How can char* be a condition in for loop?

Question

In a book I am reading there is a piece of code :

string x;
size_t h=0;
for(const char* s=x.c_str();*s;++s)
    h=(h*17)^*s;

Regarding this code, I have two questions:

1. how can *s be a condition? what does it mean?

2. what does "h=(h*17)^*s" mean?

Thanks for help!

Answer 1

1. how can *s be a condition? what does it mean?

It means "while the value pointed to by s is not zero." C strings are null-terminated, so the last character in the string returned by c_str() will be the null character (\0, represented by all bits zero).

2. what does "h=(h*17)^*s" mean?

It multiplies h by 17 then xors it with the value pointed to by s.

Answer 2

In C (or C++) any value can be used as a "boolean". A numeric value of 0, or a NULL pointer, means "false". Anything else means "true".

Here, *s is "the character value currently pointed to by s". The loop stops if that character is a 0 (not the "0" digit, with ASCII encoding 48, but the byte with ASCII encoding 0). This is conventionally the "end-of-string" marker, so the loop stops when it reaches the end of the string.

"^" is the bitwise XOR operator. The left "*" is a plain multiplication, while the other "*" is the pointer dereference operator (i.e. the thing which takes the pointer s and looks at the value to which this pointer points). "=" is assignment. In brief, the value of h is multiplied by 17, then XORed with the character pointed to by s, and the result becomes the new value of h.