse type erasure](#Rt-erasure)).
 
+    #include <memory>
+
+    class Object {
+    public:
+        template<typename T>
+        Object(T&& obj)
+            : concept_(std::make_shared<ConcreteCommand<T>>(std::forward<T>(obj))) {}
+
+        int get_id() const { return concept_->get_id(); }
+
+    private:
+        struct Command {
+            virtual ~Command() {}
+            virtual int get_id() const = 0;
+        };
+
+        template<typename T>
+        struct ConcreteCommand final : Command {
+            ConcreteCommand(T&& obj) noexcept : object_(std::forward<T>(obj)) {}
+            int get_id() const final { return object_.get_id(); }
+
+        private:
+            T object_;
+        };
+
+        std::shared_ptr<Command> concept_;
+    };
+
+    class Bar {
+    public:
+        int get_id() const { return 1; }
+    };
+
+    struct Foo {
+    public:
+        int get_id() const { return 2; }
+    };
+
+    Object o(Bar{});
+    Object o2(Foo{});
+
 ##### Note
 
 In a class template, non-virtual functions are only instantiated if they're used -- but virtual functions are instantiated every time.