keep changing pointer internally to the class - c++

Question

the best way to keep a pointer in main changes reflected inside the class?
static unsigned char tmp[][20] = {"hello world", "bye world"};
class X {
unsigned char ** buffer;
public:
X(unsigned char* buff)
{
buffer = &buff;
}
void printThis()
{
DBG_MSG_FORMATED(".......> %s", *buffer);
}
};
int main (int argc, char * const argv[]) {
unsigned char * buff = new unsigned char[20];
memcpy(buff, tmp[0], 12);
X x(buff);
x.printThis();
memcpy(buff, tmp[1], 12);
x.printThis();
delete [] buff;
return EXIT_SUCCESS;
}
this works, but when I do the follow
buff = tmp[0];
x.printThis();
the printout doesnt print hello world again??? how to fix that

Answer 1

You'll need to use a pointer to pointer in your class (gulp!):
class X {
unsigned char ** buffer;
public:
X(unsigned char** buff)
{
buffer = buff;
}
void printThis()
{
DBG_MSG_FORMATED(".......> %s", *buffer);
}
};
And then pass in the address of the pointer during construction:
X x(&buff);

Answer 2

int main (int argc, char * const argv[]) {
unsigned char * buff = new unsigned char[20];
memcpy(buff, tmp[0], 12);
X x(buff);
x.printThis();
delete [] buff;
buff = tmp[1];
x.printThis();
return EXIT_SUCCESS;
}
After you have done delete buff;, your pointer buffer in the class is pointing at memory that has been deleted, which is very bad news.
If you want to store the actual address of buff, you would need to pass the address of buff and store that, like this:
char **buffer;
X(unsigned char** buff)
{
buffer = buff;
}
void printThis()
{
DBG_MSG_FORMATED(".......> %s", *buffer);
}
...
X x(&buff);
Or you could make buffer a reference to buff:
char*& buffer;
X(unsigned char*& buff) : buffer(buff) {}
(No other changes needed in class or other code - but note that you can't do buffer = some_other_buffer; at a later stage - that will change the value of buff to some_other_buffer, which is probably not what you expected).

Answer 3

You can do something thing as bellow (using a pointer to a pointer), but sincerally, this more a problem than a solution because you are unable to delete tmp without a good care with the pointer in class X
#include <cstdio>
#include <cstring>
static unsigned char tmp[][20] = {"hello world", "bye world"};
class X {
unsigned char ** buffer;
public:
X(unsigned char** buff)
{
buffer = buff;
}
void printThis()
{
printf(".......> %s", *buffer);
}
};
int main (int argc, char * const argv[]) {
unsigned char * buff = new unsigned char[20];
memcpy(buff, tmp[0], 12);
X x(&buff);
x.printThis();
buff = NULL;
buff = tmp[1];
x.printThis();
}