Question about Null vs zero

I have this function:

void ToUpper(char * S)
{
    while (*S!=0)
    {
       *S=(*S >= 'a' && *S <= 'z')?(*S-'a'+'A'):*S;
       S开发者_如何学运维++;
    }
} 

What does it mean for *S != 0, should it be null instead?

That is checking for the end of the string which is a character which has the value of zero. It is not connected to NULL pointers in any way.

I would write it *S != '\0' as I feel that is more idiomatic, but that is really just personal style preference. You are checking for the null character (ASCII NUL).

You might also consider checking S != 0 before any of that code as the pointer itself may be null, and you don't want to dereference a null pointer.

NULL is defined differently in C and C++

in C

#define NULL 0

in C++

#define NULL (void*) 0

I like algorithms better than loops:

#include <algorithm>
#include <cstring>
#include <cctype>

void ToUpper(char* p)
{
    std::transform(p, p + strlen(p), p, toupper);
}

This solution also works for character encodings where a to z aren't sequential.

Just for fun, here is an experiment that only does one iteration with algorithms:

#include <algorithm>
#include <cassert>
#include <cstring>
#include <cctype>
#include <iostream>
#include <iterator>

struct cstring_iterator : std::iterator<std::random_access_iterator_tag, char>
{
    char* p;

    cstring_iterator(char* p = 0) : p(p) {}

    char& operator*()
    {
        return *p;
    }

    cstring_iterator& operator++()
    {
        ++p;
        return *this;
    }

    bool operator!=(cstring_iterator that) const
    {
        assert(p);
        assert(!that.p);
        return *p != '\0';
    }
};

void ToUpper(char* p)
{
    std::transform(cstring_iterator(p), cstring_iterator(),
                   cstring_iterator(p), toupper);
}

int main()
{
    char test[] = "aloha";
    ToUpper(test);
    std::cout << test << std::endl;
}

NULL is a pointer while *S is the value stored at the pointer. Thankes to Dennis Ritchie, the digit 0 is acceotable as both.