Add License header + elaborate on the placement new

1 files changed, 32 insertions, 2 deletions

diff --git a/framework/src/errors.h b/framework/src/errors.h
index ec5bed7b..d95272da 100644
--- a/framework/src/errors.h
+++ b/framework/src/errors.h
@@ -1,3 +1,21 @@
+/*
+    Copyright (c) 2018 Christian Mollekopf <mollekopf@kolabsys.com>
+
+    This library is free software; you can redistribute it and/or modify it
+    under the terms of the GNU Library General Public License as published by
+    the Free Software Foundation; either version 2 of the License, or (at your
+    option) any later version.
+
+    This library is distributed in the hope that it will be useful, but WITHOUT
+    ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+    FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Library General Public
+    License for more details.
+
+    You should have received a copy of the GNU Library General Public License
+    along with this library; see the file COPYING.LIB.  If not, write to the
+    Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
+    02110-1301, USA.
+*/
 #pragma once
 
 #include <memory>
@@ -92,6 +110,19 @@ protected:
     {
         // This is a constructor, you have to construct object, not assign them
         // (hence the placement new)
+        //
+        // Here's the problem:
+        //
+        // Object that are part of a union are not initialized (which is
+        // normal). If we replaced the placement new by a line like this:
+        //
+        // ```
+        // mValue = other.mValue;
+        // ```
+        //
+        // If overloaded, this will call `mValue.operator=(other.mValue);`, but
+        // since we're in the constructor, mValue is not initialized. This can
+        // cause big issues if the Type is not trivially (move) assignable.
         if (mIsValue) {
             new (std::addressof(mValue)) Type(other.mValue);
         } else {
@@ -101,8 +132,7 @@ protected:
 
     StorageBase(StorageBase &&other) : mIsValue(other.mIsValue)
     {
-        // This is a constructor, you have to construct object, not assign them
-        // (hence the placement new)
+        // If you're thinking WTF, see the comment in the copy constructor above.
         if (mIsValue) {
             new (std::addressof(mValue)) Type(std::move(other.mValue));
         } else {