Why and when is worth using pointer to pointer? [duplicate]

Question

Possible Duplicate:
How do pointer to pointers work in C?

Hello,

Altough I think I passed the newbie phase in programming I still have some questions that somehow I can't find them explained. Yes, there are plenty of "how to"s overthere but almost always nobody explains why and/or when one technique is useful.

In my case I have discovered that in some cases a pointer to a pointer is used in C++. Is not a pointer to an object enough? Which are the benefits? Where or when should be used a pointer to a pointer? I feel little bit desorientated in this matter.

I hope time experienced expert could respond to this concerns that hopefully is shared by other no so experienced programers. ;-)

Thank you everyone.

Julen.

Answer 1

Well, it is somehow hard to answer to such a general question.

First answer of a C++ programmer will certainly be : Do not use pointers in C++ ! As you have a lot of safer ways to handle problems than pointers, one of your goal will be to avoid them in the first place :)

So pointers to pointers are seldom used in C++. They are mainly used in C. First, because in C, strings are "char*" so when you need a "pointer to a C string" you end with a "char**". Second, as you do not have references in C, when you need to have a function that modify a pointer or that give a pointer as an output value, you need to give a pointer to a pointer parameter. You typically find that in functions that allocate memory, as they give you a pointer to the allocated memory.

If you go the C++ way, try to avoid pointers, you usually have better ways.

my2c

Answer 2

In C, an argument is passed to a function that changes it, through a pointer. You will see the same with C++ for old or legacy code (int main(int argc, char** argv)) , for code that will be accessed from C (COM / XPCOM) or with code that was written by someone used to C (or the C style).

From a "purely C++" standpoint, using pointer to pointer is in most situations a sign of poor coding style, as most situations that require a ** construct can (and should) be refactored to use safer alternatives (like std:: containers, or *& parameters).