Add field-initializing constructors to generated mojo structs.

mojo/public/cpp/bindings/struct_ptr.h

Index: mojo/public/cpp/bindings/struct_ptr.h
diff --git a/mojo/public/cpp/bindings/struct_ptr.h b/mojo/public/cpp/bindings/struct_ptr.h
index dbc3256ea782eed9d1950fee35552c77ceb347d4..238cacf9bd0338c72f967b14343c8b48cb089fd1 100644
--- a/mojo/public/cpp/bindings/struct_ptr.h
+++ b/mojo/public/cpp/bindings/struct_ptr.h
@@ -21,9 +21,9 @@ constexpr size_t kHashSeed = 31;
 template <typename Struct>
 class StructHelper {
  public:
-  template <typename Ptr>
-  static void Initialize(Ptr* ptr) {
-    ptr->Initialize();
+  template <typename Ptr, typename... Args>
+  static void Initialize(Ptr* ptr, Args&&... args) {
+    ptr->Initialize(std::forward<Args>(args)...);
   }
 };
 
@@ -109,9 +109,10 @@ class StructPtr {
   friend class internal::StructHelper<Struct>;
   friend class internal::StructPtrWTFHelper<Struct>;
 
-  void Initialize() {
+  template <typename... Args>
+  void Initialize(Args&&... args) {
     DCHECK(!ptr_);
-    ptr_ = new Struct();
+    ptr_ = new Struct(std::forward<Args>(args)...);
   }
 
   void Take(StructPtr* other) {
@@ -207,7 +208,12 @@ class InlinedStructPtr {
   friend class internal::StructHelper<Struct>;
   friend class internal::InlinedStructPtrWTFHelper<Struct>;
 
-  void Initialize() { state_ = VALID; }
+  template <typename... Args>
+  void Initialize(Args&&... args) {
+    state_ = VALID;
+    value_.~Struct();

[yzshen1, 2017/02/13 17:45:53]

It seems we always default construct value_, destruct it and re-construct it
(with/without) arguments. Could we optimize it?

[Sam McNally, 2017/02/14 02:32:24]

Done.

+    new (&value_) Struct(std::forward<Args>(args)...);
+  }
 
   void Take(InlinedStructPtr* other) {
     reset();