Fix issue where compiler cannot determine address of ::free at compile time

[folly.git] / folly / Memory.h

diff --git a/folly/Memory.h b/folly/Memory.h
index f5e92e7588ab6c9e50d5781f8f14d404b2054bf4..272d64dc68da5b6e997f6acb49e92be5d0d0a4b2 100644 (file)
--- a/folly/Memory.h
+++ b/folly/Memory.h
@@ -1,5 +1,5 @@
 /*
- * Copyright 2016 Facebook, Inc.
+ * Copyright 2017 Facebook, Inc.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -36,9 +36,8 @@ namespace folly {
  * @author Xu Ning (xning@fb.com)
  */
 
-#if __cplusplus >= 201402L ||                                              \
-    (defined __cpp_lib_make_unique && __cpp_lib_make_unique >= 201304L) || \
-    (defined(_MSC_VER) && _MSC_VER >= 1900)
+#if __cplusplus >= 201402L || __cpp_lib_make_unique >= 201304L || \
+    (__ANDROID__ && __cplusplus >= 201300L) || _MSC_VER >= 1900
 
 /* using override */ using std::make_unique;
 
@@ -138,7 +137,18 @@ std::weak_ptr<T> to_weak_ptr(const std::shared_ptr<T>& ptr) {
   return std::weak_ptr<T>(ptr);
 }
 
-using SysBufferDeleter = static_function_deleter<void, ::free>;
+namespace detail {
+/**
+ * Not all STL implementations define ::free in a way that its address can be
+ * determined at compile time. So we must wrap ::free in a function whose
+ * address can be determined.
+ */
+inline void SysFree(void* p) {
+  ::free(p);
+}
+}
+
+using SysBufferDeleter = static_function_deleter<void, &detail::SysFree>;
 using SysBufferUniquePtr = std::unique_ptr<void, SysBufferDeleter>;
 inline SysBufferUniquePtr allocate_sys_buffer(size_t size) {
   return SysBufferUniquePtr(::malloc(size));
@@ -447,4 +457,93 @@ std::shared_ptr<T> allocate_shared(Allocator&& allocator, Args&&... args) {
  */
 template <class T> struct IsArenaAllocator : std::false_type { };
 
+/*
+ * folly::enable_shared_from_this
+ *
+ * To be removed once C++17 becomes a minimum requirement for folly.
+ */
+#if __cplusplus >= 201700L || \
+    __cpp_lib_enable_shared_from_this >= 201603L
+
+// Guaranteed to have std::enable_shared_from_this::weak_from_this(). Prefer
+// type alias over our own class.
+/* using override */ using std::enable_shared_from_this;
+
+#else
+
+/**
+ * Extends std::enabled_shared_from_this. Offers weak_from_this() to pre-C++17
+ * code. Use as drop-in replacement for std::enable_shared_from_this.
+ *
+ * C++14 has no direct means of creating a std::weak_ptr, one must always
+ * create a (temporary) std::shared_ptr first. C++17 adds weak_from_this() to
+ * std::enable_shared_from_this to avoid that overhead. Alas code that must
+ * compile under different language versions cannot call
+ * std::enable_shared_from_this::weak_from_this() directly. Hence this class.
+ *
+ * @example
+ *   class MyClass : public folly::enable_shared_from_this<MyClass> {};
+ *
+ *   int main() {
+ *     std::shared_ptr<MyClass> sp = std::make_shared<MyClass>();
+ *     std::weak_ptr<MyClass> wp = sp->weak_from_this();
+ *   }
+ */
+template <typename T>
+class enable_shared_from_this : public std::enable_shared_from_this<T> {
+public:
+  constexpr enable_shared_from_this() noexcept = default;
+
+  std::weak_ptr<T> weak_from_this() noexcept {
+    return weak_from_this_<T>(this);
+  }
+
+  std::weak_ptr<T const> weak_from_this() const noexcept {
+    return weak_from_this_<T>(this);
+  }
+
+private:
+  // Uses SFINAE to detect and call
+  // std::enable_shared_from_this<T>::weak_from_this() if available. Falls
+  // back to std::enable_shared_from_this<T>::shared_from_this() otherwise.
+  template <typename U>
+  auto weak_from_this_(std::enable_shared_from_this<U>* base_ptr)
+  noexcept -> decltype(base_ptr->weak_from_this()) {
+    return base_ptr->weak_from_this();
+  }
+
+  template <typename U>
+  auto weak_from_this_(std::enable_shared_from_this<U> const* base_ptr)
+  const noexcept -> decltype(base_ptr->weak_from_this()) {
+    return base_ptr->weak_from_this();
+  }
+
+  template <typename U>
+  std::weak_ptr<U> weak_from_this_(...) noexcept {
+    try {
+      return this->shared_from_this();
+    } catch (std::bad_weak_ptr const&) {
+      // C++17 requires that weak_from_this() on an object not owned by a
+      // shared_ptr returns an empty weak_ptr. Sadly, in C++14,
+      // shared_from_this() on such an object is undefined behavior, and there
+      // is nothing we can do to detect and handle the situation in a portable
+      // manner. But in case a compiler is nice enough to implement C++17
+      // semantics of shared_from_this() and throws a bad_weak_ptr, we catch it
+      // and return an empty weak_ptr.
+      return std::weak_ptr<U>{};
+    }
+  }
+
+  template <typename U>
+  std::weak_ptr<U const> weak_from_this_(...) const noexcept {
+    try {
+      return this->shared_from_this();
+    } catch (std::bad_weak_ptr const&) {
+      return std::weak_ptr<U const>{};
+    }
+  }
+};
+
+#endif
+
 }  // namespace folly