Pointer to Object Questions

Kirk McDonald kirklin.mcdonald at gmail.com
Sat Dec 30 16:55:35 PST 2006

Thomas Kuehne wrote:
> Kirk McDonald schrieb am 2006-12-30:
> <snip>
>> Since classes are by reference, anyway, there's no good reason to throw 
>> around pointers to objects.
> There is at least one valid use case for pointers to objects:
> #
> # ClassType[IndexType] aa;
> # //...
> # ClassType* c = index in aa;
> #
> Thomas

But what you have there isn't a pointer to an object, it's a pointer to 
a reference. This is an important distinction.

import std.stdio;

class Foo{}

void main() {
     Foo f = new Foo;
     void* ptr = cast(void*)f;
     Foo* fptr = &f;

     writefln("ptr:   %s", ptr);
     writefln("fptr:  %s", fptr);
     writefln("*fptr: %s", *cast(void**)fptr);

$ ./test
ptr:   B7CBAFE0
fptr:  BF96E110
*fptr: B7CBAFE0

In this example, ptr is a pointer to the actual object. fptr is a 
pointer to the /reference/. Because variables of type 'Foo' are actually 
pointers, variables of type Foo* are pointers to pointers.

The following, therefore, is a very bad idea:

Foo* func() {
     Foo f = new Foo;
     return &f;

This returns a pointer to a stack variable (the Foo reference f). This 
is a bad idea for the same reason returning a pointer to any stack 
variable is a bad idea.

Kirk McDonald
Pyd: Wrapping Python with D

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