“C is how the computer works” is a dangerous mindset for C programmers

Re:“C is how the computer works” is a dangerous mindset for ... by saagarjha on Tuesday March 31, 2020 @04:04PM (#22741878)

Rust has both static (based on monomorphization, similar to a template instantiation in C++) and dynamic (based on vtables, similar to inheritance in C++) dispatch: https://doc.rust-lang.org/1.8.0/book/trait-objects.html. Both Rust and C++ are moving in the direction of encouraging static dispatch.

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Re:“C is how the computer works” is a dangerous mindset for ... by steveklabnik on Tuesday March 31, 2020 @04:48PM (#22742393)

It may be similar, but it is implemented very differently, in exactly the "so can always work on unmodified structures" sense.

Trait objects are a double pointer: one to the vtable, and one to the data. C++'s dynamic dispatch uses a single pointer to a structure that has the vtable and the data. The memory layouts are very different.

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Re:“C is how the computer works” is a dangerous mindset for ... by jeffdavis on Tuesday March 31, 2020 @05:53PM (#22743077)

"The memory layouts are very different."

Clarification: the memory layouts between C++ dynamic dispatch and rust dynamic dispatch are very different.

Rust imposes no requirements on the structure layout, whereas C++ adds some magic data. The magic data means it's awkward to make C++ do dynamic dispatch on a C struct pointer; but rust can do dynamic dispatch on a C struct pointer with no problem.

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