习题2.25应该还要补充变量的值

顺便调整了一下格式
>If no initializer is specified for an object, and the object is of (possibly cv-qualified) non-POD class type (or array thereof), the object shall be default-initialized; if the object is of const-qualified type, the underlying class type shall have a user-declared default constructor. **Otherwise, if no initializer is specified for a nonstatic object, the object and its subobjects, if any, have an indeterminate initial value);** if the object or any of its subobjects are of const-qualified type, the program is ill-formed.

--from C++ standard 2003.
C++11,14 and 1y should be almost the same as above.