Add base::MakeUnique to mirror C++14's std::make_unique.

base/memory/ptr_util.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef BASE_MEMORY_PTR_UTIL_H_
 #define BASE_MEMORY_PTR_UTIL_H_
 
 #include <memory>
+#include <utility>
 
 namespace base {
 
 // Helper to transfer ownership of a raw pointer to a std::unique_ptr<T>.
 // Note that std::unique_ptr<T> has very different semantics from
 // std::unique_ptr<T[]>: do not use this helper for array allocations.
 template <typename T>
 std::unique_ptr<T> WrapUnique(T* ptr) {
   return std::unique_ptr<T>(ptr);
 }
 
+namespace internal {
+
+template <typename T>
+struct MakeUniqueResult {
+  using Scalar = std::unique_ptr<T>;
+};
+
+template <typename T>
+struct MakeUniqueResult<T[]> {
+  using Array = std::unique_ptr<T[]>;
+};
+
+template <typename T, size_t N>
+struct MakeUniqueResult<T[N]> {
+  using Invalid = void;
+};
+
+}  // namespace internal
+
+// Helper to construct an object wrapped in a std::unique_ptr. This is an
+// implementation of C++14's std::make_unique that can be used in Chrome.
+//
+// MakeUnique<T>(args) should be preferred over WrapUnique(new T(args)): bare
+// calls to `new` should be treated with scrutiny.

[Ryan Sleevi, 2015/12/15 23:53:40]

Is this level of warning needed?

That is, it seems like it'd be a common pattern for factory-creation where
"CreateOrFail" is a thing, where the ctor is private but a unique_ptr<> is
returned.

class Foo {
 public:
  static std::unique_ptr<Foo> CreateFooWithA(...);
  static std::unique_ptr<Foo> CreateFooWithB(...);

 private:
  Foo();
  bool Init(...);
};

Or is this pattern proscribed, in favour of

class Foo {
 public:
  Foo(WithA, ...);
  Foo(WithB, ...);

  bool Init(...);
};

?

[dcheng, 2015/12/15 23:59:50]

It seems like you could still get the former pattern if you're willing to friend
MakeUnique... in some sense, it seems similar to how we force protected/private
destructors for refcounted types, which then have to friend RefCounted.

+//
+// Usage:
+//   // ptr is a std::unique_ptr<std::string>
+//   auto ptr = MakeUnique<std::string>("hello world!");
+//
+//   // arr is a std::unique_ptr<int[]>
+//   auto arr = MakeUnique<int[]>(5);
+
+// Overload for non-array types. Arguments are forwarded to T's constructor.
+template <typename T, typename... Args>
+typename internal::MakeUniqueResult<T>::Scalar MakeUnique(Args&&... args) {
+  return std::unique_ptr<T>(new T(std::forward<Args>(args)...));
+}
+
+// Overload for array types of unknown bound, e.g. T[]. The array is allocated
+// with `new T[n]()` and value-initialized: note that this is distinct from
+// `new T[n]`, which default-initializes.
+template <typename T>
+typename internal::MakeUniqueResult<T>::Array MakeUnique(size_t size) {
+  return std::unique_ptr<T>(new typename std::remove_extent<T>::type[size]());
+}
+
+// Overload to reject array types of known bound, e.g. T[n].
+template <typename T, typename... Args>
+typename internal::MakeUniqueResult<T>::Invalid MakeUnique(Args&&... args) =
+    delete;
+
 }  // namespace base
 
 #endif  // BASE_MEMORY_PTR_UTIL_H_