Is the memory address of classinfo the same for all instances of a class?

[Heywood Floyd]

On Jul 2, 2010, at 15:34 , Steven Schveighoffer wrote:

> On Fri, 02 Jul 2010 09:32:39 -0400, Steven Schveighoffer <schveiguy at yahoo.com> wrote:
> 
>> On Fri, 02 Jul 2010 09:24:20 -0400, Heywood Floyd <soul8o8 at gmail.com> wrote:
>> 
>>> 
>>> Good day!
>>> 
>>> 
>>> Consider
>>> 
>>> // - - - -
>>> class Foo{}
>>> auto one = new Foo();
>>> auto two = new Foo();
>>> writefln("one: %x  two: %x", &one.classinfo, &two.classinfo);
>>> // - - - -
>>> 
>>> For me this results in two identical memory addresses "every time".
>>> 
>>> Can I rely on this?
>>> Can I design software based on the assumption that these addresses are always the same?
>>> 
>>> (I'd like to be able to use the memory address as the key in an associative array, for quick by-class
>>> lookups.)
>> 
>> Use classinfo.name.  The classinfo is the same memory address in the same executable/dynamic library.  If you open another D dynamic library, the classinfo address for the same class may be different, but the name will be the same.
>> 
>> Note that comparing classinfo.names will be just as fast as comparing classinfo addresses if the names are at the same address (which will be true if the classinfo is at the same address) because the string comparison function short-circuits if the addresses are the same.
> 
> Duh, just realized, classinfos should use this same method to compare.  Just use the whole class info as the key, don't take the address.
> 
> -Steve


Alright thanks!

Ok, loading in code dynamically changes the addresses. Good point. Thanks!

I looked up the TypeInfo_Class-implementation and it seems to compare class names. So that looks good. Will use the classinfos directly like you suggested. Seems proper.


***

Hm, but still, I can't quite let go of this.
Even if the string comparer can short-circuit, it still has to go through strings that are _not_ of the same address untill it spots a difference, as they could potentially be equal anyway?

I noted that the classinfo.name-strings typically looks like this:

	classtype.Foo
	classtype.Bar
	classtype.Cat
	classtype.Dog

Doesn't this first "classtype."-part introduce overhead when these strings are used as keys in an AA? The string comparer more or less always have to check the first 10 chars, which are equal for all. (I know I'm being picky here. But the whole using memory addresses-thing came from the fear of string comparisons being suboptimal.)

/heywood

(PS. Feature-request: move the "classtype."-part of classinfo names to the end ; )