Should I use static_cast or reinterpret_cast when casting a void* to whatever

Question

Both static_cast and reinterpret_cast seem to work fine for casting void* to another pointer type. Is there a good reason to favor one over the other?

Answer 1

Use static_cast: it is the narrowest cast that exactly describes what conversion is made here.

There’s a misconception that using reinterpret_cast would be a better match because it means “completely ignore type safety and just cast from A to B”.

However, this doesn’t actually describe the effect of a reinterpret_cast. Rather, reinterpret_cast has a number of meanings, for all of which holds that “the mapping performed by reinterpret_cast is implementation-defined.” [5.2.10.3]

But in the particular case of casting from void* to T* the mapping is completely well-defined by the standard; namely, to assign a type to a typeless pointer without changing its address.

This is a reason to prefer static_cast.

Additionally, and arguably more important, is the fact that every use of reinterpret_cast is downright dangerous because it converts anything to anything else really (for pointers), while static_cast is much more restrictive, thus providing a better level of protection. This has already saved me from bugs where I accidentally tried to coerce one pointer type into another.

Answer 2

My personal preference is based on code literacy like this:

void* data = something;
MyClass* foo = reinterpret_cast<MyClass*>(data);
foo->bar();

or

typedef void* hMyClass; //typedef as a handle or reference
hMyClass = something;
const MyClass& foo = static_cast<MyClass&>(*hMyClass);
foo.bar();

They both do the same in the end, but static_cast seems more appropriate in a middle-ware, app enviroment, while reinterpret cast seems more like something you'd see in a lower-level library IMHO.

Answer 3

This is a tough question. On the one hand, Konrad makes an excellent point about the spec definition for reinterpret_cast, although in practice it probably does the same thing. On the other hand, if you're casting between pointer types (as is fairly common when indexing in memory via a char*, for example), static_cast will generate a compiler error and you'll be forced to use reinterpret_cast anyway.

In practice I use reinterpret_cast because it's more descriptive of the intent of the cast operation. You could certainly make a case for a different operator to designate pointer reinterprets only (which guaranteed the same address returned), but there isn't one in the standard.

Answer 4

I suggest using the weakest possible cast always.

reinterpret_cast may be used to cast a pointer to a float. The more structure-breaking the cast is, the more attention using it requires.

In case of char*, I'd use c-style cast, until we have some reinterpret_pointer_cast, because it's weaker and nothing else is sufficient.