Re: Finding out about D - 101

[Steven Schveighoffer]

On Mon, 11 May 2009 12:42:17 -0400, Steve Teale  
steve.te...@britseyeview.com wrote:


OK, so let's find out about strings. D does not have 'string' as a built  
in type. It is just an alias for invariant(char[]). So what is it?


Well, you can find out quite a bit by compiling and running:

import std.stdio;

struct S
{
   size_t len;
   void* p;
   int xtra;
}

void main()
{
   string a;
   string b = ;
   char[] ca = ['a','b','c','d'];
   string c = abcd;

   writefln((a == b) %s, (a == b));

   S* s = cast(S*) b;

   writefln(a: [%s] %s %08x, %d, a, (a is null), a.ptr, a.length);
   writefln(b: [%s] %s %08x, %d, b, (b is null), b.ptr, b.length);

   writefln(b as S: [%08x] %d [%08x], s.p, s.len, s.xtra);
   writefln(char at *b.ptr = [%s] (%d), *b.ptr, cast(ubyte) *b.ptr);
   s = cast(S*) ca;
   writefln(ca as S: [%08x] %d [%08x], s.p, s.len, s.xtra);
   s = cast(S*) c;
   writefln(c as S: [%08x] %d [%08x], s.p, s.len, s.xtra);
}

This tells me that a string in D is a sequence of characters (whatever  
that might mean) in memory, prefixed by a size_t length. Of  course,  
that's not to say there is no more to it, but the values for 'xtra'  
don't give us much clue. If there was some other member of struct S, I'd  
kind of expect to see bit flags there indicating whether the array of  
char was invariant, const, or fair game.


So I'm left with the question as to how does 'invariant' have teeth?


invariant is not a runtime flag, but rather a type modifier.  The teeth of  
invariance only exist during compilation, once you're in runtime, the  
expectation is that the compiler will have weeded out all your attempts to  
change the invariant data, so no need to keep track of its invariance.  It  
is not immune to casting.


And BTW, string is invariant(char)[], not invariant(char[]), which is a  
distinct difference.  The former is a mutable array of invariant  
characters, the latter is an invariant array of invariant characters.  You  
can't change the length or reassign an invariant(char[]), but you can  
reassign/append to an invariant(char)[].


The xtra flags do not exist.  You are printing out garbage data on the  
stack (most likely the pointer field of ca).


-Steve

Re: Finding out about D - 101

[BCS]

Hello Steve,


This tells me that a string in D is a sequence of characters (whatever
that might mean) in memory, prefixed by a size_t length. Of  course,
that's not to say there is no more to it, but the values for 'xtra'
don't give us much clue. If there was some other member of struct S,
I'd kind of expect to see bit flags there indicating whether the array
of char was invariant, const, or fair game.


T[] is a length/pointer pair. If the data ends up being right after that 
bit, then it's just a coincidence.




So I'm left with the question as to how does 'invariant' have teeth?



const and invariant are both implemented completely at compile time so they 
don't show up the final code at all.

Re: Finding out about D - 101

[Steve Teale]

Steven Schveighoffer Wrote:

  This tells me that a string in D is a sequence of characters (whatever  
  that might mean) in memory, prefixed by a size_t length. Of  course,  
  that's not to say there is no more to it, but the values for 'xtra'  
  don't give us much clue. If there was some other member of struct S, I'd  
  kind of expect to see bit flags there indicating whether the array of  
  char was invariant, const, or fair game.
 
  So I'm left with the question as to how does 'invariant' have teeth?
 
 invariant is not a runtime flag, but rather a type modifier.  The teeth of  
 invariance only exist during compilation, once you're in runtime, the  
 expectation is that the compiler will have weeded out all your attempts to  
 change the invariant data, so no need to keep track of its invariance.  It  
 is not immune to casting.
 
 And BTW, string is invariant(char)[], not invariant(char[]), which is a  
 distinct difference.  The former is a mutable array of invariant  
 characters, the latter is an invariant array of invariant characters.  You  
 can't change the length or reassign an invariant(char[]), but you can  
 reassign/append to an invariant(char)[].
 
 The xtra flags do not exist.  You are printing out garbage data on the  
 stack (most likely the pointer field of ca).
 
 -Steve

Great, so there are experienced D programmers checking on this newsgroup!

I'd kind of figured out from the test program that the invariant thing was just 
compile time, but  thanks for pointing out my misconception about the string 
alias. I should read things more carefully.

- Also Steve