A POD (plain old data) type is a union, struct or class with no user-declared constructors, no private or protected non-static data members, no base classes, and no virtual functions. To default-initialize an object of type T means the following:if T is a non-POD class type, the default constructor for T is called. If T is an array type, each element is default-initialized. Otherwise, the storage for the object is zero-initialized.

An object whose initializer is an empty set of parentheses, i.e., (), shall be default-initialized. For example:

   struct Date // POD type  {    int day;    int month;    int year;  };  Date d = Date();  // default initialize d

Because d is of a POD type, the explicit default initialization of d initializes all of its members to 0. However, the default initialization rule was added recently to C++, so at present, very few compilers implement it. Therefore, you should use an explicit initialization list to ensure that d is zero-initialized:

   Date d = {0}; // guarantee zero initialization