SatView: Pointer Perfect, Part 2: Construction / Destruction - Pointer Pointers
(Page 4 of 4 )
So a pointer is a variable that stores an address and can be null. This means that a pointer can store the address to another pointer. That’s right: a pointer to a pointer.
If the term pointer pointers sounds exotic to you, don’t worry. You have been using them all along:
int main(int argc, char *argv[]) { return 0; }
See, you have been using them all the time! Argv is a pointer to an array of char and could have been written as char **argv (main is written many different ways, actually). If you define an array char arry[25]; then arry can be used and seen as a char* simply because an array points to the address of the first element; thus argv is a pointer to a char*. You can roughly translate argv as a list of a list of characters.
Using the techniques we already know we can create pointer pointers expanding on the first example from Part 1:
int main(int argc, char *argv[]) {
int myVar = 10;
int *pPtr = &myVar;
int *ppPtr = &pPtr;
return 0;
}
Again we compile it and gaze at it in the debugger:
0x0012FEB8 cccccccc ÌÌÌÌ
0x0012FEBC 0012fec8 Èþ.. ppPtr addr:0x0012FEBC value:0x0012FEC8
0x0012FEC0 cccccccc ÌÌÌÌ
0x0012FEC4 cccccccc ÌÌÌÌ
0x0012FEC8 0012fed4 Ôþ.. pPtr addr:0x0012FEC8 value:0x0012FED4
0x0012FECC cccccccc ÌÌÌÌ
0x0012FED0 cccccccc ÌÌÌÌ
0x0012FED4 0000000a .... myVar addr:0x0012FED4 value:10
0x0012FED8 cccccccc ÌÌÌÌ
You can see that ppPtr holds the address to pPtr (0x0012FEC8), which holds the address to myVar (0x0012FED4). The concept holds!
Let's take a little sidestep here. If you were not allowed to use references, but would like to return a value via a function parameter, how would you do it? Here is a function declaration that returns a boolean value to tell you whether the function succeeded or not and a counter:
bool foo(int *count);
By passing the address of an int variable you declared before calling the function, a result can be returned from inside the function by assigning a value to be held at that address. What if you want to return an address to something that is much larger than a primitive? Then you can use pointer pointers. Use them when a function wants to return the address to a dynamically allocated piece of memory through one of its function parameters. (COM actually forces you to do it this way, since pointers are capable of crossing programming language barriers and C++ references not.)
Since functions in COM always return HRESULT, the only way to return a polymorphic object (e.g. the pointer to the base class of a newly created derived object) is by storing that pointer in a pointer that can hold it:
HRESULT foo (MyBaseClass **newObj);
In the foo function you can create a new MyBaseClass inside the newObj pointer pointer:
*newobj = reinterpret_cast<MyBaseClass*>(new MyDerivedClass);
Pointer arithmetic combined with pointer pointers allow you to do some pretty nifty things. If we assume pMem to be a character pointer to a piece of memory that was read binary from a file and pOffset a character pointer to some memory that contains the offsets of objects inside that file, you could calculate pMem to certain offsets with the following statement (using C-style casting on purpose to maintain readability):
*((char **)(&pMem))=((char *)((unsigned int)pOffset)+((unsigned int)pMem));
There are better ways to construct binary file formats, but this one shows nicely how complicated the use of pointers can become. Next time we will look at smart pointers; C++ classes that help you manage all these crazy pointers.
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