问题
My understanding is that C++ reinterpret_cast and C pointer cast is a just a compile-time functionality and that it has no performance cost at all.
Is this true?
回答1:
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.
回答2:
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.
回答3:
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.
回答4:
Yes, this is true. Casting type which has runtime cost is dynamic_cast.
回答5:
You're right, but think about it: reinterpret_cast means maybe a bad design or that you're doing something very low level.
dynamic-cast instead it will cost you something, because it has to look in a lookup table at runtime.
回答6:
reinterpret_cast does not incur runtime cost.. however you have to be careful, as every use of reinterpret_cast is implementation defined. For example, it is possible reinterpreting a char array as an int array could cause the target architecture to throw an interrupt, because different types may have different alignment rules.
Get correct first, then worry about efficiency.
来源：https://stackoverflow.com/questions/3575234/reinterpret-cast-cast-cost