Why is this error on using strtok function showing when I use a different version of GCC?

Question

I have this piece of C++ code:

const char *Delim = "some text";
char *token;
...
token = strtok('\0', Delim); // error here

The error is as follows:

invalid conversion from 'char' to 'char*' [-fpermissive]

I tested this in GCC 5.3 and it worked. But in GCC 8.1 it fails and gives me the error.

I understand I hard-code the char value. However, the Delim variable is also hard-coded. What would be the right replacement for it to work regardless of GCC versions? And why?

Note: I tested the following to get it to compile, but I want to understand it a bit more

const char *Delim = "some text";
char *str_0 = "\0"; // still receive warning that ISO C++ forbids converting a string constant to 'char*'
car *token;
...
token = strtok(str_0, Delim);

Interestingly, the single quote did not work either:

const char *Delim = "some text";
char *str_0 = '\0'; // fails
car *token;
...
token = strtok(str_0, Delim);

Answer 1

It's because the char expression '\0' is no longer a null pointer constant (and thus no longer convertible to a pointer), since C++11. Your newer compiler uses a new version of C++ by default.

You're supposed to pass an actual null pointer (i.e. nullptr, or NULL in the old days) to that argument, not '\0'. In the past your code only worked because of a weird kind of implicit conversion that eventually got removed. It's possible that this conversion masked a misunderstanding of what passing a null pointer there means (it means "continue the previous tokenisation session please", not "use '\0' as delimiter").

So:

token = strtok(nullptr, Delim);

I also recommend that you tell GCC exactly which version of C++ you're writing, with the -std flag.

char *str_0 = "\0"; // still receive warning that ISO C++ forbids converting a string constant to 'char*'

This has been advised against for a long time, and illegal for almost a decade. String literals should be stored in a const char*, not a char*. They're immutable (constant).

Regardless, this variant of the code does a different thing; you're now tokenising the string str_0, not continuing tokenising the original string.

Finally, I think you may have got the arguments backwards in your mind. Delim is supposed to be a list of delimiters, not an input string. It's hard to know for sure without a concrete example of your problem, but "some text" doesn't even look like a placeholder for a list of delimiters.

Please review some strtok documentation, or the section on it in your favourite book, to refresh your memory on how it's supposed to be used.