POD（ Plain Old Data）概念：
Arithmetic types (3.9.1), enumeration types, pointer types, and pointer to member types (3.9.2), and cv-qualified（注2） versions of these types (3.9.3) are collectively called scalar types. Scalar types, POD-struct types,POD-union types (clause 9), arrays of such types and cv-qualified versions of these types (3.9.3) are collectively called POD types.
如果一个class没有non-trivial 的 constructor, destructor, 以及 copy assignmet operator这些东西的话，这个class事实上已经退化成一个POD类型了，就相当于C中的struct。
POD类型包括下面类型：
1、标量类型，c/c++的基本类型
- signed integer types (signed char, short, int, long),
- unsigned integer types (unsigned char,unsigned short, unsigned int,unsigned long),
- char and wchar_t, and
- bool.
- float, double, and long double
- pointer-to-void (void *),
- pointer-to-object and pointer-to-static-member-data (both of the form T* when pointing to an object of typeT), and
- pointer-to-function and pointer-to-static-member-function (both of the form T (*)(...) when pointing to a function that returns an object of typeT).
- pointer-to-nonstatic-member-data (of the form T C::* when pointing to one of classC's data members that has type T), and
- pointer-to-nonstatic-member-functions (of the form T (C::*)(...) when pointing to one of classC's member functions that returns an object of typeT).
2、用户自定义的类类型：
- non-static data (including arrays) of any pointer-to-member type,
- non-static data (including arrays) of any non-POD class type,
- non-static data of any reference type,
- user-defined copy assignment operator, nor
- user-defined destructor.
- user-declared constructors,
- private or protected non-static data members,
- base classes, nor
- virtual functions.