From da5817816d4fd433ced56576d2cad98df5b238d2 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Steven=20Dee=20=28J=C5=8Dshin=29?= <git@wholezero.org>
Date: Wed, 19 Jun 2024 22:53:37 -0700
Subject: [PATCH] Basics of make_shared with anonymous union

---
 ctl/shared_ptr.h            | 52 +++++++++++++++++++++++++++++++++++++
 test/ctl/shared_ptr_test.cc | 11 ++++++++
 2 files changed, 63 insertions(+)

diff --git a/ctl/shared_ptr.h b/ctl/shared_ptr.h
index f902b7c65..7702a99b0 100644
--- a/ctl/shared_ptr.h
+++ b/ctl/shared_ptr.h
@@ -2,6 +2,7 @@
 // vi: set et ft=cpp ts=4 sts=4 sw=4 fenc=utf-8 :vi
 #ifndef COSMOPOLITAN_CTL_SHARED_PTR_H_
 #define COSMOPOLITAN_CTL_SHARED_PTR_H_
+#include "new.h"
 #include "unique_ptr.h"
 
 namespace ctl {
@@ -72,6 +73,39 @@ struct shared_pointer : shared_control
     }
 };
 
+template <typename T>
+struct shared_emplace : shared_control
+{
+    union {
+        T t;
+    };
+
+    template <typename... Args>
+    void construct(Args&&... args) {
+        ::new (&t) T(ctl::forward<Args>(args)...);
+    }
+
+    static unique_ptr<shared_emplace> make_unique()
+    {
+        return new shared_emplace();
+    }
+
+private:
+    explicit constexpr shared_emplace() noexcept
+    {
+    }
+
+    void on_zero_shared() noexcept override
+    {
+        t.~T();
+    }
+
+    void on_zero_weak() noexcept override
+    {
+        delete this;
+    }
+};
+
 } // namespace __
 
 struct bad_weak_ptr : ctl::exception
@@ -203,10 +237,28 @@ class shared_ptr
     // TODO(mrdomino): owner_before(weak_ptr const&)
 
   private:
+    constexpr shared_ptr(T* const p, __::shared_control* rc) noexcept
+      : p(p), rc(rc)
+    {
+    }
+
+    template <typename U, typename... Args>
+    friend shared_ptr<U> make_shared(Args&&... args);
+
     T* p;
     __::shared_control* rc;
 };
 
+template <typename T, typename... Args>
+shared_ptr<T> make_shared(Args&&... args)
+{
+    auto rc = __::shared_emplace<T>::make_unique();
+    rc->construct(ctl::forward<Args>(args)...);
+    auto r = shared_ptr<T>(&rc->t, rc.get());
+    rc.release();
+    return r;
+}
+
 // TODO(mrdomino): non-member functions (make_shared et al)
 // TODO(mrdomino): weak_ptr
 
diff --git a/test/ctl/shared_ptr_test.cc b/test/ctl/shared_ptr_test.cc
index 41cf41ffa..8fee9361f 100644
--- a/test/ctl/shared_ptr_test.cc
+++ b/test/ctl/shared_ptr_test.cc
@@ -24,6 +24,11 @@
 template<typename T>
 using Ptr = ctl::shared_ptr<T>;
 
+template <typename T, typename... Args>
+Ptr<T> Mk(Args&&... args) {
+    return ctl::make_shared<T>(ctl::forward<Args>(args)...);
+}
+
 #undef ctl
 
 int
@@ -62,6 +67,12 @@ main()
     }
 #endif
 
+    {
+        auto x = Mk<int>(5);
+        if (x.use_count() != 1)
+            return 8;
+    }
+
     // TODO(mrdomino): exercise more of API
     // TODO(mrdomino): threading stress-test