You did, except that it’s not one pointer, it’s two. This is one way that rust is different than C++; the vtable isn’t stored with the data, but separately, with the “trait object” itself being two pointers to the two things.
Which “data” is the vtable stored with in C++? An object contains only a pointer to its vtable, not the vtable itself...
struct ClassVTable {
void (*firstMethod)();
void (*secondMethod)();
}
struct Class {
struct ClassVTable *virt;
int firstMember;
int secondMember;
};
struct Class *objectRef;
struct Class {
int firstMember;
int secondMember;
};
struct FatPointerToClass {
struct ClassVTable *virt;
struct Class *data;
}
FatPointerToClass objectRef; // note lack of pointer, it'd be stored on the stack directly for eg.
(please note that I'm using C struct to be precise about the concept, and this should not be taken as a perfectly verbatim description of the actual object layouts in memory)
In other words, in Rust, a pointer to a shape is
(pointer to vtable, pointer to data)
(pointer to shape)
(pointer to vtable, data)
I get it now, that you meant the “pointer to the vtable” under “vtable” in your original post.