Safely cast to unsigned char *

Question

If I need to treat something as an unsigned char array, is this the correct way for a function:

int main() {
    int i = 500;
    f(&i);
}

void f(void *ptr) {
    unsigned char *byteptr = static_cast<unsigned char *>(ptr);
    ...
}

What if I don't have a function? Should I use two static_cast, first to void * and then to unsigned char *?

What are you actually trying to do?

Lightness Races in Orbit
– Lightness Races in Orbit

2013-11-28 17:08:01 +00:00
Commented Nov 28, 2013 at 17:08 — Lightness Races in Orbit
– Lightness Races in Orbit, Commented Nov 28, 2013 at 17:08

Tim Pierce · Accepted Answer · 2013-11-28 17:13:17Z

2

To cast a pointer to your int variable to an unrelated type like unsigned char, you will need to use reinterpret_cast:

unsigned char *byteptr = reinterpret_cast<unsigned char *>(&i);

But I echo the commenter's question: what are you actually trying to do? It seems unlikely that anything useful can come of casting int * to unsigned char * like this.

answered Nov 28, 2013 at 17:13

Tim Pierce

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6 Comments

Lightness Races in Orbit Over a year ago

That shall be my new name. "The Commenter". :D

n. m. could be an AI Over a year ago

C++ objects are made up of bytes, which can be accessed via character types. You can copy those bytes around, and sometimes even copy them back to the original object. This is explicitly permitted by the standard.

Lightness Races in Orbit Over a year ago

@n.m.: Explicitly? Chapter and verse, please.

n. m. could be an AI Over a year ago

@LightnessRacesinOrbit: 3.9.2

Konrad Rudolph Over a year ago

@potrzebie No it isn’t. A reinterpret_cast between (unsigned) char* and the original object type is explicitly allowed by the standard. By contrast, I’m actually not sure if your original code is – as far as I remember you may only cast between void* and the original type, your code thus violates strict aliasing rules.

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