From 652d6ef2a312eba17adfa25790c23425c96c6af3 Mon Sep 17 00:00:00 2001
From: evoskuil <eric@voskuil.org>
Date: Fri, 1 Mar 2024 20:18:09 -0500
Subject: [PATCH] Add enable_shared_from_sibling.

---
 .../network/async/enable_shared_from_base.hpp | 61 ++++++++++++++++++-
 .../impl/async/enable_shared_from_base.ipp    | 44 ++++++++++++-
 2 files changed, 101 insertions(+), 4 deletions(-)

diff --git a/include/bitcoin/network/async/enable_shared_from_base.hpp b/include/bitcoin/network/async/enable_shared_from_base.hpp
index 53b6ec120..d091a03d0 100644
--- a/include/bitcoin/network/async/enable_shared_from_base.hpp
+++ b/include/bitcoin/network/async/enable_shared_from_base.hpp
@@ -25,8 +25,18 @@
 namespace libbitcoin {
 namespace network {
 
-/// Thread safe, base class.
-/// Empty base optimization using CRTP.
+// "The constructors of std::shared_ptr detect the presence of an unambiguous
+// and accessible (i.e. public inheritance is mandatory)
+// enable_shared_from_this base and assign the newly created std::shared_ptr
+// to weak-this if not already owned by a live std::shared_ptr.
+// Constructing a std::shared_ptr for an object that is already managed by
+// another std::shared_ptr will not consult weak-this and is undefined behavior.
+// It is permitted to call shared_from_this only on a previously shared object,
+// i.e. on an object managed by std::shared_ptr<T>. Otherwise,
+// std::bad_weak_ptr is thrown(by the shared_ptr constructor).."
+// en.cppreference.com/w/cpp/memory/enable_shared_from_this
+
+/// Base instance must be downcastable to Derived.
 /// Because enable_shared_from_this does not support inheritance.
 template <class Base>
 class enable_shared_from_base
@@ -42,6 +52,53 @@ class enable_shared_from_base
     std::shared_ptr<Derived> shared_from_base() NOEXCEPT;
 };
 
+/// Sibling instance must be castable to Derived:Base.
+/// Because enable_shared_from_this/base do not support multiple inheritance.
+/// Avoids diamond inheritance ambiguity by requiring a primary inheritance
+/// linear path (sibling) from which classes in an independent path (Base)
+/// may obtain a shared pointer within their own path (Derived).
+template <class Base, class Sibling>
+class enable_shared_from_sibling
+{
+public:
+    DELETE_COPY_MOVE(enable_shared_from_sibling);
+
+    enable_shared_from_sibling() NOEXCEPT;
+
+    /// Must be a polymorphic type (to use dynamic_cast).
+    virtual ~enable_shared_from_sibling() NOEXCEPT;
+
+    /// Simplifies capture of the shared pointer for a nop handler.
+    void nop() volatile NOEXCEPT;
+
+protected:
+    /// Sibling (not Derived:Base) implements enable_shared_from...
+    /// Use in Derived to create shared instance of Derived from its Sibling.
+    /// Undefined behavior if cast from Sibling* to Derived* not well formed.
+    template <class Derived, bc::if_base_of<Base, Derived> = true>
+    std::shared_ptr<Derived> shared_from_sibling() NOEXCEPT;
+};
+
+// class foo__ : enabled_shared_from_base<foo__>
+// class foo_ : foo__
+// class foo: foo_
+// auto fooptr = foo.shared_from_this()
+// auto foo_ptr = foo.shared_from_base<foo_>()
+// auto foo__ptr = foo.shared_from_base<foo__>()
+//
+// class bar__ : enabled_shared_from_sibling<bar__, foo>
+// class bar_ : bar__
+// class bar : bar_
+// bar__/bar_/bar must be joined with foo.
+//
+// class foobar : public foo, public bar
+// auto barptr = foobar.shared_from_sibling<bar>()
+// auto bar_ptr = foobar.shared_from_sibling<bar_>()
+// auto bar__ptr = foobar.shared_from_sibling<bar__>()
+// auto fooptr = foobar.shared_from_base<foo>()
+// auto foo_ptr = foobar.shared_from_base<foo_>()
+// auto foo__ptr = foobar.shared_from_base<foo__>()
+
 } // namespace network
 } // namespace libbitcoin
 
diff --git a/include/bitcoin/network/impl/async/enable_shared_from_base.ipp b/include/bitcoin/network/impl/async/enable_shared_from_base.ipp
index 8ab481136..cae5dfc17 100644
--- a/include/bitcoin/network/impl/async/enable_shared_from_base.ipp
+++ b/include/bitcoin/network/impl/async/enable_shared_from_base.ipp
@@ -24,9 +24,13 @@
 
 namespace libbitcoin {
 namespace network {
+    
+// enable_shared_from_base : enable_shared_from_this
+// ----------------------------------------------------------------------------
 
 template <class Base>
-void enable_shared_from_base<Base>::nop() volatile NOEXCEPT
+void enable_shared_from_base<Base>::
+nop() volatile NOEXCEPT
 {
 }
 
@@ -35,8 +39,44 @@ template <class Derived, bc::if_base_of<Base, Derived>>
 std::shared_ptr<Derived> enable_shared_from_base<Base>::
 shared_from_base() NOEXCEPT
 {
-    // Instance must be downcastable to Derived.
+    BC_PUSH_WARNING(NO_THROW_IN_NOEXCEPT)
     return std::static_pointer_cast<Derived>(this->shared_from_this());
+    BC_POP_WARNING()
+}
+
+// enable_shared_from_sibling
+// ----------------------------------------------------------------------------
+
+template <class Base, class Sibling>
+enable_shared_from_sibling<Base, Sibling>::
+enable_shared_from_sibling() NOEXCEPT
+{
+}
+
+template <class Base, class Sibling>
+enable_shared_from_sibling<Base, Sibling>::
+~enable_shared_from_sibling() NOEXCEPT
+{
+}
+
+template <class Base, class Sibling>
+void enable_shared_from_sibling<Base, Sibling>::
+nop() volatile NOEXCEPT
+{
+}
+
+template <class Base, class Sibling>
+template <class Derived, bc::if_base_of<Base, Derived>>
+std::shared_ptr<Derived> enable_shared_from_sibling<Base, Sibling>::
+shared_from_sibling() NOEXCEPT
+{
+    BC_PUSH_WARNING(NO_THROW_IN_NOEXCEPT)
+    // Obtain shared pointer to object from Sibling class inheritance path.
+    const auto sibling = dynamic_cast<Sibling*>(this)->shared_from_this();
+    BC_POP_WARNING()
+
+    // Cast pointer back to Derived (from Base) class it was created from.
+    return std::dynamic_pointer_cast<Derived>(sibling);
 }
 
 } // namespace network