Buggy code in “A Tour of C++” or non-compliant compiler?

Question

I saw the following function in \"A Tour of C++\", page 12:

int count_x(char const* p, char x)
{
   int count = 0;
   while (p)
   {
      if (*p == x) ++cou         


        

Answer 1

This is listed in the Errata for 2nd printing of A Tour of C++:

Chapter 1:

pp 11-12: The code for count_if() is wrong (doesn't do what it claims to), but the points made about the language are correct.

The code in the 2nd Printing now reads:

int count_x(char* p, char x)
    // count the number of occurences of x in p[]
    // p is assumed to point to a zero-terminated array of char (or to nothing)
{
    if (p==nullptr) return 0;
    int count = 0;
    while(*p) {
        if(*p==x)
            ++count;
        ++p;
    }
    return count;
}

There is a similar example in The C++ Programming Language (4th Edition) on which A Tour of C++ was based but it does not have this bug.

Answer 2

gcc has good compatibility at 4.7.3, but you have to compile with -std=c++11. There are charts on the gnu webpage. But yeah, that's not a standard thing, that pointer is just never going to be NULL, at least not until it overflows, so unless you've allocated all the memory above the char *, it's going to segfault. It's just a bug.