C++ Primer, Fifth Edition

7.5.1. Constructor Initializer List

When we define variables, we typically initialize them immediately rather than defining them and then assigning to them:

string foo = "Hello World!"; // define and initializestring bar;                  // default initialized to the empty stringbar = "Hello World!";        // assign a new value to bar

Exactly the same distinction between initialization and assignment applies to the data members of objects. If we do not explicitly initialize a member in the constructor initializer list, that member is default initialized before the constructor body starts executing. For example:

// legal but sloppier way to ...