saved_handler is a public interface:

This container allows completion handlers to be safely
suspended and resumed later, or destroyed.

include/boost/beast/core/impl/saved_handler.hpp

@@ -0,0 +1,193 @@
+//
+// Copyright (c) 2016-2017 Vinnie Falco (vinnie dot falco at gmail dot com)
+//
+// Distributed under the Boost Software License, Version 1.0. (See accompanying
+// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+//
+// Official repository: https://github.com/boostorg/beast
+//
+
+#ifndef BOOST_BEAST_CORE_IMPL_SAVED_HANDLER_HPP
+#define BOOST_BEAST_CORE_IMPL_SAVED_HANDLER_HPP
+
+#include <boost/beast/core/detail/allocator.hpp>
+#include <boost/asio/associated_allocator.hpp>
+#include <boost/asio/associated_executor.hpp>
+#include <boost/asio/executor_work_guard.hpp>
+#include <boost/assert.hpp>
+#include <boost/core/empty_value.hpp>
+#include <boost/core/exchange.hpp>
+#include <memory>
+#include <utility>
+
+namespace boost {
+namespace beast {
+
+//------------------------------------------------------------------------------
+
+class saved_handler::base
+{
+protected:
+    ~base() = default;
+
+public:
+    base() = default;
+    virtual void destroy() = 0;
+    virtual void invoke() = 0;
+};
+
+//------------------------------------------------------------------------------
+
+template<class Handler, class Alloc>
+class saved_handler::impl final : public base
+{
+    using alloc_type = typename
+        beast::detail::allocator_traits<
+            Alloc>::template rebind_alloc<impl>;
+
+    using alloc_traits =
+        beast::detail::allocator_traits<alloc_type>;
+
+    struct ebo_pair : boost::empty_value<alloc_type>
+    {
+        Handler h;
+
+        template<class Handler_>
+        ebo_pair(
+            alloc_type const& a,
+            Handler_&& h_)
+            : boost::empty_value<alloc_type>(
+                boost::empty_init_t{}, a)
+            , h(std::forward<Handler_>(h_))
+        {
+        }
+    };
+
+    ebo_pair v_;
+    net::executor_work_guard<
+        net::associated_executor_t<Handler>> wg_;
+
+public:
+    template<class Handler_>
+    impl(alloc_type const& a, Handler_&& h)
+        : v_(a, std::forward<Handler_>(h))
+        , wg_(net::get_associated_executor(v_.h))
+    {
+    }
+
+    void
+    destroy() override
+    {
+        auto v = std::move(v_);
+        alloc_traits::destroy(v.get(), this);
+        alloc_traits::deallocate(v.get(), this, 1);
+    }
+
+    void
+    invoke() override
+    {
+        auto v = std::move(v_);
+        alloc_traits::destroy(v.get(), this);
+        alloc_traits::deallocate(v.get(), this, 1);
+        v.h();
+    }
+};
+
+//------------------------------------------------------------------------------
+
+saved_handler::
+~saved_handler()
+{
+    if(p_)
+        p_->destroy();
+}
+
+saved_handler::
+saved_handler(saved_handler&& other) noexcept
+    : p_(boost::exchange(other.p_, nullptr))
+{
+}
+
+saved_handler&
+saved_handler::
+operator=(saved_handler&& other) noexcept
+{
+    // Can't delete a handler before invoking
+    BOOST_ASSERT(! has_value());
+    p_ = boost::exchange(other.p_, nullptr);
+    return *this;
+}
+
+template<class Handler, class Allocator>
+void
+saved_handler::
+emplace(Handler&& handler, Allocator const& alloc)
+{
+    // Can't delete a handler before invoking
+    BOOST_ASSERT(! has_value());
+    using alloc_type = typename
+        beast::detail::allocator_traits<Allocator>::
+            template rebind_alloc<impl<Handler, Allocator>>;
+    using alloc_traits =
+        beast::detail::allocator_traits<alloc_type>;
+    struct storage
+    {
+        alloc_type a;
+        impl<Handler, Allocator>* p;
+
+        explicit
+        storage(Allocator const& a_)
+            : a(a_)
+            , p(alloc_traits::allocate(a, 1))
+        {
+        }
+
+        ~storage()
+        {
+            if(p)
+                alloc_traits::deallocate(a, p, 1);
+        }
+    };
+    storage s(alloc);
+    alloc_traits::construct(s.a, s.p,
+        s.a, std::forward<Handler>(handler));
+    p_ = boost::exchange(s.p, nullptr);
+}
+
+template<class Handler>
+void
+saved_handler::
+emplace(Handler&& handler)
+{
+    // Can't delete a handler before invoking
+    BOOST_ASSERT(! has_value());
+    emplace(
+        std::forward<Handler>(handler),
+        net::get_associated_allocator(handler));
+}
+
+void
+saved_handler::
+invoke()
+{
+    // Can't invoke without a value
+    BOOST_ASSERT(has_value());
+    boost::exchange(
+        p_, nullptr)->invoke();
+}
+
+bool
+saved_handler::
+maybe_invoke()
+{
+    if(! p_)
+        return false;
+    boost::exchange(
+        p_, nullptr)->invoke();
+    return true;
+}
+
+} // beast
+} // boost
+
+#endif