Casting a const void*

Question

I have a function that returns a const void* and I'd like to use its information as a char*. I can cast it C-style fine as (char *)variable but when I try to use reinterpret_cast like reinterpret_cast<char *>(variable), I get a compilation error.

Should I use a different casting method?

Given `const void*` points to `const` data, and `char*` doesn't, that cast would be a bad idea to begin with. — Baum mit Augen, Feb 14 '18 at 03:28
@jijemi: Because C is like the psycho ex you hope you never hear from again — Lightness Races in Orbit, Feb 14 '18 at 22:52

Sid S · Answer 1 · 2018-02-14T22:40:38.467

5

const void *p;
void *q = const_cast<void *>(p);
char *r = static_cast<char *>(q);

The first cast gets rid of const and yields a void *

The second cast changes the data type and yields a char *

Read about the different C++ casts here

edited Feb 14 '18 at 22:40

answered Feb 14 '18 at 04:54

Sid S

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score 1 · Accepted Answer · answered Feb 14 '18 at 03:57

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Either reinterpret_cast<const char*>(variable) or, if you really are absolutely sure you can ignore the const qualifier, const_cast<char*>(reinterpret_cast<const char*>(variable)).

answered Feb 14 '18 at 03:57

Davislor

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first way doesn't work, can't cast that way; compilation error. second way works. – jijemi Feb 14 '18 at 22:02
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What do you mean the first way doesn't work? It should. The second is ... bad. – Lightness Races in Orbit Feb 14 '18 at 22:53
Sometimes you really do want to point the gun at your foot and pull the trigger. – Davislor Feb 14 '18 at 23:26
The standard says that `static_cast` and `reinterpret_cast` from `void*` to another pointer should both work, and that they are equivalent, but perhaps your compiler is not compliant for some reason? – Davislor Feb 14 '18 at 23:31
While I don’t know the context of this code, I want to echo the advice that this kind of cast is a code smell. – Davislor Feb 14 '18 at 23:33

score 0 · Answer 3 · answered Feb 14 '18 at 04:01

0

Using reinterpret_cast<char*>() for simple types is not a good idea. Use const char *new_variable = static_cast<const char*>(variable).

Also it is a bad idea to remove const. If you absolutely sure that the returned memory is writable, you can use then const_cast<char*>(new_variable).

answered Feb 14 '18 at 04:01

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doesn't work this way either, can't cast that way. compilation error. – jijemi Feb 14 '18 at 22:03
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Is the compilation error a secret? This should work. Please share with the class. – Lightness Races in Orbit Feb 14 '18 at 22:53

Casting a const void*

3 Answers3

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