Undefined behavior when adding a const with const_cast?

Question

This question is regarding the behavior I observed while using const_cast for making a char * const char *. I am aware that this casting is done implicitly and t working for me when the cast is being done implicitly.

The problematic code is:

#include <cstdlib>
int main() {
    const char * org_str = NULL;
    org_str = const_cast<const char*>(getenv("ENV_VAR")); // PROBLEM !!
}

As per the Linux man page getenv() takes const char * and returns char*. So, as per my understanding of const-correctness, I can do a const cast on char* without any issues.

So, my question is, why const_cast here giving me a UB (code is crashing) but as expected without const_cast(implicit casting) its working fine(So the problem has to be with the use of const_cast) ?

Please note, I know implicit cast is the way to go here, through this post I require the answer specifically for the behavior observed here.

EDIT:

Since the bug is non reproducible by fellow So'ers, I am assuming this as some weird runtime/compiler issue. But, do let me know if there is any mention of problems such as this in the standard.

For the time being I am accepting Mike's answer.

Answer 1

You are casting the function pointer, not the pointer returned by the function. Call the function first with (), then cast the result.

EDIT: I can't reproduce the problem. Here's the code I used:

#include <cstdlib>
#include <iostream>

using namespace std;
int main() {
    const char * org_str = NULL;
    org_str = const_cast<const char*>(getenv("PATH"));
    cout << "Got: " << org_str << endl;
}

Here's what I got:

$ g++ foo.cc -o foo.app
$ ./foo.app
Got: /usr/bin:/bin:/usr/sbin:/sbin:/usr/local/bin:/usr/X11/bin:/usr/X11R6/bin
$

BTW, the assignment to NULL is unnecessary; recommended practice is to use one of:

const char *org_str = const_cast<const char*>(getenv("PATH"));

const char *org_str(const_cast<const char*>(getenv("PATH")));

const char *org_str(getenv("PATH"));

Answer 2

You don't need a const_cast<> to make something const, you only need it to take away the const-ness.

Also I don't believe the code you have there is correct at all, since getenv is a function and it looks like you're using it as a variable. Perhaps something like this would work:

const char * org_str = getenv("name-of-env");

Answer 3

It's, as far as I understand, not the return value of getenv you should cast, but the const char you have. As org_str is constant, you can't assign to it without using const_cast, which means you would need to do something like:

#include <cstdlib>
int main() {
    const char * org_str = NULL;
    const_cast<char*>(org_str) = getenv("ENV_VAR"); // NO PROBLEM !!
}

EDIT: As for having const_cast on the getenv, it makes no sense, as you don't assign to that and therefor will not have any violations of the const expression, as

org_str = getenv("ENV_VAR") will give you.