reinterpret_cast cast cost

Yes, this is true. Casting type which has runtime cost is dynamic_cast.

C style casts in C++ will attempt a static_cast first and only perform a reinterpret_cast if a static cast cannot be performed. A static_cast may change the value of the pointer in the case of multiple inheritance (or when casting an interface to a concrete type), this offset calculation may involve an extra machine instruction. This will at most be 1 machine instruction so really very small.

It's a good assumption to start with. However, the optimizer may be restricted in what it can assume in the presence of a reinterpret_cast<> or C pointer cast. Then, even though the cast itself has no associated instructions, the resulting code is slower.

For instance, if you cast an int to a pointer, the optimizer likely will have no idea what that pointer could be pointing to. As a result, it probably has to assume that a write through that pointer can change any variable. That beats very common optimizations such as storing variables in registers.

That's right. No cost other than any gain/loss in performance for performing instructions at the new width, which I might add, is only a concern in rare cases. Casting between pointers on every platform I've ever heard of has zero cost, and no performance change whatsoever.