Clarifying performance in POD vs other in initializer lists.

Author: olivif

03-Style.md

@@ -223,7 +223,9 @@ It also makes it possible to have two separate files next to each other on one s
 ```
 
 ## Initialize Member Variables
-...with the member initializer list.
+...with the member initializer list. 
+
+For POD types, the performance of an initializer list is the same as manual initialization, but for other types there is a clear performance gain, see below.
 
 ```cpp
 // Bad Idea
@@ -239,11 +241,23 @@ private:
  int m_value;
 };
 
+// Bad Idea
+// This leads to an additional constructor call for m_myOtherClass
+// before the assignment.
+class MyClass
+{
+public:
+ MyClass(MyOtherClass t_myOtherClass)
+ {
+ m_myOtherClass = t_myOtherClass;
+ }
+
+private:
+ MyOtherClass m_myOtherClass;
+};
 
 // Good Idea
-// C++'s member initializer list is unique to the language and leads to
-// cleaner code and potential performance gains that other languages cannot
-// match.
+// There is no performance gain here but the code is cleaner.
 class MyClass
 {
 public:
@@ -255,6 +269,21 @@ public:
 private:
  int m_value;
 };
+
+// Good Idea
+// There is a performance gain here because the default constructor 
+// for m_myOtherClass is never called. 
+class MyClass
+{
+public:
+ MyClass(MyOtherClass t_myOtherClass)
+ : m_myOtherClass(t_myOtherClass)
+ {
+ }
+
+private:
+ MyOtherClass m_myOtherClass;
+};
 ```
 
 In C++11 you may consider always giving each member a default value, e.g. by writing