Discussion

Constructors generate initialization code under the covers. Consider:

class A {


  string s1_, s2_;





public:


  A() {s1_ = "Hello, "; s2_ = "world"; }


};

In reality, constructor's code is generated as if you wrote:

A() : s1_(), s2_() {s1 = "Hello, "; s2 = "world"; }

That is, the objects not explicitly initialized by you are automatically initialized using their default constructors, and then assigned to using their assignment operators. Most often, the assignment operator of a nontrivial object needs to do slightly more than a constructor because it needs to deal with an already-constructed object.

Say what you mean: Initialize member variables in the initializer list, with code that better expresses intent and in addition, as a perk, is usually smaller and faster:

A() : s1_("Hello, "), s2_("world") {}

This isn't premature optimization; it's avoiding premature pessimization. (See Item 9)