3 ms·

Doesn't a vtable imply an extra level of indirection? You have to find where the vtable is in the object, then the function within the vtable, right? Is that no

Doesn't a vtable imply an extra level of indirection? You have to find where the vtable is in the object, then the function within the vtable, right? Is that not slower?

mcguire 9y ago
Yes, although (cache behavior notwithstanding) it's a single pointer indirection that can frequently be optimized away by the compiler.
- khitchdee 9y ago
  My point is simply this -- adding protection mechanisms and inheritance to classes neccesitates adding more complexity to the structure used to represent them (such as a vtable) which does add performance overheads. If you dont need those features, you can go leaner and faster with a C structure that includes function pointers to give you the basic packaging of data and functions that an object has.
nly 9y ago
In common implementations the vtable pointer is always the first word inside the object. Given an arbitrary pointer to an object, the offset of this vtable pointer relative to what you're pointing to is always computed statically at compile time. Unless you're using multiple inheritance, this offset is usually zero because derived object pointers in a single-inheritance hierarchy actually always point to their base. If you're using multiple inheritance then an object can have multiple vtable pointers, but again which one you need to use is known at compile-time based on which class the virtual function you're calling is declared within, and the type of pointer you have. Once you have the vtable you then have to locate the function pointer for the function you're calling. Again, this is usually a compile-time constant offset from the start of the vtable. This ceases to be true when you have 'virtual inheritance' (not to be confused with virtual functions), when another indirection to find this function pointer is required. Here are some examples: https://godbolt.org/g/N2XcV7 https://godbolt.org/g/N2XcV7 You'll notice that the get_square() function, which returns a member function pointer to the virtual square function, doesn't even return any memory addresses, just metadata and an offset

Doesn't a vtable imply an extra level of indirection? You have to find where the vtable is in the object, then the function within the vtable, right? Is that not slower?

mcguire 9y ago

Yes, although (cache behavior notwithstanding) it's a single pointer indirection that can frequently be optimized away by the compiler.

khitchdee 9y ago
My point is simply this -- adding protection mechanisms and inheritance to classes neccesitates adding more complexity to the structure used to represent them (such as a vtable) which does add performance overheads. If you dont need those features, you can go leaner and faster with a C structure that includes function pointers to give you the basic packaging of data and functions that an object has.

nly 9y ago

In common implementations the vtable pointer is always the first word inside the object. Given an arbitrary pointer to an object, the offset of this vtable pointer relative to what you're pointing to is always computed statically at compile time. Unless you're using multiple inheritance, this offset is usually zero because derived object pointers in a single-inheritance hierarchy actually always point to their base. If you're using multiple inheritance then an object can have multiple vtable pointers, but again which one you need to use is known at compile-time based on which class the virtual function you're calling is declared within, and the type of pointer you have. Once you have the vtable you then have to locate the function pointer for the function you're calling. Again, this is usually a compile-time constant offset from the start of the vtable. This ceases to be true when you have 'virtual inheritance' (not to be confused with virtual functions), when another indirection to find this function pointer is required. Here are some examples: https://godbolt.org/g/N2XcV7 https://godbolt.org/g/N2XcV7 You'll notice that the get_square() function, which returns a member function pointer to the virtual square function, doesn't even return any memory addresses, just metadata and an offset