c++ brush up

#include <iostream>
using namespace std;

class Blah;

class String {
    int len;
    char *data;

    public:
        static int CStringLength( const char* c ) {
            int count = 0;
            while( *c != '\0' ) {
                ++c;
                ++count;
            }//while
            return count;
        }//CStringLength

        static char* CStringCopy( char *to, const char* from ) {
            //copies from to to.
            //from: '\0' terminated c string
            //to: will point to a new c string with same content as from

            if( to != NULL ) {
                delete [] to;
            }//if
            /*
            for( int i=0; i<len; ++i ) {
                *(data+i) = *(d+i);
            }//for
            *(data+len) = '\0';
            */
            return NULL;
        }//CStringCopy

        String() {
            len = 0;
            data = NULL;
        }//default constructor

        String( const String& cp ) : len(cp.len) {
            data = new char[ len+1 ];
        }//copy constructor



        String( const char *d ) {
            len = String::CStringLength( d );
            data = new char[ len+1 ];
            for( int i=0; i<len; ++i ) {
                *(data+i) = *(d+i);
            }//for
            *(data+len) = '\0';
            /*
            while ( d != NULL ) {
                *(data+len) = *(d+len);
                ++len;
            }//while
            */
        }//String

        int length() const {
            return len;
        }//length

        char* begin() {
            return data;
        }//begin

        friend ostream& operator<<(ostream& os, String& s) {
            for( int i=0; i<s.length(); ++i ) {
                os<<*(s.begin()+i);
            }//for
            os<<" ("<<s.length()<<")";
            return os;
        }//<<

};//String

#define HOLA
int main() {
#ifndef HOLA
    cout<<"hola"<<endl;
#define HOLA
#endif
    char *c = new char('c');
    char *d = c;
    cout<<"c-> "<<*c<<endl;
    cout<<"d-> "<<*d<<endl;
    delete [] d;
    cout<<"c-> "<<*c<<endl;
    cout<<"d-> "<<*d<<endl;

    char tmp = 'c';
    *d = tmp;
    cout<<"d-> "<<*d<<endl;
    d = &tmp;
    cout<<"d-> "<<*d<<endl;
    delete [] d;
    cout<<"d-> "<<*d<<endl;

    cout<<"String is "<<sizeof(String)<<" bytes or "<<sizeof(String)*8<<" bits long"<<endl;
    String str("hello");
    cout<<str<<endl;
    return 0;
}//main