Benefits of Initialization lists

Question

Of what I know of benefits of using initialization list is that they provide efficiency when initializing class members which are not build-in. For example,

Fred::Fred() : x_(whatever) { }

is preferable to,

Fred::Fred() { x_ = whatever; }

if x is an object of a custom class. Other than that, this style is used even with built-in types for the sake of consistency.

The most common benefit of doing this is improved performance. If the expression whatever is the same type as member variable x_, the result of the whatever expression is constructed directly inside x_ — the compiler does not make a separate copy of the object.

With the other style, the expression whatever causes a separate, temporary object to be created, and this temporary object is passed into the x_ object's assignment operator. Then that temporary object is destructed at the ;. That's inefficient.

Question
Is there any efficiency gain in the following example with using initialization list. I think there is no gain. The first version calls string's copy constructor and the other calls string's assignment operator (there isn't any temporary thats created). It that correct?

class MyClass { public:     MyClass(string n):name(n) { } private:     string name; };  class MyClass { public:     MyClass(string n)     {         name=n;     } private:     string name; }; 

Answer 1

The second version is calling string's default ctor and then string's copy-assignment operator -- there could definitely be (minor) efficiency losses compared to the first one, which directly calls c's copy-ctor (e.g., depending on string's implementation, there might be useless allocation-then-release of some tiny structure). Why not just always use the right way?-)

Answer 2

I think the only way to initialize const data members is in the initialization list

Eg. in the header:

class C {     C(); private:     const int x;     int y; } 

And the in the cpp file:

C::C() :     x( 10 ),     y( 10 ) {     x = 20; // fails     y = 20; }