const_cast with two levels pointers

Question

I want to do this conversion using C++ format, it works on the C way. but it fails when I try on C++ format.

It works!

void req_password(const void *data, size_t datalen)
{
    char *password_old = ((char **) data)[0];
    char *password_new = ((char **) data)[1];
    ...
}

It fails

void req_password(const void *data, size_t datalen)
{
    char *password_old = (const_cast<char **>(data))[0];
    char *password_old = (const_cast<char **>(data))[1];
    ...
}

error:

error: invalid const_cast from type 'const void*' to type 'char**'

So my doubt is, how could I do this conversion using the C++ way?

PS: This code is part from a API, I can't control the the input of data.

This is the format of the input that I receive, I cant change the format of the input parameters. — Alex, Dec 09 '14 at 18:45
You need to first `const_cast` away the `const`ness, and then `reinterpret_cast` to `char **`. But this is some of the ugliest code I've seen on SO for a while. — Praetorian, Dec 09 '14 at 18:48
Looks like you have a terrible API to work with, so I suppose it makes sense that you go about following a terrible style guide as well. — Praetorian, Dec 09 '14 at 18:53
@Praetorian: You don't need to cast away any `const`ness at all. — Lightness Races in Orbit, Dec 09 '14 at 18:53
@LightnessRacesinOrbit Probably not (you already got +1 from me for pointing that out), I didn't put much effort into trying to figure out the intent before commenting. Shame on me :) — Praetorian, Dec 09 '14 at 18:56
The API is from Android low level to work with Radio Layer Interface. — Alex, Dec 09 '14 at 18:57

score 3 · Accepted Answer · edited Dec 09 '14 at 18:56

Don't.

If you are being given immutable data, then you are being given immutable data and that is the end of it!

First, here's what I suggest for maximum safety. Coercing data into its real type is a little tricky, alas:

void req_password(const void* data, size_t datalen)
{
    const char* password_old = (reinterpret_cast<const char* const*>(data)[0]);
    const char* password_new = (reinterpret_cast<const char* const*>(data)[1]);
    // ...
}

(I've actually added some constness in the above, as it seems to be the intent of having const void* in the first place.)

But, if you really want the strings to be mutable, then this is fine too:

void req_password(const void* data, size_t datalen)
{
    char* password_old = (reinterpret_cast<char* const*>(data)[0]);
    char* password_new = (reinterpret_cast<char* const*>(data)[1]);
    // ...
    // More obvious when you recall that `const void*` is actually `void const*`;
    // So:
    //   void  const*
    // becomes:
    //   char* const*
}

Notice how you don't even need const_cast here, because you're not modifying the thing that data points to: you are dereferencing it and getting its pointee.

Of course, ideally, data would point to a const std::string instance.

should it be `(reinterpret_cast(data))[0]`? – Alex Dec 10 '14 at 15:22 — Alex, Dec 10 '14 at 15:22
@Alex: Either is fine. – Lightness Races in Orbit Dec 10 '14 at 15:29 — Lightness Races in Orbit, Dec 10 '14 at 15:29

const_cast with two levels pointers

1 Answers1