Question

During compilation of a call to the following function:

char* process_array_of_strings(const char** strings);

GCC complains when a char** is passed as argument:

note: expected ‘const char **’ but argument is of type ‘char **’

While the function does not alter the characters (hence the const) it does duplicate the array of pointers in order to modify the character pointers themselves, so constant pointers are definitely undesirable here.

Compilation succeeds and the program appears to work. So how is the programmer supposed to handle this warning?

Answer 1

This is why char ** is not automatically converted to const char ** in C++, and why the C compiler issues a warning while allowing it.

/* This function returns a pointer to a string through its output parameter: */
void get_some_string(const char ** p) {
    /* I can do this because p is const char **, so the string won't be modified. */
    *p = "unchangeable string in program core";
}

void f() {
    char * str;
    /* First, I'll call this function to obtain a pointer to a string: */
    get_some_string(&str);
    /* Now, modify the string: */
    for (char * p = str; *p; p++)
        *p = toupper(*p);
    /* We have just overwritten a constant string in program core (or crashed). */
}

From your description of what process_array_of_strings() does, it could just as well take const char * const * because it modifies neither the pointers nor the characters (but duplicates the pointers elsewhere). In that case, the above scenario would not be possible, and compiler theoretically could allow you to convert char ** to const char * const * automatically without warnings, but that’s not how the language is defined.

So the answer is obviously that you need a cast (explicit). I’ve written up this expanation so that you can fully understand why the warning appears, which is important when you decide to silence one.