In C++, why struct is in fact class?

Question

The other topic and responses there made me ask this question:

Why does C++ allow struct to behave just like class? At one hand, C++ made

Answer 1

1. "public" default for struct fields is required for compatibility with C code which may access struct fields.
2. "public" default for struct inheritance is required so that the child class could be used in place of base struct.
3. It could be possible to make struct into a different data type than classes (ie disallow access type specifiers and methods in it), but that would be both inconvenient for programming and add a lot of unnecessary work for compiler developers.