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
