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I am porting some C code to C++ right now. The C code is using multiple defines like:

#define IPADDRESS "fd9e:21a7:a92c:2323::1"

The problem that i have is that when i am calling C functions with the defines that are now in the C++ file i get:

warning: ISO C++ forbids converting a string constant to ‘char*’

I don't want to modify the C functions in this case and since I am still new to C++ and i was wondering how to handle this problem. I guess it isn't possible to tell C++ to handle these defines as a char* and not as a string constant so i was wondering if it is safe to cast the string constant to a char* in this case or if there is a function that i should use for this?

I appreciate your help!

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    The correct type for a string constant is const char *. The absence of const is the problem. Commented Jun 14, 2021 at 12:41
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    If you could change the defines themselves, I suppose you could write something like #define IPADDRESS (char*)"fd9e:21a7:a92c:2323::1" Commented Jun 14, 2021 at 12:42
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    You can throw away the constness by a const_cast<char *>(...) but if something tries to write to them, you may get a crash. Commented Jun 14, 2021 at 12:44

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The problem is that string literals "this is a string literal" are of type char[] in C but const char[] in C++. So if you have sloppily written C code which doesn't use const correctness of function parameters, that code will break when ported to C++. Because you can't pass a const char* to a function expecting char*.

And no, it is generally not safe to "cast away" const - doing so is undefined behavior in C and C++ both.

The solution is to fix the original C code. Not using const correctness is incorrect design, both in C and C++. If the C++ compiler supports compound literals, then one possible fix could also be:

#define IPADDRESS (char[]){"fd9e:21a7:a92c:2323::1"}
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6 Comments

Worth noting that compound literals are not standard C++.
Thank you very much @Lundin for this answer. I didn't know that the const was the problem here. The C code is using char * const ipAddress in the function calls for example. I think changing it to const char * const ipAddress could be the best option. I also appreciate that you added the other option with the compount literal. I didn't know that that this is possible in C++.
@mab0189 if you can change function argument types to char const* ..., then that's the best solution.
@mab0189 char * const ipAddress as a function parameter usually means that the person who wrote the function didn't have a clue. Or in some very rare cases, that the function expects a read-only storage pointer as input. Compound literals is a C feature, it may not be available in C++ other than as a non-standard extension.
@Lundin @AyxanHaqverdili I changed the functions to char const *const and i it works great. Thank you very much!
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