Reinterpret_cast use in C++

Question

Just a simple question,having this:

fftw_complex *H_cast;
H_cast = (fftw_complex*) fftw_malloc(sizeof(fftw_complex)*M*N);

what is the difference between:

H_cast=  reinterpret_cast<fftw_complex*> (H); 

and

H_cast= reinterpret_cast<fftw_complex*> (&H); 

Thanks so much in advance

Antonio

Answer 1

Answer to current question

The difference is that they do two completely different things!

Note: you do not tell us what H is, so it's impossible to answer the question with confidence. But general principles apply.

For the first case to be sensible code, H should be a pointer (typed as void* possibly?) to a fftw_complex instance. You would do this to tell the compiler that H is really a fftw_complex* , so you can then use it.

For the second case to be sensible code, H should be an instance of a class with a memory layout identical to that of class fftw_complex . I can't think of a compelling reason to put yourself in this situation, it is very unnatural. Based on this, and since you don't give us information regarding H , I think it's almost certainly a bug.

Original answer

The main difference is that in the second case you can search your source code for reinterpret_cast (and hopefully ensure that every use is clearly documented and a necessary evil).

However, if you are casting from void* to another pointer type (is this the case here?) then it's preferable to use static_cast instead (which can also be easily searched for).

Answer 2

H_cast=  reinterpret_cast<fftw_complex*> (H); 

This converts the pointer-ish type inside H (or the integer itself, if H is an integer type) and tells the compiler "this is a pointer. Stop thinking whatever it was, it's a pointer now". H is used as something where you had stored a pointer-like address.

H_cast= reinterpret_cast<fftw_complex*> (&H); 

This converts the address of H (which is a pointer to whatever type H is) into a pointer to "fftw_complex". Modifying the contents of H_cast will now change H itself.

You'll want the second if H is not a pointer and usually the first if it is. There are use cases for the other way around but they're uncommon and ugly (especially reinterpreting an int or - god forbid - a double as a pointer).

Answer 3

Pointer casts are always executed as a reinterpret_cast, so when casting from or to a void * there's no difference between a c-style cast, a static_cast or a reinterpret_cast.

Reinterpret_casts are usually reserved for the ugliest of locations where c-style casts and static_casts are used for innocuous casts. You basically use reinterpret_cast to tag some code as really-ugly:

float f = 3.1415f;
int x = *reinterpret_cast<int *>(&f);

That way, these ugly unsafe casts are searchable/greppable.