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

Allowing something you declare as struct to really be a class allows type-safety when creating a C interface.

You can forwardly declare your struct for your C interface:

struct Foo;

You can declare methods on it

void doStuffThatModifiesFoo( struct Foo * foo, ... );
struct Bar getStuffFromFoo( const struct Foo * foo );

You can also write create and destroy methods for it.

Underneath you implement Foo not as a C struct but as a class but your C clients do not need to know that. This is better than passing it around as a void * then casting (not safe if someone passes you a void* to a completely different type and you cast it).