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>This is one way that rust is different than C++; the vtable isn’t stored with the data Which “data” is the vtable stored with in C++? An object contains only

>This is one way that rust is different than C++; the vtable isn’t stored with the data Which “data” is the vtable stored with in C++? An object contains only a pointer to its vtable, not the vtable itself...

steveklabnik 8y ago
My understanding is that, generally (because (again in my understanding) this is all compiler-specific, not mandated by the standard) is that in C++, there's a single pointer that points to the combo of the vtable and then the data, after it. See http://www.drdobbs.com/cpp/storage-layout-of-polymorphic-objects/240012098 http://www.drdobbs.com/cpp/storage-layout-of-polymorphic-obj... for example. While a Shape is a position, outline, and fill, if you have virtual functions, the pointer points to a vtable pointer, and then all of the data. In other words, in Rust, a pointer to a shape is (pointer to vtable, pointer to data) whereas, in C++, a pointer to a shape is (pointer to shape) where shape is (pointer to vtable, data) If that's incorrect, I'm quite happy to be corrected! This isn't an area I'm an expert in.
- tzahola 8y ago
  That’s absolutely correct. I get it now, that you meant the “pointer to the vtable” under “vtable” in your original post.
  - steveklabnik 8y ago
    Ah, I see. Yes, I should probably have been more clear there. Thanks for pointing that out.
stormbrew 8y ago
A C++ object with virtual methods and some members looks like this, if you put it in terms of C: struct ClassVTable { void (*firstMethod)(); void (*secondMethod)(); } struct Class { struct ClassVTable *virt; int firstMember; int secondMember; }; and a pointer to it looks like: struct Class *objectRef; A language that uses fat pointers, like rust, has this separated completely: 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. This means a much simpler object layout in exchange for passing around larger pointers, and it's a good fit for trait-based typing since it doesn't require you to know all possible interface subtypes of the object in order to describe or use its layout. (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)
- Coding_Cat 8y ago
  Oh, I didn't know that: this is also great for serializing code where you might have an array of `Class`, it would still be a POD-type to use C++ language.

steveklabnik 8y ago

My understanding is that, generally (because (again in my understanding) this is all compiler-specific, not mandated by the standard) is that in C++, there's a single pointer that points to the combo of the vtable and then the data, after it. See http://www.drdobbs.com/cpp/storage-layout-of-polymorphic-objects/240012098 http://www.drdobbs.com/cpp/storage-layout-of-polymorphic-obj... for example. While a Shape is a position, outline, and fill, if you have virtual functions, the pointer points to a vtable pointer, and then all of the data. In other words, in Rust, a pointer to a shape is (pointer to vtable, pointer to data) whereas, in C++, a pointer to a shape is (pointer to shape) where shape is (pointer to vtable, data) If that's incorrect, I'm quite happy to be corrected! This isn't an area I'm an expert in.

tzahola 8y ago
That’s absolutely correct. I get it now, that you meant the “pointer to the vtable” under “vtable” in your original post.
- steveklabnik 8y ago
  Ah, I see. Yes, I should probably have been more clear there. Thanks for pointing that out.

stormbrew 8y ago

A C++ object with virtual methods and some members looks like this, if you put it in terms of C: struct ClassVTable { void (*firstMethod)(); void (*secondMethod)(); } struct Class { struct ClassVTable *virt; int firstMember; int secondMember; }; and a pointer to it looks like: struct Class *objectRef; A language that uses fat pointers, like rust, has this separated completely: 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. This means a much simpler object layout in exchange for passing around larger pointers, and it's a good fit for trait-based typing since it doesn't require you to know all possible interface subtypes of the object in order to describe or use its layout. (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)

Coding_Cat 8y ago
Oh, I didn't know that: this is also great for serializing code where you might have an array of `Class`, it would still be a POD-type to use C++ language.