如何在C ++中实现static_cast？

Question

How does static_cast work? If you are doing something like...

If D inherits from B via some unspecified hierarchy (not necessarily directly), and you do:

B* b = new D();
D* d = static_cast<D*>(b);

what is happening? Is it simply calculating an offset at compile time and applying that offset to the pointer? Or is there something happening at runtime in order to do the cast?

Answer 1

what is happening?

The compiler assumes that you know what you're doing, so that the pointer really does point to a D object, and changes the pointer type accordingly, adjusting the value if necessary to point to the complete D object rather than the B sub-object.

If you get it wrong, and use a D* pointer that doesn't really point to a D object, then you'll get undefined behaviour; so be careful.

Is it simply calculating an offset at compile time and applying that offset to the pointer?

Yes.

Or is there something happening at runtime in order to do the cast?

No; "static" implies that it uses only compile-time information. The only runtime activity is adding the fixed offset if necessary.

Use dynamic_cast if you want a runtime check that the conversion is valid (as long as the types are polymorphic). It will give a null pointer (or throw a bad_cast exception if you're casting a reference rather than a pointer) if there isn't really a D object there.