In C++03, a class or struct must follow a number of rules for it to be considered a plain old data (POD) type. Types that fit this definition produce object layouts that are compatible with C, and they could also be initialized statically. The C++03 standard has restrictions on what types are compatible with C or can be statically initialized despite there being no technical reason a compiler couldn't accept the program; if someone were to create a C++03 POD type and add a non-virtual member function, this type would no longer be a POD type, could not be statically initialized, and would be incompatible with C despite no change to the memory layout.
