The Standard does not specify the memory layout of base class subobjects in a derived class. In practice, however, all C++ compilers use the same convention: The base class subobject appears first (in left-to-right order in case of multiple inheritance), and data members of the derived class follow. C code can access derived objects, as long as the derived class abides by the same restrictions that were specified in the Tip “Accessing a C++ Object in C Code.” For example, consider a non-polymorphic class that inherits from Date (the declaration of Date is repeated here for convenience) and has additional data members:
class Date{public: int day; int month; int year; Date(); //current date ~Date(); bool isLeap() const; bool operator == (const Date& other);};class DateTime: public Date{public: long time; //additional members char AM // AM or PM? DateTime(); ~DateTime(); long getTime() const;};The two additional data members of DateTime, namely ‘time’ and ‘AM’, are appended after the three members of the base class Date, so the memory layout of a DateTime object is equivalent to this C struct:
struct POD_DateTime{ int day; int month; int year; long time char AM;};As with class Date, the non-polymorphic member functions of DateTime have no effect on the size or memory layout of the object.
